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>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index e6fbb94..20061b9 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -582,12 +582,13 @@ void __math_state_restore(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. safe_address 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" (safe_address));
+       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" (safe_address));
+       }
 
        /*
         * Paranoid restore. send a SIGSEGV if we fail to restore the state.