upstream: add explict check for server hostkey type against

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

OpenBSD-Commit-ID: 364087e4a395ff9b2f42bf3aefdb2090bb23643a

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
@@ -2441,25 +2441,6 @@ 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
@@ -2504,7 +2485,7 @@ 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;

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
@@ -57,6 +57,7 @@
 #include "sshkey.h"
 #include "sshconnect.h"
 #include "log.h"
+#include "match.h"
 #include "misc.h"
 #include "readconf.h"
 #include "atomicio.h"
@@ -717,6 +718,29 @@ 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)
 {
@@ -1017,6 +1041,12 @@ 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);

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 *);