From ecd1891c5128b5293138c3f350b6e68bce5ca579 Mon Sep 17 00:00:00 2001 From: Andres Amaya Garcia Date: Thu, 26 Oct 2017 22:43:41 +0100 Subject: [PATCH] Change mbedtls_zeroize() to prevent optimizations Change mbedtls_zeroize() implementation to use memset() instead of a custom implementation for performance reasons. Furthermore, we would also like to prevent as much as we can compiler optimisations that remove zeroization code. The implementation of mbedtls_zeroize() now uses a volatile function pointer to memset() as suggested by Colin Percival at: http://www.daemonology.net/blog/2014-09-04-how-to-zero-a-buffer.html --- library/utils.c | 29 ++++++++++++++++++++++++----- 1 file changed, 24 insertions(+), 5 deletions(-) diff --git a/library/utils.c b/library/utils.c index 3819558f4..1adf8adf4 100644 --- a/library/utils.c +++ b/library/utils.c @@ -28,14 +28,33 @@ #include "mbedtls/utils.h" #include +#include #if !defined(MBEDTLS_UTILS_ZEROIZE_ALT) -/* This implementation should never be optimized out by the compiler */ +/* + * This implementation should never be optimized out by the compiler + * + * This implementation for mbedtls_zeroize() uses a volatile function pointer. + * We always know that it points to memset(), but because it is volatile the + * compiler expects it to change at any time and will not optimize out the + * call that could potentially perform other operations on the input buffer + * instead of just setting it to 0. Nevertheless, optimizations of the + * following form are still possible: + * + * if( memset_func != memset ) + * memset_func( buf, 0, len ); + * + * Note that it is extremely difficult to guarantee that mbedtls_zeroize() + * will not be optimized out by aggressive compilers in a portable way. For + * this reason, mbed TLS also provides the configuration option + * MBEDTLS_UTILS_ZEROIZE_ALT, which allows users to configure + * mbedtls_zeroize() to use a suitable implementation for their platform and + * needs. + */ +static void * (* const volatile memset_func)( void *, int, size_t ) = memset; + void mbedtls_zeroize( void *buf, size_t len ) { - volatile unsigned char *p = (unsigned char *)buf; - - while( len-- ) - *p++ = 0; + memset_func( buf, 0, len ); } #endif /* MBEDTLS_UTILS_ZEROIZE_ALT */