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Fix uninitialised memory access in constant time functions

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Fix an issue reported by Coverity whereby some constant time functions
called from the ssl decrypt code could potentially access uninitialised
memory.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
This commit is contained in:
Paul Elliott 2022-05-09 18:15:54 +01:00
parent a745c7d439
commit 5260ce27ed
3 changed files with 21 additions and 2 deletions
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12
library/constant_time.c
View file

@ -514,6 +514,12 @@ int mbedtls_ct_hmac( mbedtls_svc_key_id_t key,
PSA_CHK( psa_hash_update( &operation, add_data, add_data_len ) );
PSA_CHK( psa_hash_update( &operation, data, min_data_len ) );
/* Fill the hash buffer in advance with something that is
* not a valid hash (barring an attack on the hash and
* deliberately-crafted input), in case the caller doesn't
* check the return status properly. */
memset( output, '!', hash_size );
/* For each possible length, compute the hash up to that point */
for( offset = min_data_len; offset <= max_data_len; offset++ )
{
@ -609,6 +615,12 @@ int mbedtls_ct_hmac( mbedtls_md_context_t *ctx,
MD_CHK( mbedtls_md_update( ctx, add_data, add_data_len ) );
MD_CHK( mbedtls_md_update( ctx, data, min_data_len ) );
/* Fill the hash buffer in advance with something that is
* not a valid hash (barring an attack on the hash and
* deliberately-crafted input), in case the caller doesn't
* check the return status properly. */
memset( output, '!', hash_size );
/* For each possible length, compute the hash up to that point */
for( offset = min_data_len; offset <= max_data_len; offset++ )
{

7
library/constant_time_internal.h
View file

@ -221,6 +221,13 @@ void mbedtls_ct_memcpy_if_eq( unsigned char *dest,
* offset_secret, but only on \p offset_min, \p offset_max and \p len.
* Functionally equivalent to `memcpy(dst, src + offset_secret, len)`.
*
* \note This function reads from \p dest, but the value that
* is read does not influence the result and this
* function's behavior is well-defined regardless of the
* contents of the buffers. This may result in false
* positives from static or dynamic analyzers, especially
* if \p dest is not initialized.
*
* \param dest The destination buffer. This must point to a writable
* buffer of at least \p len bytes.
* \param src The base of the source buffer. This must point to a

4
library/ssl_msg.c
View file

@ -1634,8 +1634,8 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_MAC)
if( auth_done == 0 )
{
unsigned char mac_expect[MBEDTLS_SSL_MAC_ADD];
unsigned char mac_peer[MBEDTLS_SSL_MAC_ADD];
unsigned char mac_expect[MBEDTLS_SSL_MAC_ADD] = { 0 };
unsigned char mac_peer[MBEDTLS_SSL_MAC_ADD] = { 0 };
/* If the initial value of padlen was such that
* data_len < maclen + padlen + 1, then padlen