From b37860d2714333ca20eeb45f13c9ea72c1a9174a Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 18 Feb 2015 14:07:21 +0100
Subject: [PATCH 344/414] arm/futex: disable preemption during
 futex_atomic_cmpxchg_inatomic()

The ARM UP implementation of futex_atomic_cmpxchg_inatomic() assumes that
pagefault_disable() inherits a preempt disabled section. This assumtion
is true for mainline but -RT reverts this and allows preemption in
pagefault disabled regions.
The code sequence of futex_atomic_cmpxchg_inatomic():

|   x = *futex;
|   if (x == oldval)
|           *futex = newval;

The problem occurs if the code is preempted after reading the futex value or
after comparing it with x. While preempted, the futex owner has to be
scheduled which then releases the lock (in userland because it has no waiter
yet). Once the code is back on the CPU, it overwrites the futex value
with with the old PID and the waiter bit set.

The workaround is to explicit disable code preemption to avoid the
described race window.

Debugged-by:  Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 arch/arm/include/asm/futex.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
index 2aff798fbef4..54d26a98ff84 100644
--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -90,6 +90,8 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
 		return -EFAULT;
 
+	preempt_disable_rt();
+
 	__asm__ __volatile__("@futex_atomic_cmpxchg_inatomic\n"
 	"1:	" TUSER(ldr) "	%1, [%4]\n"
 	"	teq	%1, %2\n"
@@ -101,6 +103,8 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 	: "cc", "memory");
 
 	*uval = val;
+
+	preempt_enable_rt();
 	return ret;
 }
 
-- 
2.10.2

