linux-kernel - [PATCH 3.16 13/76] x86/alternatives: Guard NOPs optimization

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Message-ID: <lsq.1520823972.987771650@decadent.org.uk>
Date:   Mon, 12 Mar 2018 03:06:12 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Borislav Petkov" <bp@...e.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Andy Lutomirski" <luto@...capital.net>,
        "Denys Vlasenko" <dvlasenk@...hat.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...nel.org>, "Borislav Petkov" <bp@...en8.de>,
        "Brian Gerst" <brgerst@...il.com>
Subject: [PATCH 3.16 13/76] x86/alternatives: Guard NOPs optimization

3.16.56-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Borislav Petkov <bp@...e.de>

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@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1428181662-18020-1-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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: ",