x86-64: fix FPU corruption with signals and preemption

In 64bit signal delivery path, clear_used_math() was happening before saving
the current active FPU state on to the user stack for signal handling. Between
clear_used_math() and the state store on to the user stack, potentially we
can get a page fault for the user address and can block. Infact, while testing
we were hitting the might_fault() in __clear_user() which can do a schedule().

At a later point in time, we will schedule back into this process and
resume the save state (using "xsave/fxsave" instruction) which can lead
to DNA fault. And as used_math was cleared before, we will reinit the FP state
in the DNA fault and continue. This reinit will result in loosing the
FPU state of the process.

Move clear_used_math() to a point after the FPU state has been stored
onto the user stack.

This issue is present from a long time (even before the xsave changes
and the x86 merge). But it can easily be exposed in 2.6.28.x and 2.6.29.x
series because of the __clear_user() in this path, which has an explicit
__cond_resched() leading to a context switch with CONFIG_PREEMPT_VOLUNTARY.

[ Impact: fix FPU state corruption ]
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: <stable@kernel.org>			[2.6.28.x, 2.6.29.x]
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/xsave.c

@@ -89,7 +89,7 @@ int save_i387_xstate(void __user *buf)
 
 	if (!used_math())
 		return 0;
-	clear_used_math(); /* trigger finit */
+
 	if (task_thread_info(tsk)->status & TS_USEDFPU) {
 		/*
 	 	 * Start with clearing the user buffer. This will present a
@@ -114,6 +114,8 @@ int save_i387_xstate(void __user *buf)
 			return -1;
 	}
 
+	clear_used_math(); /* trigger finit */
+
 	if (task_thread_info(tsk)->status & TS_XSAVE) {
 		struct _fpstate __user *fx = buf;
 		struct _xstate __user *x = buf;