upstream: Enforce a maximum size for usernames in agent key use

constraints

Along with the match_pattern() performance change that was just
committed this avoids a denial-of-service where an agent client could
waste CPU on an agent by sending user constraints with lots of
wildcards.

Reported by Huzaifa Sidhpurwala of Redhat

ok markus

OpenBSD-Commit-ID: 0483817f1a8accf4dbff42b7073ee4d119105d71

diff --git a/ssh-agent.c b/ssh-agent.c
index 4a1781360f4edcc7ae0c230d9ef8c2ce531e1eb2..151b2b324b65b95b1fccde972ff7ca0757085321 100644 (file)
--- a/ssh-agent.c
+++ b/ssh-agent.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-agent.c,v 1.327 2026/05/27 03:28:07 tb Exp $ */
+/* $OpenBSD: ssh-agent.c,v 1.328 2026/05/31 04:31:04 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -101,6 +101,8 @@
 #define AGENT_MAX_DEST_CONSTRAINTS     1024
 /* Maximum number of associated certificate constraints to accept on a key */
 #define AGENT_MAX_EXT_CERTS            1024
+/* Max length of username constraint */
+#define AGENT_USER_CONSTRAINT_MAX_LEN  256
 
 /* XXX store hostkey_sid in a refcounted tree */
 
@@ -1078,13 +1080,13 @@ static int
 parse_dest_constraint_hop(struct sshbuf *b, struct dest_constraint_hop *dch)
 {
        u_char key_is_ca;
-       size_t elen = 0;
+       size_t elen = 0, userlen = 0;
        int r;
        struct sshkey *k = NULL;
        char *fp;
 
        memset(dch, '\0', sizeof(*dch));
-       if ((r = sshbuf_get_cstring(b, &dch->user, NULL)) != 0 ||
+       if ((r = sshbuf_get_cstring(b, &dch->user, &userlen)) != 0 ||
            (r = sshbuf_get_cstring(b, &dch->hostname, NULL)) != 0 ||
            (r = sshbuf_get_string_direct(b, NULL, &elen)) != 0) {
                error_fr(r, "parse");
@@ -1102,6 +1104,10 @@ parse_dest_constraint_hop(struct sshbuf *b, struct dest_constraint_hop *dch)
        if (*dch->user == '\0') {
                free(dch->user);
                dch->user = NULL;
+       } else if (userlen > AGENT_USER_CONSTRAINT_MAX_LEN) {
+               error_f("user match pattern too long");
+               r = SSH_ERR_INVALID_FORMAT;
+               goto out;
        }
        while (sshbuf_len(b) != 0) {
                dch->keys = xrecallocarray(dch->keys, dch->nkeys,