Check that the subject name in a proxy cert complies to RFC 3820

The subject name MUST be the same as the issuer name, with a single CN
entry added.

RT#1852
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NStephen Henson <steve@openssl.org>

crypto/x509/x509_txt.c

@@ -167,6 +167,8 @@ const char *X509_verify_cert_error_string(long n)
         return ("Issuer certificate lookup error");
     case X509_V_ERR_NO_VALID_SCTS:
         return ("Certificate Transparency required, but no valid SCTs found");
+    case X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION:
+        return ("proxy subject name violation");
 
     default:
         /* Printing an error number into a static buffer is not thread-safe */

crypto/x509/x509_vfy.c

@@ -558,6 +558,79 @@ static int check_name_constraints(X509_STORE_CTX *ctx)
         /* Ignore self issued certs unless last in chain */
         if (i && (x->ex_flags & EXFLAG_SI))
             continue;
+
+        /*
+         * Proxy certificates policy has an extra constraint, where the
+         * certificate subject MUST be the issuer with a single CN entry
+         * added.
+         * (RFC 3820: 3.4, 4.1.3 (a)(4))
+         */
+        if (x->ex_flags & EXFLAG_PROXY) {
+            X509_NAME *tmpsubject = X509_get_subject_name(x);
+            X509_NAME *tmpissuer = X509_get_issuer_name(x);
+            X509_NAME_ENTRY *tmpentry = NULL;
+            int last_object_nid = 0;
+            int err = X509_V_OK;
+            int last_object_loc = X509_NAME_entry_count(tmpsubject) - 1;
+
+            /* Check that there are at least two RDNs */
+            if (last_object_loc < 1) {
+                err = X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION;
+                goto proxy_name_done;
+            }
+
+            /*
+             * Check that there is exactly one more RDN in subject as
+             * there is in issuer.
+             */
+            if (X509_NAME_entry_count(tmpsubject)
+                != X509_NAME_entry_count(tmpissuer) + 1) {
+                err = X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION;
+                goto proxy_name_done;
+            }
+
+            /*
+             * Check that the last subject component isn't part of a
+             * multivalued RDN
+             */
+            if (X509_NAME_ENTRY_set(X509_NAME_get_entry(tmpsubject,
+                                                        last_object_loc))
+                == X509_NAME_ENTRY_set(X509_NAME_get_entry(tmpsubject,
+                                                           last_object_loc - 1))) {
+                err = X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION;
+                goto proxy_name_done;
+            }
+
+            /*
+             * Check that the last subject RDN is a commonName, and that
+             * all the previous RDNs match the issuer exactly
+             */
+            tmpsubject = X509_NAME_dup(tmpsubject);
+            if (tmpsubject == NULL) {
+                X509err(X509_F_CHECK_NAME_CONSTRAINTS, ERR_R_MALLOC_FAILURE);
+                ctx->error = X509_V_ERR_OUT_OF_MEM;
+                return 0;
+            }
+
+            tmpentry =
+                X509_NAME_delete_entry(tmpsubject, last_object_loc);
+            last_object_nid =
+                OBJ_obj2nid(X509_NAME_ENTRY_get_object(tmpentry));
+
+            if (last_object_nid != NID_commonName
+                || X509_NAME_cmp(tmpsubject, tmpissuer) != 0) {
+                err = X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION;
+            }
+
+            X509_NAME_ENTRY_free(tmpentry);
+            X509_NAME_free(tmpsubject);
+
+         proxy_name_done:
+            if (err != X509_V_OK
+                && !verify_cb_cert(ctx, x, i, err))
+                return 0;
+        }
+
         /*
          * Check against constraints for all certificates higher in chain
          * including trust anchor. Trust anchor not strictly speaking needed

include/openssl/x509_vfy.h

@@ -165,6 +165,8 @@ void X509_STORE_CTX_set_depth(X509_STORE_CTX *ctx, int depth);
 /* Certificate transparency */
 # define         X509_V_ERR_NO_VALID_SCTS                        71
 
+# define         X509_V_ERR_PROXY_SUBJECT_NAME_VIOLATION         72
+
 /* Certificate verify flags */
 
 # if OPENSSL_API_COMPAT < 0x10100000L