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/oid.h

@@ -426,14 +426,25 @@ int mbedtls_oid_get_numeric_string( char *buf, size_t size, const mbedtls_asn1_b
 
 #if defined(MBEDTLS_X509_USE_C) || defined(MBEDTLS_X509_CREATE_C)
 /**
- * \brief          Translate an X.509 extension OID into local values
+ * \brief          Translate supported X.509 extension OID into local values
  *
  * \param oid      OID to use
  * \param ext_type place to store the extension type
  *
  * \return         0 if successful, or MBEDTLS_ERR_OID_NOT_FOUND
  */
 int mbedtls_oid_get_x509_ext_type( const mbedtls_asn1_buf *oid, int *ext_type );
+
+/**
+ * \brief              Translate supported and unsupported X.509 extension OID into local values
+ *
+ * \param oid          OID to use
+ * \param ext_type     place to store the extension type
+ * \param is_supported place to store flag if extension is supported (1 - supported, 0 otherwise)
+ *
+ * \return             0 if successful, or MBEDTLS_ERR_OID_NOT_FOUND
+ */
+int mbedtls_oid_get_x509_ext_type_supported( const mbedtls_asn1_buf *oid, int *ext_type, int *is_supported );
 #endif
 
 /**

include/mbedtls/ssl.h

@@ -954,6 +954,10 @@ 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
@@ -2728,6 +2732,24 @@ 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

@@ -90,6 +90,8 @@ 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/oid.c

@@ -257,38 +257,95 @@ FN_OID_GET_ATTR1(mbedtls_oid_get_attr_short_name, oid_x520_attr_t, x520_attr, co
 typedef struct {
     mbedtls_oid_descriptor_t    descriptor;
     int                 ext_type;
+    int                 is_supported;
 } oid_x509_ext_t;
 
 static const oid_x509_ext_t oid_x509_ext[] =
 {
     {
-        { ADD_LEN( MBEDTLS_OID_BASIC_CONSTRAINTS ),    "id-ce-basicConstraints",   "Basic Constraints" },
-        MBEDTLS_X509_EXT_BASIC_CONSTRAINTS,
+        { ADD_LEN( MBEDTLS_OID_AUTHORITY_KEY_IDENTIFIER ),    "id-ce-authorityKeyIdentifier",      "Authority Key Identifier" },
+        MBEDTLS_X509_EXT_AUTHORITY_KEY_IDENTIFIER, 0,
     },
     {
-        { ADD_LEN( MBEDTLS_OID_KEY_USAGE ),            "id-ce-keyUsage",           "Key Usage" },
-        MBEDTLS_X509_EXT_KEY_USAGE,
+        { ADD_LEN( MBEDTLS_OID_SUBJECT_KEY_IDENTIFIER ),      "id-ce-subjectKeyIdentifier",        "Subject Key Identifier" },
+        MBEDTLS_X509_EXT_SUBJECT_KEY_IDENTIFIER, 0,
     },
     {
-        { ADD_LEN( MBEDTLS_OID_EXTENDED_KEY_USAGE ),   "id-ce-extKeyUsage",        "Extended Key Usage" },
-        MBEDTLS_X509_EXT_EXTENDED_KEY_USAGE,
+        { ADD_LEN( MBEDTLS_OID_KEY_USAGE ),                   "id-ce-keyUsage",                    "Key Usage" },
+        MBEDTLS_X509_EXT_KEY_USAGE, 1,
     },
     {
-        { ADD_LEN( MBEDTLS_OID_SUBJECT_ALT_NAME ),     "id-ce-subjectAltName",     "Subject Alt Name" },
-        MBEDTLS_X509_EXT_SUBJECT_ALT_NAME,
+        { ADD_LEN( MBEDTLS_OID_CERTIFICATE_POLICIES ),        "id-ce-certificatePolicies",         "Certificate Policies" },
+        MBEDTLS_X509_EXT_CERTIFICATE_POLICIES, 0,
     },
     {
-        { ADD_LEN( MBEDTLS_OID_NS_CERT_TYPE ),         "id-netscape-certtype",     "Netscape Certificate Type" },
-        MBEDTLS_X509_EXT_NS_CERT_TYPE,
+        { ADD_LEN( MBEDTLS_OID_POLICY_MAPPINGS ),             "id-ce-policyMappings",              "Policy Mapping" },
+        MBEDTLS_X509_EXT_POLICY_MAPPINGS, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_ISSUER_ALT_NAME ),             "id-ce-issuerAltName",               "Issuer Alt Name" },
+        MBEDTLS_X509_EXT_ISSUER_ALT_NAME, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_SUBJECT_DIRECTORY_ATTRS ),     "id-ce-subjectDirectoryAttributes",  "Subject Directory Attributes" },
+        MBEDTLS_X509_EXT_SUBJECT_DIRECTORY_ATTRS, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_BASIC_CONSTRAINTS ),           "id-ce-basicConstraints",            "Basic Constraints" },
+        MBEDTLS_X509_EXT_BASIC_CONSTRAINTS, 1,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_NAME_CONSTRAINTS ),            "id-ce-nameConstraints",             "Name Constraints" },
+        MBEDTLS_X509_EXT_NAME_CONSTRAINTS, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_POLICY_CONSTRAINTS ),          "id-ce-policyConstraints",           "Policy Constraints" },
+        MBEDTLS_X509_EXT_POLICY_CONSTRAINTS, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_EXTENDED_KEY_USAGE ),          "id-ce-extKeyUsage",                 "Extended Key Usage" },
+        MBEDTLS_X509_EXT_EXTENDED_KEY_USAGE, 1
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_CRL_DISTRIBUTION_POINTS ),     "id-ce-cRLDistributionPoints",       "CRL Distribution Points" },
+        MBEDTLS_X509_EXT_CRL_DISTRIBUTION_POINTS, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_INIHIBIT_ANYPOLICY ),          "id-ce-inhibitAnyPolicy",            "Inhibit Any Policy" },
+        MBEDTLS_X509_EXT_INIHIBIT_ANYPOLICY, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_FRESHEST_CRL ),                "id-ce-freshestCRL",                 "Freshest CRL" },
+        MBEDTLS_X509_EXT_FRESHEST_CRL, 0,
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_SUBJECT_ALT_NAME ),            "id-ce-subjectAltName",              "Subject Alt Name" },
+        MBEDTLS_X509_EXT_SUBJECT_ALT_NAME, 1
+    },
+    {
+        { ADD_LEN( MBEDTLS_OID_NS_CERT_TYPE ),                "id-netscape-certtype",              "Netscape Certificate Type" },
+        MBEDTLS_X509_EXT_NS_CERT_TYPE, 1
     },
     {
         { NULL, 0, NULL, NULL },
-        0,
+        0, 0
     },
 };
 
