3.16.56-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Borislav Petkov <bp@xxxxxxx> 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@xxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1428181662-18020-1-git-send-email-bp@xxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- arch/x86/kernel/alternative.c | 3 +++ 1 file changed, 3 insertions(+) --- 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: ",