Commit message: Prevent bounds check bypass through overflow in PSK identity parsing

The check `if( *p + n > end )` in `ssl_parse_client_psk_identity` is
unsafe because `*p + n` might overflow, thus bypassing the check. As
`n` is a user-specified value up to 65K, this is relevant if the
library happens to be located in the last 65K of virtual memory.

This commit replaces the check by a safe version.
CVE: CVE-2017-18187
CWE: CWE-190
Vulnerability description: In ARM mbed TLS before 2.7.0, there is a bounds-check bypass through an integer overflow in PSK identity parsing in the ssl_parse_client_psk_identity() function in library/ssl_srv.c.

Project: mbedtls
File: ssl_srv.c

static int ssl_parse_client_hello_v2( mbedtls_ssl_context *ssl )
{
    int ret, got_common_suite;
    unsigned int i, j;
    size_t n;
    unsigned int ciph_len, sess_len, chal_len;
    unsigned char *buf, *p;
    const int *ciphersuites;
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info;

    MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> parse client hello v2" ) );

#if defined(MBEDTLS_SSL_RENEGOTIATION)
    if( ssl->renego_status != MBEDTLS_SSL_INITIAL_HANDSHAKE )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "client hello v2 illegal for renegotiation" ) );
        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                        MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }
#endif /* MBEDTLS_SSL_RENEGOTIATION */

    buf = ssl->in_hdr;

    MBEDTLS_SSL_DEBUG_BUF( 4, "record header", buf, 5 );

    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello v2, message type: %d",
                   buf[2] ) );
    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello v2, message len.: %d",
                   ( ( buf[0] & 0x7F ) << 8 ) | buf[1] ) );
    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello v2, max. version: [%d:%d]",
                   buf[3], buf[4] ) );

    /*
     * SSLv2 Client Hello
     *
     * Record layer:
     *     0  .   1   message length
     *
     * SSL layer:
     *     2  .   2   message type
     *     3  .   4   protocol version
     */
    if( buf[2] != MBEDTLS_SSL_HS_CLIENT_HELLO ||
        buf[3] != MBEDTLS_SSL_MAJOR_VERSION_3 )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    n = ( ( buf[0] << 8 ) | buf[1] ) & 0x7FFF;

    if( n < 17 || n > 512 )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    ssl->major_ver = MBEDTLS_SSL_MAJOR_VERSION_3;
    ssl->minor_ver = ( buf[4] <= ssl->conf->max_minor_ver )
                     ? buf[4]  : ssl->conf->max_minor_ver;

    if( ssl->minor_ver < ssl->conf->min_minor_ver )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "client only supports ssl smaller than minimum"
                            " [%d:%d] < [%d:%d]",
                            ssl->major_ver, ssl->minor_ver,
                            ssl->conf->min_major_ver, ssl->conf->min_minor_ver ) );

        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                     MBEDTLS_SSL_ALERT_MSG_PROTOCOL_VERSION );
        return( MBEDTLS_ERR_SSL_BAD_HS_PROTOCOL_VERSION );
    }

    ssl->handshake->max_major_ver = buf[3];
    ssl->handshake->max_minor_ver = buf[4];

    if( ( ret = mbedtls_ssl_fetch_input( ssl, 2 + n ) ) != 0 )
    {
        MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_fetch_input", ret );
        return( ret );
    }

    ssl->handshake->update_checksum( ssl, buf + 2, n );

    buf = ssl->in_msg;
    n = ssl->in_left - 5;

    /*
     *    0  .   1   ciphersuitelist length
     *    2  .   3   session id length
     *    4  .   5   challenge length
     *    6  .  ..   ciphersuitelist
     *   ..  .  ..   session id
     *   ..  .  ..   challenge
     */
    MBEDTLS_SSL_DEBUG_BUF( 4, "record contents", buf, n );

    ciph_len = ( buf[0] << 8 ) | buf[1];
    sess_len = ( buf[2] << 8 ) | buf[3];
    chal_len = ( buf[4] << 8 ) | buf[5];

    MBEDTLS_SSL_DEBUG_MSG( 3, ( "ciph_len: %d, sess_len: %d, chal_len: %d",
                   ciph_len, sess_len, chal_len ) );

    /*
     * Make sure each parameter length is valid
     */
    if( ciph_len < 3 || ( ciph_len % 3 ) != 0 )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    if( sess_len > 32 )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    if( chal_len < 8 || chal_len > 32 )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    if( n != 6 + ciph_len + sess_len + chal_len )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    MBEDTLS_SSL_DEBUG_BUF( 3, "client hello, ciphersuitelist",
                   buf + 6, ciph_len );
    MBEDTLS_SSL_DEBUG_BUF( 3, "client hello, session id",
                   buf + 6 + ciph_len, sess_len );
    MBEDTLS_SSL_DEBUG_BUF( 3, "client hello, challenge",
                   buf + 6 + ciph_len + sess_len, chal_len );

    p = buf + 6 + ciph_len;
    ssl->session_negotiate->id_len = sess_len;
    memset( ssl->session_negotiate->id, 0,
            sizeof( ssl->session_negotiate->id ) );
    memcpy( ssl->session_negotiate->id, p, ssl->session_negotiate->id_len );

    p += sess_len;
    memset( ssl->handshake->randbytes, 0, 64 );
    memcpy( ssl->handshake->randbytes + 32 - chal_len, p, chal_len );

