upstream: process security key provider via realpath() in agent,

avoids malicious client from being able to cause agent to load arbitrary
libraries into ssh-sk-helper.

reported by puck AT puckipedia.com; ok markus

OpenBSD-Commit-ID: 1086643df1b7eee4870825c687cf0c26a6145d1c

1 files changed, 19 insertions, 7 deletions

diff --git a/ssh-agent.c b/ssh-agent.c
index 6092f19dc..5c9a9de60 100644
--- a/ssh-agent.c
+++ b/ssh-agent.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-agent.c,v 1.253 2020/01/25 00:03:36 djm Exp $ */
+/* $OpenBSD: ssh-agent.c,v 1.254 2020/01/25 00:06:48 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -430,6 +430,7 @@ process_add_identity(SocketEntry *e)
         int success = 0, confirm = 0;
         u_int seconds, maxsign;
         char *fp, *comment = NULL, *ext_name = NULL, *sk_provider = NULL;
+        char canonical_provider[PATH_MAX];
         time_t death = 0;
         struct sshkey *k = NULL;
         u_char ctype;
@@ -514,14 +515,25 @@ process_add_identity(SocketEntry *e)
                         free(sk_provider);
                         goto send;
                 }
-                if (strcasecmp(sk_provider, "internal") == 0)
+                if (strcasecmp(sk_provider, "internal") == 0) {
                         debug("%s: internal provider", __func__);
-                else if (match_pattern_list(sk_provider,
-                    provider_whitelist, 0) != 1) {
-                        error("Refusing add key: provider %s not whitelisted",
-                            sk_provider);
+                } else {
+                        if (realpath(sk_provider, canonical_provider) == NULL) {
+                                verbose("failed provider \"%.100s\": "
+                                    "realpath: %s", sk_provider,
+                                    strerror(errno));
+                                free(sk_provider);
+                                goto send;
+                        }
                         free(sk_provider);
-                        goto send;
+                        sk_provider = xstrdup(canonical_provider);
+                        if (match_pattern_list(sk_provider,
+                            provider_whitelist, 0) != 1) {
+                                error("Refusing add key: "
+                                    "provider %s not whitelisted", sk_provider);
+                                free(sk_provider);
+                                goto send;
+                        }
                 }
         }
         if ((r = sshkey_shield_private(k)) != 0) {