x86/alternatives: Fix optimize_nops() checking

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@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Andrew Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: https://lkml.kernel.org/r/20180110112815.mgciyf5acwacphkq@pd.tnic
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 25f909362b7a89c42f32c8ebe74fb91239df3ca8..d6f375f1b9282c05512b87c5201b3ce9ba22cd55 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -339,9 +339,12 @@ 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);