Enable allowing unsupported critical extensions in runtime

When compile time flag MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION
is not set, certificate parsing fails if certificate contains unsupported critical extension.

This patch allows to modify this behavior in runtime.

Signed-off-by: Lev Stipakov <[email protected]>

include/mbedtls/ssl.h

@@ -1063,6 +1063,10 @@ struct mbedtls_ssl_config
                                          retransmission timeout (ms)        */
 #endif
 
+    uint32_t allowed_unsupported_critical_exts; /*!< Bit flags which represent runtime-enabled
+                                                     unsupported critical extensions, e.g.
+                                                     MBEDTLS_X509_EXT_NAME_CONSTRAINTS */
+
 #if defined(MBEDTLS_SSL_RENEGOTIATION)
     int renego_max_records;         /*!< grace period for renegotiation     */
     unsigned char renego_period[8]; /*!< value of the record counters
@@ -3392,6 +3396,24 @@ void mbedtls_ssl_conf_renegotiation_period( mbedtls_ssl_config *conf,
                                    const unsigned char period[8] );
 #endif /* MBEDTLS_SSL_RENEGOTIATION */
 
+/**
+ * \brief          Allows unsupported critical extensions
+ *
+ *                 Without compile-time flag MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION
+ *                 mbedTLS fails certificate verification if certificate contains
+ *                 unsupported critical extensions.
+ *
+ *                 This method allows to modify behavior in runtime by providing
+ *                 bit flags which represent unsupported extensions (for example MBEDTLS_X509_EXT_NAME_CONSTRAINTS)
+ *                 which should be allowed despite missing above mentioned compile-time flag.
+ *
+ * \param conf     SSL configuration
+ * \param exts     Bit flags which represent runtime-enabled unsupported critical extensions,
+ *                 e.g. MBEDTLS_X509_EXT_NAME_CONSTRAINTS
+ *
+ */
+void mbedtls_ssl_conf_allow_unsupported_critical_exts( mbedtls_ssl_config *conf, uint32_t exts );
+
 /**
  * \brief          Check if there is data already read from the
  *                 underlying transport but not yet processed.

include/mbedtls/x509_crt.h

@@ -96,6 +96,8 @@ typedef struct mbedtls_x509_crt
     mbedtls_pk_type_t sig_pk;           /**< Internal representation of the Public Key algorithm of the signature algorithm, e.g. MBEDTLS_PK_RSA */
     void *sig_opts;             /**< Signature options to be passed to mbedtls_pk_verify_ext(), e.g. for RSASSA-PSS */
 
+    uint32_t allowed_unsupported_critical_exts; /**< Optional Bit flags which represent runtime-enabled unsupported critical extensions, e.g. MBEDTLS_X509_EXT_NAME_CONSTRAINTS */
+
     struct mbedtls_x509_crt *next;     /**< Next certificate in the CA-chain. */
 }
 mbedtls_x509_crt;

library/ssl_tls.c

@@ -2250,6 +2250,12 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl,
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
     }
 
+    if (ssl->session_negotiate->peer_cert != NULL)
+    {
+        ssl->session_negotiate->peer_cert->allowed_unsupported_critical_exts =
+            ssl->conf->allowed_unsupported_critical_exts;
+    }
+
     /* Make &ssl->in_msg[i] point to the beginning of the CRT chain. */
     i += 3;
 
@@ -4670,6 +4676,11 @@ void mbedtls_ssl_conf_renegotiation_period( mbedtls_ssl_config *conf,
 }
 #endif /* MBEDTLS_SSL_RENEGOTIATION */
 
+void mbedtls_ssl_conf_allow_unsupported_critical_exts( mbedtls_ssl_config *conf, uint32_t exts )
+{
+    conf->allowed_unsupported_critical_exts = exts;
+}
+
 #if defined(MBEDTLS_SSL_SESSION_TICKETS)
 #if defined(MBEDTLS_SSL_CLI_C)
 void mbedtls_ssl_conf_session_tickets( mbedtls_ssl_config *conf, int use_tickets )

library/x509_crt.c

@@ -891,6 +891,7 @@ static int x509_get_crt_ext( unsigned char **p,
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len;
     unsigned char *end_ext_data, *end_ext_octet;
+    int is_supported;
 
     if( *p == end )
         return( 0 );
@@ -945,16 +946,20 @@ static int x509_get_crt_ext( unsigned char **p,
         /*
          * Detect supported extensions
          */
-        ret = mbedtls_oid_get_x509_ext_type( &extn_oid, &ext_type );
+        ret = mbedtls_oid_get_x509_ext_type_supported( &extn_oid, &ext_type, &is_supported );
 
-        if( ret != 0 )
+        if( ( ret != 0 ) || ( is_supported == 0 ) )
         {
             /* No parser found, skip extension */
             *p = end_ext_octet;
 
 #if !defined(MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION)
             if( is_critical )
             {
+                /* Do not fail if extension is found, but unsupported and allowed in runtime */
+                if( ( ret == 0 ) && ( ext_type & crt->allowed_unsupported_critical_exts ) )
+                    continue;
+
                 /* Data is marked as critical: fail */
                 return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
                         MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
@@ -1346,6 +1351,7 @@ static int mbedtls_x509_crt_parse_der_internal( mbedtls_x509_crt *chain,
 
         prev = crt;
         mbedtls_x509_crt_init( crt->next );
+        crt->next->allowed_unsupported_critical_exts = crt->allowed_unsupported_critical_exts;
         crt = crt->next;
     }
 

tests/data_files/test-ca-nc.crt

@@ -0,0 +1,20 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----

tests/suites/test_suite_x509parse.data

@@ -2518,6 +2518,12 @@ X509 File parse (trailing spaces, OK)
 depends_on:MBEDTLS_ECDSA_C:MBEDTLS_ECP_DP_SECP256R1_ENABLED:MBEDTLS_SHA256_C:MBEDTLS_RSA_C
 x509parse_crt_file:"data_files/server7_trailing_space.crt":0
 
+X509 File parse (unsupported critical ext Name Constraints, fail)
+x509parse_crt_file:"data_files/test-ca-nc.crt":MBEDTLS_ERR_X509_INVALID_EXTENSIONS + MBEDTLS_ERR_ASN1_UNEXPECTED_TAG
+
+X509 File parse (allowed unsupported critical ext Name Constraints, ok)
+x509parse_crt_file_allow_exts:"data_files/test-ca-nc.crt":MBEDTLS_X509_EXT_NAME_CONSTRAINTS:0
+
 X509 Get time (UTC no issues)
 depends_on:MBEDTLS_X509_USE_C
 x509_get_time:MBEDTLS_ASN1_UTC_TIME:"500101000000Z":0:1950:1:1:0:0:0

tests/suites/test_suite_x509parse.function

@@ -735,6 +735,21 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE depends_on:MBEDTLS_X509_CRT_PARSE_C:MBEDTLS_FS_IO */
+void x509parse_crt_file_allow_exts( char *crt_file, int exts, int result )
+{
+    mbedtls_x509_crt crt;
+
+    mbedtls_x509_crt_init( &crt );
+    crt.allowed_unsupported_critical_exts = exts;
+
+    TEST_ASSERT( mbedtls_x509_crt_parse_file( &crt, crt_file ) == result );
+
+exit:
+    mbedtls_x509_crt_free( &crt );
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:MBEDTLS_X509_CRT_PARSE_C */
 void x509parse_crt( data_t * buf, char * result_str, int result )
 {