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

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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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 e25cc33ec54d..e72b2e41499e 100644
--- a/arch/x86/include/asm/fpu-internal.h
+++ b/arch/x86/include/asm/fpu-internal.h
@@ -295,12 +295,13 @@ 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);
 }