x86/alternatives: Guard NOPs optimization
authorBorislav Petkov <bp@suse.de>
Sat, 4 Apr 2015 21:07:42 +0000 (23:07 +0200)
committerIngo Molnar <mingo@kernel.org>
Mon, 6 Apr 2015 07:24:09 +0000 (09:24 +0200)
commit69df353ff305805fc16082d0c5bfa6e20fa8b863
tree2d6377b1a094d38e0993ba94c0b9a72d3ff716b5
parentfc3e958a2b552fe6210e6de3bebb4348156a0b5f
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