Messages in this thread Patch in this message | | | Date | Tue, 7 Apr 2015 02:41:04 -0700 | From | tip-bot for Borislav Petkov <> | Subject | [tip:x86/asm] x86/alternatives: Guard NOPs optimization |
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Commit-ID: 69df353ff305805fc16082d0c5bfa6e20fa8b863 Gitweb: http://git.kernel.org/tip/69df353ff305805fc16082d0c5bfa6e20fa8b863 Author: Borislav Petkov <bp@suse.de> AuthorDate: Sat, 4 Apr 2015 23:07:42 +0200 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Mon, 6 Apr 2015 09:24:09 +0200
x86/alternatives: Guard NOPs optimization
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> --- arch/x86/kernel/alternative.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 7c4ad00..aef6531 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -325,6 +325,9 @@ 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: ",
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