| From: Borislav Petkov <bp@suse.de> |
| Date: Sat, 4 Apr 2015 23:07:42 +0200 |
| Subject: x86/alternatives: Guard NOPs optimization |
| |
| commit 69df353ff305805fc16082d0c5bfa6e20fa8b863 upstream. |
| |
| Take a look at the first instruction byte before optimizing the NOP - |
| there might be something else there already, like the ALTERNATIVE_2() |
| in rdtsc_barrier() which NOPs out on AMD even though we just |
| patched in an MFENCE. |
| |
| This happens because the alternatives sees X86_FEATURE_MFENCE_RDTSC, |
| AMD CPUs set it, we patch in the MFENCE and right afterwards it sees |
| X86_FEATURE_LFENCE_RDTSC which AMD CPUs don't set and we blindly |
| optimize the NOP. |
| |
| Checking whether at least the first byte is 0x90 prevents that. |
| |
| Signed-off-by: Borislav Petkov <bp@suse.de> |
| Cc: Andy Lutomirski <luto@amacapital.net> |
| Cc: Borislav Petkov <bp@alien8.de> |
| Cc: Brian Gerst <brgerst@gmail.com> |
| Cc: Denys Vlasenko <dvlasenk@redhat.com> |
| Cc: H. Peter Anvin <hpa@zytor.com> |
| Cc: Linus Torvalds <torvalds@linux-foundation.org> |
| Cc: Thomas Gleixner <tglx@linutronix.de> |
| Link: http://lkml.kernel.org/r/1428181662-18020-1-git-send-email-bp@alien8.de |
| Signed-off-by: Ingo Molnar <mingo@kernel.org> |
| Signed-off-by: Ben Hutchings <ben@decadent.org.uk> |
| --- |
| arch/x86/kernel/alternative.c | 3 +++ |
| 1 file changed, 3 insertions(+) |
| |
| --- a/arch/x86/kernel/alternative.c |
| +++ b/arch/x86/kernel/alternative.c |
| @@ -323,6 +323,9 @@ done: |
| |
| static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr) |
| { |
| + if (instr[0] != 0x90) |
| + return; |
| + |
| add_nops(instr + (a->instrlen - a->padlen), a->padlen); |
| |
| DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ", |
