[PATCH 3.16 16/76] x86/alternatives: Fix optimize_nops() checking

[Ben Hutchings]

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

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

From: Borislav Petkov <bp@xxxxxxx>

commit 612e8e9350fd19cae6900cf36ea0c6892d1a0dca upstream.

The alternatives code checks only the first byte whether it is a NOP, but
with NOPs in front of the payload and having actual instructions after it
breaks the "optimized' test.

Make sure to scan all bytes before deciding to optimize the NOPs in there.

Reported-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jiri Kosina <jikos@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Andrew Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20180110112815.mgciyf5acwacphkq@xxxxxxx
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -326,9 +326,12 @@ done:
 static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
 {
 	unsigned long flags;
+	int i;
 
-	if (instr[0] != 0x90)
-		return;
+	for (i = 0; i < a->padlen; i++) {
+		if (instr[i] != 0x90)
+			return;
+	}
 
 	local_irq_save(flags);
 	add_nops(instr + (a->instrlen - a->padlen), a->padlen);