From 92a2f01ea40ec52f8f77893ff433dc47c1f5b9ef Mon Sep 17 00:00:00 2001 From: Bernd Edlinger Date: Fri, 21 Jun 2019 21:26:19 +0200 Subject: [PATCH] Add value_barriers in constant time select functions The barriers prevent the compiler from narrowing down the possible value range of the mask and ~mask in the select statements, which avoids the recognition of the select and turning it into a conditional load or branch. Reviewed-by: Paul Dale (Merged from https://github.com/openssl/openssl/pull/9418) --- include/internal/constant_time_locl.h | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) diff --git a/include/internal/constant_time_locl.h b/include/internal/constant_time_locl.h index 18d2f56825..b702675db8 100644 --- a/include/internal/constant_time_locl.h +++ b/include/internal/constant_time_locl.h @@ -158,11 +158,29 @@ static ossl_inline unsigned char constant_time_eq_int_8(int a, int b) return constant_time_eq_8((unsigned)(a), (unsigned)(b)); } +/* + * Returns the value unmodified, but avoids optimizations. + * The barriers prevent the compiler from narrowing down the + * possible value range of the mask and ~mask in the select + * statements, which avoids the recognition of the select + * and turning it into a conditional load or branch. + */ +static ossl_inline unsigned int value_barrier(unsigned int a) +{ +#if !defined(OPENSSL_NO_ASM) && defined(__GNUC__) + unsigned int r; + __asm__("" : "=r"(r) : "0"(a)); +#else + volatile unsigned int r = a; +#endif + return r; +} + static ossl_inline unsigned int constant_time_select(unsigned int mask, unsigned int a, unsigned int b) { - return (mask & a) | (~mask & b); + return (value_barrier(mask) & a) | (value_barrier(~mask) & b); } static ossl_inline unsigned char constant_time_select_8(unsigned char mask, -- 2.25.1