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>
(cherry picked from commit 26bef1318adc1b3a530ecc807ef99346db2aa8b0)
[wt: in 2.6.32, patch applies to arch/x86/include/asm/i387.h. There's
 no static_cpu_has() so we use boot_cpu_has() like other kernels do
 with gcc3.
]
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 0b20bbb758f26594358f55c8ceb827dc2d35bdaf..cb42fad1f6fde5fc74b739c41add4b1a987ccccb 100644 (file)
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -242,12 +242,13 @@ clear_state:
        /* 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(
-               GENERIC_NOP8 GENERIC_NOP2,
-               "emms\n\t"              /* clear stack tags */
-               "fildl %[addr]",        /* set F?P to defined value */
-               X86_FEATURE_FXSAVE_LEAK,
-               [addr] "m" (safe_address));
+       if (unlikely(boot_cpu_has(X86_FEATURE_FXSAVE_LEAK))) {
+               asm volatile(
+                       "fnclex\n\t"
+                       "emms\n\t"
+                       "fildl %[addr]"        /* set F?P to defined value */
+                       : : [addr] "m" (safe_address));
+       }
 end:
        task_thread_info(tsk)->status &= ~TS_USEDFPU;
 }