upstream: add a RequiredRSASize for checking RSA key length in

ssh(1). User authentication keys that fall beneath this limit will be
ignored. If a host presents a host key beneath this limit then the connection
will be terminated (unfortunately there are no fallbacks in the protocol for
host authentication).

feedback deraadt, Dmitry Belyavskiy; ok markus@

OpenBSD-Commit-ID: 430e339b2a79fa9ecc63f2837b06fdd88a7da13a

Author: djmdjm

readconf.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.368 2022/06/03 04:30:47 djm Exp $ */
+/* $OpenBSD: readconf.c,v 1.369 2022/09/17 10:33:18 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -174,7 +174,7 @@ typedef enum {
 	oStreamLocalBindMask, oStreamLocalBindUnlink, oRevokedHostKeys,
 	oFingerprintHash, oUpdateHostkeys, oHostbasedAcceptedAlgorithms,
 	oPubkeyAcceptedAlgorithms, oCASignatureAlgorithms, oProxyJump,
-	oSecurityKeyProvider, oKnownHostsCommand,
+	oSecurityKeyProvider, oKnownHostsCommand, oRequiredRSASize,
 	oIgnore, oIgnoredUnknownOption, oDeprecated, oUnsupported
 } OpCodes;
 
@@ -320,6 +320,7 @@ static struct {
 	{ "proxyjump", oProxyJump },
 	{ "securitykeyprovider", oSecurityKeyProvider },
 	{ "knownhostscommand", oKnownHostsCommand },
+	{ "requiredrsasize", oRequiredRSASize },
 
 	{ NULL, oBadOption }
 };
@@ -2176,6 +2177,10 @@ process_config_line_depth(Options *options, struct passwd *pw, const char *host,
 			*charptr = xstrdup(arg);
 		break;
 
+	case oRequiredRSASize:
+		intptr = &options->required_rsa_size;
+		goto parse_int;
+
 	case oDeprecated:
 		debug("%s line %d: Deprecated option \"%s\"",
 		    filename, linenum, keyword);
@@ -2423,6 +2428,7 @@ initialize_options(Options * options)
 	options->hostbased_accepted_algos = NULL;
 	options->pubkey_accepted_algos = NULL;
 	options->known_hosts_command = NULL;
+	options->required_rsa_size = -1;
 }
 
 /*
@@ -2619,6 +2625,8 @@ fill_default_options(Options * options)
 	if (options->sk_provider == NULL)
 		options->sk_provider = xstrdup("$SSH_SK_PROVIDER");
 #endif
+	if (options->required_rsa_size == -1)
+		options->required_rsa_size = SSH_RSA_MINIMUM_MODULUS_SIZE;
 
 	/* Expand KEX name lists */
 	all_cipher = cipher_alg_list(',', 0);
@@ -3308,6 +3316,7 @@ dump_client_config(Options *o, const char *host)
 	dump_cfg_int(oNumberOfPasswordPrompts, o->number_of_password_prompts);
 	dump_cfg_int(oServerAliveCountMax, o->server_alive_count_max);
 	dump_cfg_int(oServerAliveInterval, o->server_alive_interval);
+	dump_cfg_int(oRequiredRSASize, o->required_rsa_size);
 
 	/* String options */
 	dump_cfg_string(oBindAddress, o->bind_address);

readconf.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.h,v 1.147 2022/06/03 04:30:47 djm Exp $ */
+/* $OpenBSD: readconf.h,v 1.148 2022/09/17 10:33:18 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <[email protected]>
@@ -176,6 +176,8 @@ typedef struct {
 
 	char   *known_hosts_command;
 
+	int	required_rsa_size;	/* minimum size of RSA keys */
+
 	char	*ignored_unknown; /* Pattern list of unknown tokens to ignore */
 }       Options;
 

ssh.1

@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh.1,v 1.431 2022/05/28 05:57:56 jmc Exp $
-.Dd $Mdocdate: May 28 2022 $
+.\" $OpenBSD: ssh.1,v 1.432 2022/09/17 10:33:18 djm Exp $
+.Dd $Mdocdate: September 17 2022 $
 .Dt SSH 1
 .Os
 .Sh NAME
@@ -571,6 +571,7 @@ For full details of the options listed below, and their possible values, see
 .It RemoteCommand
 .It RemoteForward
 .It RequestTTY
+.It RequiredRSASize
 .It SendEnv
 .It ServerAliveInterval
 .It ServerAliveCountMax

ssh.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh.c,v 1.575 2022/07/01 00:36:30 djm Exp $ */
+/* $OpenBSD: ssh.c,v 1.576 2022/09/17 10:33:18 djm Exp $ */
 /*
  * Author: Tatu Ylonen <[email protected]>
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
@@ -516,14 +516,22 @@ resolve_canonicalize(char **hostp, int port)
 }
 
