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Add counter-measure to cache-based Lucky 13

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The basis for the Lucky 13 family of attacks is for an attacker to be able to
distinguish between (long) valid TLS-CBC padding and invalid TLS-CBC padding.
Since our code sets padlen = 0 for invalid padding, the length of the input to
the HMAC function, and the location where we read the MAC, give information
about that.

A local attacker could gain information about that by observing via a
cache attack whether the bytes at the end of the record (at the location of
would-be padding) have been read during MAC verification (computation +
comparison).

Let's make sure they're always read.
This commit is contained in:
Manuel Pégourié-Gonnard 2018-06-28 10:38:35 +02:00
parent 1cc1fb0599
commit 7b42030b5d
2 changed files with 59 additions and 1 deletions
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52
library/ssl_tls.c
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@ -1276,6 +1276,27 @@ static void ssl_mac( mbedtls_md_context_t *md_ctx,
#define SSL_SOME_MODES_USE_MAC
#endif
/* The function below is only used in the Lucky 13 counter-measure in
* ssl_decrypt_buf(). These are the defines that guard the call site. */
#if defined(SSL_SOME_MODES_USE_MAC) && \
( defined(MBEDTLS_SSL_PROTO_TLS1) || \
defined(MBEDTLS_SSL_PROTO_TLS1_1) || \
defined(MBEDTLS_SSL_PROTO_TLS1_2) )
/* This function makes sure every byte in the memory region is accessed
* (in ascending addresses order) */
static void ssl_read_memory( unsigned char *p, size_t len )
{
unsigned char acc = 0;
volatile unsigned char force;
for( ; len != 0; p++, len-- )
acc ^= *p;
force = acc;
(void) force;
}
#endif /* SSL_SOME_MODES_USE_MAC && ( TLS1 || TLS1_1 || TLS1_2 ) */
/*
* Encryption/decryption functions
*/
@ -2011,6 +2032,20 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
* linking an extra division function in some builds).
*/
size_t j, extra_run = 0;
/*
* The next two sizes are the minimum and maximum values of
* in_msglen over all padlen values.
*
* They're independent of padlen, since we previously did
* in_msglen -= padlen.
*
* Note that max_len + maclen is never more than the buffer
* length, as we previously did in_msglen -= maclen too.
*/
const size_t max_len = ssl->in_msglen + padlen;
const size_t min_len = ( max_len > 256 ) ? max_len - 256 : 0;
switch( ssl->transform_in->ciphersuite_info->mac )
{
#if defined(MBEDTLS_MD5_C) || defined(MBEDTLS_SHA1_C) || \
@ -2042,12 +2077,25 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
mbedtls_md_hmac_update( &ssl->transform_in->md_ctx_dec, ssl->in_len, 2 );
mbedtls_md_hmac_update( &ssl->transform_in->md_ctx_dec, ssl->in_msg,
ssl->in_msglen );
/* Make sure we access everything even when padlen > 0. This
* makes the synchronisation requirements for just-in-time
* Prime+Probe attacks much tighter and hopefully impractical. */
ssl_read_memory( ssl->in_msg + ssl->in_msglen, padlen );
mbedtls_md_hmac_finish( &ssl->transform_in->md_ctx_dec, mac_expect );
/* Call mbedtls_md_process at least once due to cache attacks */
/* Call mbedtls_md_process at least once due to cache attacks
* that observe whether md_process() was called of not */
for( j = 0; j < extra_run + 1; j++ )
mbedtls_md_process( &ssl->transform_in->md_ctx_dec, ssl->in_msg );
mbedtls_md_hmac_reset( &ssl->transform_in->md_ctx_dec );
/* Make sure we access all the memory that could contain the MAC,
* before we check it in the next code block. This makes the
* synchronisation requirements for just-in-time Prime+Probe
* attacks much tighter and hopefully impractical. */
ssl_read_memory( ssl->in_msg + min_len,
max_len - min_len + ssl->transform_in->maclen );
}
else
#endif /* MBEDTLS_SSL_PROTO_TLS1 || MBEDTLS_SSL_PROTO_TLS1_1 || \
@ -2057,9 +2105,11 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
}
#if defined(MBEDTLS_SSL_DEBUG_ALL)
MBEDTLS_SSL_DEBUG_BUF( 4, "expected mac", mac_expect, ssl->transform_in->maclen );
MBEDTLS_SSL_DEBUG_BUF( 4, "message mac", ssl->in_msg + ssl->in_msglen,
ssl->transform_in->maclen );
#endif
if( mbedtls_ssl_safer_memcmp( ssl->in_msg + ssl->in_msglen, mac_expect,
ssl->transform_in->maclen ) != 0 )