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Zeroize local MAC variables

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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 <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-12-10 21:33:21 +01:00
parent 6ec13af413
commit 957e383f87
2 changed files with 24 additions and 3 deletions
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5
ChangeLog.d/ssl-mac-zeroize.txt Normal file
View File

@ -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.

22
library/ssl_tls.c
View File

@ -1479,6 +1479,7 @@ static int ssl_encrypt_buf( mbedtls_ssl_context *ssl )
mac );
memcpy( ssl->out_msg + ssl->out_msglen, mac, ssl->transform_out->maclen );
mbedtls_platform_zeroize( mac, ssl->transform_out->maclen );
}
else
#endif
@ -1497,6 +1498,7 @@ static int ssl_encrypt_buf( mbedtls_ssl_context *ssl )
mbedtls_md_hmac_reset( &ssl->transform_out->md_ctx_enc );
memcpy( ssl->out_msg + ssl->out_msglen, mac, ssl->transform_out->maclen );
mbedtls_platform_zeroize( mac, ssl->transform_out->maclen );
}
else
#endif
@ -1760,6 +1762,7 @@ static int ssl_encrypt_buf( mbedtls_ssl_context *ssl )
ssl->out_msglen += ssl->transform_out->maclen;
auth_done++;
mbedtls_platform_zeroize( mac, ssl->transform_out->maclen );
}
#endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */
}
@ -2156,14 +2159,21 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
MBEDTLS_SSL_DEBUG_BUF( 4, "expected mac", mac_expect,
ssl->transform_in->maclen );
ret = 0;
if( mbedtls_ssl_safer_memcmp( ssl->in_iv + ssl->in_msglen, mac_expect,
ssl->transform_in->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, ssl->transform_in->maclen );
if( ret != 0 )
return( ret );
}
#endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */
@ -2322,6 +2332,7 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
{
unsigned char mac_expect[MBEDTLS_SSL_MAC_ADD];
unsigned char mac_peer[MBEDTLS_SSL_MAC_ADD];
int ret = 0;
ssl->in_msglen -= ssl->transform_in->maclen;
@ -2345,7 +2356,6 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
defined(MBEDTLS_SSL_PROTO_TLS1_2)
if( ssl->minor_ver > MBEDTLS_SSL_MINOR_VERSION_0 )
{
int ret;
unsigned char add_data[13];
/*
@ -2373,7 +2383,7 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
if( ret != 0 )
{
MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_cf_hmac", ret );
return( ret );
goto hmac_failed_etm_disabled;
}
mbedtls_ssl_cf_memcpy_offset( mac_peer, ssl->in_msg,
@ -2403,6 +2413,12 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
correct = 0;
}
auth_done++;
hmac_failed_etm_disabled:
mbedtls_platform_zeroize( mac_peer, ssl->transform_in->maclen );
mbedtls_platform_zeroize( mac_expect, ssl->transform_in->maclen );
if( ret != 0 )
return( ret );
}
/*