[tip:x86/pti] x86/alternatives: Fix optimize_nops() checking

[tip-bot for Borislav Petkov]

Commit-ID:  d1cb4348f683d132ef2d468d4e9ad421486163f9
Gitweb:     https://git.kernel.org/tip/d1cb4348f683d132ef2d468d4e9ad421486163f9
Author:     Borislav Petkov <bp@xxxxxxx>
AuthorDate: Wed, 10 Jan 2018 12:28:16 +0100
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Wed, 10 Jan 2018 18:28:21 +0100

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 <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

---
 arch/x86/kernel/alternative.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d33..e0b97e4 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);