x86/alternatives: Fix optimize_nops() checking

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 <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andrew Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Paul Turner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d3382e91..e0b97e4d1db5 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -344,9 +344,12 @@ done:
 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);