Fix SSL_select_next_proto

Ensure that the provided client list is non-NULL and starts with a valid
entry. When called from the ALPN callback the client list should already
have been validated by OpenSSL so this should not cause a problem. When
called from the NPN callback the client list is locally configured and
will not have already been validated. Therefore SSL_select_next_proto
should not assume that it is correctly formatted.

We implement stricter checking of the client protocol list. We also do the
same for the server list while we are about it.

CVE-2024-5535

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24716)

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 37a307b9927c3f7136ce4734b561b94a649a36de..589ed2c64d4f9c9b3bcb83439378554fce63c75d 100644 (file)
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -3535,37 +3535,54 @@ int SSL_select_next_proto(unsigned char **out, unsigned char *outlen,
                           unsigned int server_len,
                           const unsigned char *client, unsigned int client_len)
 {
-    unsigned int i, j;
-    const unsigned char *result;
-    int status = OPENSSL_NPN_UNSUPPORTED;
+    PACKET cpkt, csubpkt, spkt, ssubpkt;
+
+    if (!PACKET_buf_init(&cpkt, client, client_len)
+            || !PACKET_get_length_prefixed_1(&cpkt, &csubpkt)
+            || PACKET_remaining(&csubpkt) == 0) {
+        *out = NULL;
+        *outlen = 0;
+        return OPENSSL_NPN_NO_OVERLAP;
+    }
+
+    /*
+     * Set the default opportunistic protocol. Will be overwritten if we find
+     * a match.
+     */
+    *out = (unsigned char *)PACKET_data(&csubpkt);
+    *outlen = (unsigned char)PACKET_remaining(&csubpkt);
 
     /*
      * For each protocol in server preference order, see if we support it.
      */
-    for (i = 0; i < server_len;) {
-        for (j = 0; j < client_len;) {
-            if (server[i] == client[j] &&
-                memcmp(&server[i + 1], &client[j + 1], server[i]) == 0) {
-                /* We found a match */
-                result = &server[i];
-                status = OPENSSL_NPN_NEGOTIATED;
-                goto found;
+    if (PACKET_buf_init(&spkt, server, server_len)) {
+        while (PACKET_get_length_prefixed_1(&spkt, &ssubpkt)) {
+            if (PACKET_remaining(&ssubpkt) == 0)
+                continue; /* Invalid - ignore it */
+            if (PACKET_buf_init(&cpkt, client, client_len)) {
+                while (PACKET_get_length_prefixed_1(&cpkt, &csubpkt)) {
+                    if (PACKET_equal(&csubpkt, PACKET_data(&ssubpkt),
+                                     PACKET_remaining(&ssubpkt))) {
+                        /* We found a match */
+                        *out = (unsigned char *)PACKET_data(&ssubpkt);
+                        *outlen = (unsigned char)PACKET_remaining(&ssubpkt);
+                        return OPENSSL_NPN_NEGOTIATED;
+                    }
+                }
+                /* Ignore spurious trailing bytes in the client list */
+            } else {
+                /* This should never happen */
+                return OPENSSL_NPN_NO_OVERLAP;
             }
-            j += client[j];
-            j++;
         }
-        i += server[i];
-        i++;
+        /* Ignore spurious trailing bytes in the server list */
     }
 
-    /* There's no overlap between our protocols and the server's list. */
-    result = client;
-    status = OPENSSL_NPN_NO_OVERLAP;
-
- found:
-    *out = (unsigned char *)result + 1;
-    *outlen = result[0];
-    return status;
+    /*
+     * There's no overlap between our protocols and the server's list. We use
+     * the default opportunistic protocol selected earlier
+     */
+    return OPENSSL_NPN_NO_OVERLAP;
 }
 
 #ifndef OPENSSL_NO_NEXTPROTONEG