Remove arbitrary limitation on length of common name in SSL certificates.

Both libpq and the backend would truncate a common name extracted from a
certificate at 32 bytes.  Replace that fixed-size buffer with dynamically
allocated string so that there is no hard limit.  While at it, remove the
code for extracting peer_dn, which we weren't using for anything; and
don't bother to store peer_cn longer than we need it in libpq.

This limit was not so terribly unreasonable when the code was written,
because we weren't using the result for anything critical, just logging it.
But now that there are options for checking the common name against the
server host name (in libpq) or using it as the user's name (in the server),
this could result in undesirable failures.  In the worst case it even seems
possible to spoof a server name or user name, if the correct name is
exactly 32 bytes and the attacker can persuade a trusted CA to issue a
certificate in which that string is a prefix of the certificate's common
name.  (To exploit this for a server name, he'd also have to send the
connection astray via phony DNS data or some such.)  The case that this is
a realistic security threat is a bit thin, but nonetheless we'll treat it
as one.

Back-patch to 8.4.  Older releases contain the faulty code, but it's not
a security problem because the common name wasn't used for anything
interesting.

Reported and patched by Heikki Linnakangas

Security: CVE-2012-0867

1 files changed, 66 insertions, 40 deletions

diff --git a/src/interfaces/libpq/fe-secure.c b/src/interfaces/libpq/fe-secure.c
index 3c1ca8c97fa..5c4d73c3acf 100644
--- a/src/interfaces/libpq/fe-secure.c
+++ b/src/interfaces/libpq/fe-secure.c
@@ -733,6 +733,11 @@ wildcard_certificate_match(const char *pattern, const char *string)
 static bool
 verify_peer_name_matches_certificate(PGconn *conn)
 {
+	char	   *peer_cn;
+	int			r;
+	int			len;
+	bool		result;
+
 	/*
 	 * If told not to verify the peer name, don't do it. Return true
 	 * indicating that the verification was successful.
@@ -740,33 +745,81 @@ verify_peer_name_matches_certificate(PGconn *conn)
 	if (strcmp(conn->sslmode, "verify-full") != 0)
 		return true;
 
+	/*
+	 * Extract the common name from the certificate.
+	 *
+	 * XXX: Should support alternate names here
+	 */
+	/* First find out the name's length and allocate a buffer for it. */
+	len = X509_NAME_get_text_by_NID(X509_get_subject_name(conn->peer),
+									NID_commonName, NULL, 0);
+	if (len == -1)
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("could not get server common name from server certificate\n"));
+		return false;
+	}
+	peer_cn = malloc(len + 1);
+	if (peer_cn == NULL)
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("out of memory\n"));
+		return false;
+	}
+
+	r = X509_NAME_get_text_by_NID(X509_get_subject_name(conn->peer),
+								  NID_commonName, peer_cn, len + 1);
+	if (r != len)
+	{
+		/* Got different length than on the first call. Shouldn't happen. */
+		printfPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("could not get server common name from server certificate\n"));
+		free(peer_cn);
+		return false;
+	}
+	peer_cn[len] = '\0';
+
+	/*
+	 * Reject embedded NULLs in certificate common name to prevent attacks
+	 * like CVE-2009-4034.
+	 */
+	if (len != strlen(peer_cn))
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("SSL certificate's common name contains embedded null\n"));
+		free(peer_cn);
+		return false;
+	}
+
+	/*
+	 * We got the peer's common name. Now compare it against the originally
+	 * given hostname.
+	 */
 	if (!(conn->pghost && conn->pghost[0] != '\0'))
 	{
 		printfPQExpBuffer(&conn->errorMessage,
 						  libpq_gettext("host name must be specified for a verified SSL connection\n"));
-		return false;
+		result = false;
 	}
 	else
 	{
-		/*
-		 * Compare CN to originally given hostname.
-		 *
-		 * XXX: Should support alternate names here
-		 */
-		if (pg_strcasecmp(conn->peer_cn, conn->pghost) == 0)
+		if (pg_strcasecmp(peer_cn, conn->pghost) == 0)
 			/* Exact name match */
-			return true;
-		else if (wildcard_certificate_match(conn->peer_cn, conn->pghost))
+			result = true;
+		else if (wildcard_certificate_match(peer_cn, conn->pghost))
 			/* Matched wildcard certificate */
-			return true;
+			result = true;
 		else
 		{
 			printfPQExpBuffer(&conn->errorMessage,
 							  libpq_gettext("server common name \"%s\" does not match host name \"%s\"\n"),
-							  conn->peer_cn, conn->pghost);
-			return false;
+							  peer_cn, conn->pghost);
+			result = false;
 		}
 	}
+
+	free(peer_cn);
+	return result;
 }
 
 #ifdef ENABLE_THREAD_SAFETY
@@ -1372,7 +1425,7 @@ open_client_SSL(PGconn *conn)
 	 * SSL_CTX_set_verify(), if root.crt exists.
 	 */
 
-	/* pull out server distinguished and common names */
+	/* get server certificate */
 	conn->peer = SSL_get_peer_certificate(conn->ssl);
 	if (conn->peer == NULL)
 	{
@@ -1386,33 +1439,6 @@ open_client_SSL(PGconn *conn)
 		return PGRES_POLLING_FAILED;
 	}
 
-	X509_NAME_oneline(X509_get_subject_name(conn->peer),
-					  conn->peer_dn, sizeof(conn->peer_dn));
-	conn->peer_dn[sizeof(conn->peer_dn) - 1] = '\0';
-
-	r = X509_NAME_get_text_by_NID(X509_get_subject_name(conn->peer),
-								  NID_commonName, conn->peer_cn, SM_USER);
-	conn->peer_cn[SM_USER] = '\0';		/* buffer is SM_USER+1 chars! */
-	if (r == -1)
-	{
-		/* Unable to get the CN, set it to blank so it can't be used */
-		conn->peer_cn[0] = '\0';
-	}
-	else
-	{
-		/*
-		 * Reject embedded NULLs in certificate common name to prevent attacks
-		 * like CVE-2009-4034.
-		 */
-		if (r != strlen(conn->peer_cn))
-		{
-			printfPQExpBuffer(&conn->errorMessage,
-							  libpq_gettext("SSL certificate's common name contains embedded null\n"));
-			close_SSL(conn);
-			return PGRES_POLLING_FAILED;
-		}
-	}
-
 	if (!verify_peer_name_matches_certificate(conn))
 	{
 		close_SSL(conn);