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Force some compilers to respect volatile reads

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Inspection of the generated assembly showed that before this commit, armcc 5
was optimizing away the successive reads to the volatile local variable that's
used for double-checks. Inspection also reveals that inserting a call to an
external function is enough to prevent it from doing that.

The tested versions of ARM-GCC, Clang and Armcc 6 (aka armclang) all keep the
double read, with our without a call to an external function in the middle.

The inserted function can also be changed to insert a random delay if
desired in the future, as it is appropriately places between the reads.
This commit is contained in:
Manuel Pégourié-Gonnard 2019-11-08 10:21:00 +01:00
parent ca7b5ab5ef
commit 72a8c9e7dc
4 changed files with 19 additions and 0 deletions
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2
tinycrypt/ecc_dsa.c
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@ -67,6 +67,7 @@
#if defined(MBEDTLS_USE_TINYCRYPT)
#include <tinycrypt/ecc.h>
#include <tinycrypt/ecc_dsa.h>
#include "mbedtls/platform_util.h"
#if default_RNG_defined
static uECC_RNG_Function g_rng_function = &default_CSPRNG;
@ -304,6 +305,7 @@ int uECC_verify(const uint8_t *public_key, const uint8_t *message_hash,
/* Accept only if v == r. */
diff = uECC_vli_equal(rx, r);
if (diff == 0) {
mbedtls_platform_enforce_volatile_reads();
if (diff == 0) {
return UECC_SUCCESS;
}