    /*
     * Check for TLS_EMPTY_RENEGOTIATION_INFO_SCSV
     */
    for( i = 0, p = buf + 6; i < ciph_len; i += 3, p += 3 )
    {
        if( p[0] == 0 && p[1] == 0 && p[2] == MBEDTLS_SSL_EMPTY_RENEGOTIATION_INFO )
        {
            MBEDTLS_SSL_DEBUG_MSG( 3, ( "received TLS_EMPTY_RENEGOTIATION_INFO " ) );
#if defined(MBEDTLS_SSL_RENEGOTIATION)
            if( ssl->renego_status == MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS )
            {
                MBEDTLS_SSL_DEBUG_MSG( 1, ( "received RENEGOTIATION SCSV "
                                    "during renegotiation" ) );

                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                                MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
                return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
            }
#endif /* MBEDTLS_SSL_RENEGOTIATION */
            ssl->secure_renegotiation = MBEDTLS_SSL_SECURE_RENEGOTIATION;
            break;
        }
    }

#if defined(MBEDTLS_SSL_FALLBACK_SCSV)
    for( i = 0, p = buf + 6; i < ciph_len; i += 3, p += 3 )
    {
        if( p[0] == 0 &&
            p[1] == (unsigned char)( ( MBEDTLS_SSL_FALLBACK_SCSV_VALUE >> 8 ) & 0xff ) &&
            p[2] == (unsigned char)( ( MBEDTLS_SSL_FALLBACK_SCSV_VALUE      ) & 0xff ) )
        {
            MBEDTLS_SSL_DEBUG_MSG( 3, ( "received FALLBACK_SCSV" ) );

            if( ssl->minor_ver < ssl->conf->max_minor_ver )
            {
                MBEDTLS_SSL_DEBUG_MSG( 1, ( "inapropriate fallback" ) );

                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                        MBEDTLS_SSL_ALERT_MSG_INAPROPRIATE_FALLBACK );

                return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
            }

            break;
        }
    }
#endif /* MBEDTLS_SSL_FALLBACK_SCSV */

    got_common_suite = 0;
    ciphersuites = ssl->conf->ciphersuite_list[ssl->minor_ver];
    ciphersuite_info = NULL;
#if defined(MBEDTLS_SSL_SRV_RESPECT_CLIENT_PREFERENCE)
    for( j = 0, p = buf + 6; j < ciph_len; j += 3, p += 3 )
        for( i = 0; ciphersuites[i] != 0; i++ )
#else
    for( i = 0; ciphersuites[i] != 0; i++ )
        for( j = 0, p = buf + 6; j < ciph_len; j += 3, p += 3 )
#endif
        {
            if( p[0] != 0 ||
                p[1] != ( ( ciphersuites[i] >> 8 ) & 0xFF ) ||
                p[2] != ( ( ciphersuites[i]      ) & 0xFF ) )
                continue;

            got_common_suite = 1;

            if( ( ret = ssl_ciphersuite_match( ssl, ciphersuites[i],
                                               &ciphersuite_info ) ) != 0 )
                return( ret );

            if( ciphersuite_info != NULL )
                goto have_ciphersuite_v2;
        }

    if( got_common_suite )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "got ciphersuites in common, "
                            "but none of them usable" ) );
        return( MBEDTLS_ERR_SSL_NO_USABLE_CIPHERSUITE );
    }
    else
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "got no ciphersuites in common" ) );
        return( MBEDTLS_ERR_SSL_NO_CIPHER_CHOSEN );
    }

have_ciphersuite_v2:
    MBEDTLS_SSL_DEBUG_MSG( 2, ( "selected ciphersuite: %s", ciphersuite_info->name ) );

    ssl->session_negotiate->ciphersuite = ciphersuites[i];
    ssl->transform_negotiate->ciphersuite_info = ciphersuite_info;

    /*
     * SSLv2 Client Hello relevant renegotiation security checks
     */
    if( ssl->secure_renegotiation == MBEDTLS_SSL_LEGACY_RENEGOTIATION &&
        ssl->conf->allow_legacy_renegotiation == MBEDTLS_SSL_LEGACY_BREAK_HANDSHAKE )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "legacy renegotiation, breaking off handshake" ) );
        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                        MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    ssl->in_left = 0;
    ssl->state++;

    MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= parse client hello v2" ) );

    return( 0 );
}

Project: mbedtls
File: ssl_srv.c

static int ssl_parse_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
                                      const unsigned char *buf,
                                      size_t len )
{
    if( len != 0 )
    {
        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
    }

    ((void) buf);

    if( ssl->conf->encrypt_then_mac == MBEDTLS_SSL_ETM_ENABLED &&
        ssl->minor_ver != MBEDTLS_SSL_MINOR_VERSION_0 )
    {
        ssl->session_negotiate->encrypt_then_mac = MBEDTLS_SSL_ETM_ENABLED;
    }

    return( 0 );
}

Project: mbedtls
File: ssl_srv.c

static void ssl_write_truncated_hmac_ext( mbedtls_ssl_context *ssl,
                                          unsigned char *buf,
                                          size_t *olen )
{
    unsigned char *p = buf;

    if( ssl->session_negotiate->trunc_hmac == MBEDTLS_SSL_TRUNC_HMAC_DISABLED )
    {
        *olen = 0;
        return;
    }

    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, adding truncated hmac extension" ) );

    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_TRUNCATED_HMAC >> 8 ) & 0xFF );
    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_TRUNCATED_HMAC      ) & 0xFF );

    *p++ = 0x00;
    *p++ = 0x00;

    *olen = 4;
}