 /*
- * Check the result of hostkey loading, ignoring some errors and
- * fatal()ing for others.
+ * Check the result of hostkey loading, ignoring some errors and either
+ * discarding the key or fatal()ing for others.
  */
 static void
-check_load(int r, const char *path, const char *message)
+check_load(int r, struct sshkey **k, const char *path, const char *message)
 {
 	switch (r) {
 	case 0:
+		/* Check RSA keys size and discard if undersized */
+		if (k != NULL && *k != NULL &&
+		    (r = sshkey_check_rsa_length(*k,
+		    options.required_rsa_size)) != 0) {
+			error_r(r, "load %s \"%s\"", message, path);
+			free(*k);
+			*k = NULL;
+		}
 		break;
 	case SSH_ERR_INTERNAL_ERROR:
 	case SSH_ERR_ALLOC_FAIL:
@@ -1578,15 +1586,16 @@ main(int ac, char **av)
 	if ((o) >= sensitive_data.nkeys) \
 		fatal_f("pubkey out of array bounds"); \
 	check_load(sshkey_load_public(p, &(sensitive_data.keys[o]), NULL), \
-	    p, "pubkey"); \
+	    &(sensitive_data.keys[o]), p, "pubkey"); \
 	if (sensitive_data.keys[o] != NULL) \
 		debug2("hostbased key %d: %s key from \"%s\"", o, \
 		    sshkey_ssh_name(sensitive_data.keys[o]), p); \
 } while (0)
 #define L_CERT(p,o) do { \
 	if ((o) >= sensitive_data.nkeys) \
 		fatal_f("cert out of array bounds"); \
-	check_load(sshkey_load_cert(p, &(sensitive_data.keys[o])), p, "cert"); \
+	check_load(sshkey_load_cert(p, &(sensitive_data.keys[o])), \
+	    &(sensitive_data.keys[o]), p, "cert"); \
 	if (sensitive_data.keys[o] != NULL) \
 		debug2("hostbased key %d: %s cert from \"%s\"", o, \
 		    sshkey_ssh_name(sensitive_data.keys[o]), p); \
@@ -2265,7 +2274,7 @@ load_public_identity_files(const struct ssh_conn_info *cinfo)
 		filename = default_client_percent_dollar_expand(cp, cinfo);
 		free(cp);
 		check_load(sshkey_load_public(filename, &public, NULL),
-		    filename, "pubkey");
+		    &public, filename, "pubkey");
 		debug("identity file %s type %d", filename,
 		    public ? public->type : -1);
 		free(options.identity_files[i]);
@@ -2284,7 +2293,7 @@ load_public_identity_files(const struct ssh_conn_info *cinfo)
 			continue;
 		xasprintf(&cp, "%s-cert", filename);
 		check_load(sshkey_load_public(cp, &public, NULL),
-		    filename, "pubkey");
+		    &public, filename, "pubkey");
 		debug("identity file %s type %d", cp,
 		    public ? public->type : -1);
 		if (public == NULL) {
@@ -2315,7 +2324,7 @@ load_public_identity_files(const struct ssh_conn_info *cinfo)
 		free(cp);
 
 		check_load(sshkey_load_public(filename, &public, NULL),
-		    filename, "certificate");
+		    &public, filename, "certificate");
 		debug("certificate file %s type %d", filename,
 		    public ? public->type : -1);
 		free(options.certificate_files[i]);

ssh_config.5

@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: ssh_config.5,v 1.373 2022/06/24 04:27:14 djm Exp $
-.Dd $Mdocdate: June 24 2022 $
+.\" $OpenBSD: ssh_config.5,v 1.374 2022/09/17 10:33:18 djm Exp $
+.Dd $Mdocdate: September 17 2022 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -1634,6 +1634,17 @@ and
 .Fl T
 flags for
 .Xr ssh 1 .
+.It Cm RequiredRSASize
+Specifies the minimum RSA key size (in bits) that
+.Xr ssh 1
+will accept.
+User authentication keys smaller than this limit will be ignored.
+Servers that present host keys smaller than this limit will cause the
+connection to be terminated.
+The default is
+.Cm 1024
+bits.
+Note that this limit may only be raised from the default.
 .It Cm RevokedHostKeys
 Specifies revoked host public keys.
 Keys listed in this file will be refused for host authentication.

sshconnect2.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.360 2022/08/19 06:07:47 djm Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.361 2022/09/17 10:33:18 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -96,6 +96,11 @@ static const struct ssh_conn_info *xxx_conn_info;
 static int
 verify_host_key_callback(struct sshkey *hostkey, struct ssh *ssh)
 {
+	int r;
+
+	if ((r = sshkey_check_rsa_length(hostkey,
+	    options.required_rsa_size)) != 0)
+		fatal_r(r, "Bad server host key");
 	if (verify_host_key(xxx_host, xxx_hostaddr, hostkey,
 	    xxx_conn_info) == -1)
 		fatal("Host key verification failed.");
@@ -1606,6 +1611,13 @@ load_identity_file(Identity *id)
 			private = NULL;
 			quit = 1;
 		}
+		if (!quit && (r = sshkey_check_rsa_length(private,
+		    options.required_rsa_size)) != 0) {
+			debug_fr(r, "Skipping key %s", id->filename);
+			sshkey_free(private);
+			private = NULL;
+			quit = 1;
+		}
 		if (!quit && private != NULL && id->agent_fd == -1 &&
 		    !(id->key && id->isprivate))
 			maybe_add_key_to_agent(id->filename, private, comment,
@@ -1752,6 +1764,12 @@ pubkey_prepare(struct ssh *ssh, Authctxt *authctxt)
 	/* list of keys supported by the agent */
 	if ((r = get_agent_identities(ssh, &agent_fd, &idlist)) == 0) {
 		for (j = 0; j < idlist->nkeys; j++) {
+			if ((r = sshkey_check_rsa_length(idlist->keys[j],
+			    options.required_rsa_size)) != 0) {
+				debug_fr(r, "ignoring %s agent key",
+				    sshkey_ssh_name(idlist->keys[j]));
+				continue;
+			}
 			found = 0;
 			TAILQ_FOREACH(id, &files, next) {
 				/*