[PATCH 3.8 125/166] x86, fpu, amd: Clear exceptions in AMD FXSAVE workaround

[Kamal Mostafa]

3.8.13.16 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds at linux-foundation.org>

commit 26bef1318adc1b3a530ecc807ef99346db2aa8b0 upstream.

Before we do an EMMS in the AMD FXSAVE information leak workaround we
need to clear any pending exceptions, otherwise we trap with a
floating-point exception inside this code.

Reported-by: halfdog <me at halfdog.net>
Tested-by: Borislav Petkov <bp at suse.de>
Link: http://lkml.kernel.org/r/CA%2B55aFxQnY_PCG_n4=0w-VG=YLXL-yr7oMxyy0WU2gCBAf3ydg@mail.gmail.com
Signed-off-by: H. Peter Anvin <hpa at zytor.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/include/asm/fpu-internal.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index 41ab26e..87fac9e 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -294,12 +294,13 @@ static inline int restore_fpu_checking(struct task_struct *tsk)
 	/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
 	   is pending.  Clear the x87 state here by setting it to fixed
 	   values. "m" is a random variable that should be in L1 */
-	alternative_input(
-		ASM_NOP8 ASM_NOP2,
-		"emms\n\t"		/* clear stack tags */
-		"fildl %P[addr]",	/* set F?P to defined value */
-		X86_FEATURE_FXSAVE_LEAK,
-		[addr] "m" (tsk->thread.fpu.has_fpu));
+	if (unlikely(static_cpu_has(X86_FEATURE_FXSAVE_LEAK))) {
+		asm volatile(
+			"fnclex\n\t"
+			"emms\n\t"
+			"fildl %P[addr]"	/* set F?P to defined value */
+			: : [addr] "m" (tsk->thread.fpu.has_fpu));
+	}
 
 	return fpu_restore_checking(&tsk->thread.fpu);
 }
-- 
1.8.3.2