x86, fpu, amd: Clear exceptions in AMD FXSAVE workaround

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@halfdog.net>
Tested-by: Borislav Petkov <bp@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@zytor.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
index c49a613c6452..cea1c76d49bf 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -293,12 +293,13 @@ 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);
 }