add explict check for server hostkey type against HostkeyAlgorithms.

Allows HostkeyAlgorithms to disable implicit fallback from certificate
keys to plain keys. ok markus@

diff --git a/usr.bin/ssh/clientloop.c b/usr.bin/ssh/clientloop.c
index 31eaa95..c4b37a7 100644 (file)
--- a/usr.bin/ssh/clientloop.c
+++ b/usr.bin/ssh/clientloop.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: clientloop.c,v 1.403 2024/02/21 05:57:34 djm Exp $ */
+/* $OpenBSD: clientloop.c,v 1.404 2024/04/30 02:10:49 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2426,25 +2426,6 @@ client_global_hostkeys_prove_confirm(struct ssh *ssh, int type,
        client_repledge();
 }
 
-/*
- * Returns non-zero if the key is accepted by HostkeyAlgorithms.
- * Made slightly less trivial by the multiple RSA signature algorithm names.
- */
-static int
-key_accepted_by_hostkeyalgs(const struct sshkey *key)
-{
-       const char *ktype = sshkey_ssh_name(key);
-       const char *hostkeyalgs = options.hostkeyalgorithms;
-
-       if (key->type == KEY_UNSPEC)
-               return 0;
-       if (key->type == KEY_RSA &&
-           (match_pattern_list("rsa-sha2-256", hostkeyalgs, 0) == 1 ||
-           match_pattern_list("rsa-sha2-512", hostkeyalgs, 0) == 1))
-               return 1;
-       return match_pattern_list(ktype, hostkeyalgs, 0) == 1;
-}
-
 /*
  * Handle hostkeys-00@openssh.com global request to inform the client of all
  * the server's hostkeys. The keys are checked against the user's
@@ -2489,7 +2470,7 @@ client_input_hostkeys(struct ssh *ssh)
                debug3_f("received %s key %s", sshkey_type(key), fp);
                free(fp);
 
-               if (!key_accepted_by_hostkeyalgs(key)) {
+               if (!hostkey_accepted_by_hostkeyalgs(key)) {
                        debug3_f("%s key not permitted by "
                            "HostkeyAlgorithms", sshkey_ssh_name(key));
                        continue;

diff --git a/usr.bin/ssh/sshconnect.c b/usr.bin/ssh/sshconnect.c
index 8ff9228..4cc597c 100644 (file)
--- a/usr.bin/ssh/sshconnect.c
+++ b/usr.bin/ssh/sshconnect.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.367 2024/04/23 13:34:50 jsg Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.368 2024/04/30 02:10:49 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -45,6 +45,7 @@
 #include "sshconnect.h"
 #include "hostfile.h"
 #include "log.h"
+#include "match.h"
 #include "misc.h"
 #include "readconf.h"
 #include "atomicio.h"
@@ -679,6 +680,29 @@ try_tilde_unexpand(const char *path)
        return ret;
 }
 
+/*
+ * Returns non-zero if the key is accepted by HostkeyAlgorithms.
+ * Made slightly less trivial by the multiple RSA signature algorithm names.
+ */
+int
+hostkey_accepted_by_hostkeyalgs(const struct sshkey *key)
+{
+       const char *ktype = sshkey_ssh_name(key);
+       const char *hostkeyalgs = options.hostkeyalgorithms;
+
+       if (key->type == KEY_UNSPEC)
+               return 0;
+       if (key->type == KEY_RSA &&
+           (match_pattern_list("rsa-sha2-256", hostkeyalgs, 0) == 1 ||
+           match_pattern_list("rsa-sha2-512", hostkeyalgs, 0) == 1))
+               return 1;
+       if (key->type == KEY_RSA_CERT &&
+           (match_pattern_list("rsa-sha2-512-cert-v01@openssh.com", hostkeyalgs, 0) == 1 ||
+           match_pattern_list("rsa-sha2-256-cert-v01@openssh.com", hostkeyalgs, 0) == 1))
+               return 1;
+       return match_pattern_list(ktype, hostkeyalgs, 0) == 1;
+}
+
 static int
 hostkeys_find_by_key_cb(struct hostkey_foreach_line *l, void *_ctx)
 {
@@ -979,6 +1003,12 @@ check_host_key(char *hostname, const struct ssh_conn_info *cinfo,
        }
 
  retry:
+       if (!hostkey_accepted_by_hostkeyalgs(host_key)) {
+               error("host key %s not permitted by HostkeyAlgorithms",
+                   sshkey_ssh_name(host_key));
+               goto fail;
+       }
+
        /* Reload these as they may have changed on cert->key downgrade */
        want_cert = sshkey_is_cert(host_key);
        type = sshkey_type(host_key);

diff --git a/usr.bin/ssh/sshconnect.h b/usr.bin/ssh/sshconnect.h
index 79d35cc..8b0466f 100644 (file)
--- a/usr.bin/ssh/sshconnect.h
+++ b/usr.bin/ssh/sshconnect.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.h,v 1.47 2023/10/12 02:18:18 djm Exp $ */
+/* $OpenBSD: sshconnect.h,v 1.48 2024/04/30 02:10:49 djm Exp $ */
 
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
@@ -24,6 +24,8 @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+struct sshkey;
+
 typedef struct Sensitive Sensitive;
 struct Sensitive {
        struct sshkey   **keys;
@@ -94,3 +96,5 @@ void   maybe_add_key_to_agent(const char *, struct sshkey *,
 void    load_hostkeys_command(struct hostkeys *, const char *,
     const char *, const struct ssh_conn_info *,
     const struct sshkey *, const char *);
+
+int hostkey_accepted_by_hostkeyalgs(const struct sshkey *);