From: Borislav Petkov <bp@alien8.de>
To: Ingo Molnar <mingo@kernel.org>
Cc: X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: [PATCH] x86/alternatives: Guard NOPs optimization
Date: Sat, 4 Apr 2015 23:07:42 +0200 [thread overview]
Message-ID: <1428181662-18020-1-git-send-email-bp@alien8.de> (raw)
From: Borislav Petkov <bp@suse.de>
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
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>
---
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: ",
--
2.3.3
next reply other threads:[~2015-04-04 21:09 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-04 21:07 Borislav Petkov [this message]
2015-04-07 9:41 ` [tip:x86/asm] x86/alternatives: Guard NOPs optimization tip-bot for Borislav Petkov
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