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 5cb272a7a5a32eccbbee61a23c50ab9b5a80a0fb..10d5a3d6affc454fd0f4e46208929d4560b63bfa 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -340,9 +340,12 @@ 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);