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

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>
Date: Wed, 10 Jan 2018 12:14:07 +0100

We check only the first byte whether it is a NOP but if David Woodhouse
wants to do some crazy experiments with slapping NOPs in front of the
payload and having the actual instructions after it, this "optimized"
test breaks. :-)

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

Reported-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: x86@xxxxxxxxxx
---
 arch/x86/kernel/alternative.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d3382e91..78932b283915 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -344,9 +344,11 @@ recompute_jump(struct alt_instr *a, u8 *orig_insn, u8 *repl_insn, u8 *insnbuf)
 static void __init_or_module noinline 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);
-- 
2.13.0


-- 
Regards/Gruss,
    Boris.

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