From 899635655e977a2688ec3bf2430a08444a2a0004 Mon Sep 17 00:00:00 2001 From: Cesar Eduardo Barros Date: Mon, 25 Nov 2013 22:00:41 -0200 Subject: crypto: more robust crypto_memneq Disabling compiler optimizations can be fragile, since a new optimization could be added to -O0 or -Os that breaks the assumptions the code is making. Instead of disabling compiler optimizations, use a dummy inline assembly (based on RELOC_HIDE) to block the problematic kinds of optimization, while still allowing other optimizations to be applied to the code. The dummy inline assembly is added after every OR, and has the accumulator variable as its input and output. The compiler is forced to assume that the dummy inline assembly could both depend on the accumulator variable and change the accumulator variable, so it is forced to compute the value correctly before the inline assembly, and cannot assume anything about its value after the inline assembly. This change should be enough to make crypto_memneq work correctly (with data-independent timing) even if it is inlined at its call sites. That can be done later in a followup patch. Compile-tested on x86_64. Signed-off-by: Cesar Eduardo Barros Acked-by: Daniel Borkmann Signed-off-by: Herbert Xu --- crypto/Makefile | 5 ----- 1 file changed, 5 deletions(-) (limited to 'crypto/Makefile') diff --git a/crypto/Makefile b/crypto/Makefile index 989c510d..b29402a7 100644 --- a/crypto/Makefile +++ b/crypto/Makefile @@ -2,11 +2,6 @@ # Cryptographic API # -# memneq MUST be built with -Os or -O0 to prevent early-return optimizations -# that will defeat memneq's actual purpose to prevent timing attacks. -CFLAGS_REMOVE_memneq.o := -O1 -O2 -O3 -CFLAGS_memneq.o := -Os - obj-$(CONFIG_CRYPTO) += crypto.o crypto-y := api.o cipher.o compress.o memneq.o -- cgit v1.2.3