From d5ba50e239fdb117172ba8162db607c5ad189577 Mon Sep 17 00:00:00 2001 From: Gilles Peskine Date: Fri, 10 Dec 2021 21:33:21 +0100 Subject: [PATCH] Zeroize local MAC variables Zeroize local MAC variables used for CBC+HMAC cipher suites. In encryption, this is just good hygiene but probably not needed for security since the data protected by the MAC that could leak is about to be transmitted anyway. In DTLS decryption, this could be a security issue since an adversary could learn the MAC of data that they were trying to inject. At least with encrypt-then-MAC, the adversary could then easily inject a datagram with a corrected packet. TLS would still be safe since the receiver would close the connection after the bad MAC. Signed-off-by: Gilles Peskine --- ChangeLog.d/ssl-mac-zeroize.txt | 5 +++++ library/ssl_msg.c | 19 +++++++++++++++++-- 2 files changed, 22 insertions(+), 2 deletions(-) create mode 100644 ChangeLog.d/ssl-mac-zeroize.txt diff --git a/ChangeLog.d/ssl-mac-zeroize.txt b/ChangeLog.d/ssl-mac-zeroize.txt new file mode 100644 index 0000000000..b49c7acd77 --- /dev/null +++ b/ChangeLog.d/ssl-mac-zeroize.txt @@ -0,0 +1,5 @@ +Security + * Zeroize intermediate variables used to calculate the MAC in CBC cipher + suites. This hardens the library in case stack memory leaks through a + memory disclosure vulnerabilty, which could formerly have allowed a + man-in-the-middle to inject fake ciphertext into a DTLS connection. diff --git a/library/ssl_msg.c b/library/ssl_msg.c index f7e40b123b..2d06fdd562 100644 --- a/library/ssl_msg.c +++ b/library/ssl_msg.c @@ -685,6 +685,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl, rec->data_len += transform->maclen; post_avail -= transform->maclen; auth_done++; + mbedtls_platform_zeroize( mac, transform->maclen ); } #endif /* MBEDTLS_SSL_SOME_SUITES_USE_MAC */ @@ -939,6 +940,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl, rec->data_len += transform->maclen; post_avail -= transform->maclen; auth_done++; + mbedtls_platform_zeroize( mac, transform->maclen ); } #endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */ } @@ -1222,13 +1224,20 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl, transform->maclen ); /* Compare expected MAC with MAC at the end of the record. */ + ret = 0; if( mbedtls_ct_memcmp( data + rec->data_len, mac_expect, transform->maclen ) != 0 ) { MBEDTLS_SSL_DEBUG_MSG( 1, ( "message mac does not match" ) ); - return( MBEDTLS_ERR_SSL_INVALID_MAC ); + ret = MBEDTLS_ERR_SSL_INVALID_MAC; + goto hmac_failed_etm_enabled; } auth_done++; + + hmac_failed_etm_enabled: + mbedtls_platform_zeroize( mac_expect, transform->maclen ); + if( ret != 0 ) + return( ret ); } #endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */ @@ -1420,7 +1429,7 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl, if( ret != 0 ) { MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ct_hmac", ret ); - return( ret ); + goto hmac_failed_etm_disabled; } mbedtls_ct_memcpy_offset( mac_peer, data, @@ -1443,6 +1452,12 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl, correct = 0; } auth_done++; + + hmac_failed_etm_disabled: + mbedtls_platform_zeroize( mac_peer, transform->maclen ); + mbedtls_platform_zeroize( mac_expect, transform->maclen ); + if( ret != 0 ) + return( ret ); } /*