• Borislav Petkov's avatar
    x86/alternatives: Guard NOPs optimization · 69df353f
    Borislav Petkov authored
    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: default avatarBorislav 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.deSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    69df353f
alternative.c 19.3 KB