 FN_OID_TYPED_FROM_ASN1(oid_x509_ext_t, x509_ext, oid_x509_ext)
-FN_OID_GET_ATTR1(mbedtls_oid_get_x509_ext_type, oid_x509_ext_t, x509_ext, int, ext_type)
+FN_OID_GET_ATTR2(mbedtls_oid_get_x509_ext_type_supported, oid_x509_ext_t, x509_ext, int, ext_type, int, is_supported)
+
+int mbedtls_oid_get_x509_ext_type( const mbedtls_asn1_buf *oid, int *ext_type )
+{
+    int ret = 0;
+    int is_supported = 0;
+
+    ret = mbedtls_oid_get_x509_ext_type_supported(oid, ext_type, &is_supported);
+    if( is_supported == 0 )
+        ret = MBEDTLS_ERR_OID_NOT_FOUND;
+
+    return( ret );
+}
 
 static const mbedtls_oid_descriptor_t oid_ext_key_usage[] =
 {

library/ssl_tls.c

@@ -5544,6 +5544,9 @@ static int ssl_parse_certificate_chain( mbedtls_ssl_context *ssl )
 
     mbedtls_x509_crt_init( ssl->session_negotiate->peer_cert );
 
+    ssl->session_negotiate->peer_cert->allowed_unsupported_critical_exts =
+        ssl->conf->allowed_unsupported_critical_exts;
+
     i += 3;
 
     while( i < ssl->in_hslen )
@@ -7690,6 +7693,11 @@ 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

@@ -698,6 +698,7 @@ static int x509_get_crt_ext( unsigned char **p,
     int ret;
     size_t len;
     unsigned char *end_ext_data, *end_ext_octet;
+    int is_supported;
 
     if( *p == end )
         return( 0 );
@@ -752,16 +753,20 @@ 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 );
@@ -1115,6 +1120,7 @@ int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *bu
 
         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

@@ -1810,6 +1810,12 @@ 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

@@ -506,6 +506,21 @@ 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 )
 {