[PATCH] x86/alternatives: Guard NOPs optimization

From: Borislav Petkov
Date: Sat Apr 04 2015 - 17:09:53 EST

From: Borislav Petkov <bp@xxxxxxx>

Take a look at the first insn byte before optimizing the NOP -
there might be something else already, like the ALTERNATIVE_2() in
rdtsc_barrier() which nops out on AMD even though we just patched in an

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>
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 7c4ad005d7a0..aef653193160 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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