upstream: Add RequiredRSASize for sshd(8); RSA keys that fall

beneath this limit will be ignored for user and host-based authentication.

Feedback deraadt@ ok markus@

OpenBSD-Commit-ID: 187931dfc19d51873df5930a04f2d972adf1f7f1

auth2-hostbased.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2-hostbased.c,v 1.49 2022/01/06 22:01:14 djm Exp $ */
+/* $OpenBSD: auth2-hostbased.c,v 1.50 2022/09/17 10:34:29 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -119,6 +119,11 @@ userauth_hostbased(struct ssh *ssh, const char *method)
 		    "(null)" : key->cert->signature_type);
 		goto done;
 	}
+	if ((r = sshkey_check_rsa_length(key,
+	    options.required_rsa_size)) != 0) {
+		logit_r(r, "refusing %s key", sshkey_type(key));
+		goto done;
+	}
 
 	if (!authctxt->valid || authctxt->user == NULL) {
 		debug2_f("disabled because of invalid user");

auth2-pubkey.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2-pubkey.c,v 1.116 2022/06/15 16:08:25 djm Exp $ */
+/* $OpenBSD: auth2-pubkey.c,v 1.117 2022/09/17 10:34:29 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -175,6 +175,11 @@ userauth_pubkey(struct ssh *ssh, const char *method)
 		    "(null)" : key->cert->signature_type);
 		goto done;
 	}
+	if ((r = sshkey_check_rsa_length(key,
+	    options.required_rsa_size)) != 0) {
+		logit_r(r, "refusing %s key", sshkey_type(key));
+		goto done;
+	}
 	key_s = format_key(key);
 	if (sshkey_is_cert(key))
 		ca_s = format_key(key->cert->signature_key);

servconf.c

@@ -1,5 +1,5 @@
 
-/* $OpenBSD: servconf.c,v 1.385 2022/06/03 04:30:47 djm Exp $ */
+/* $OpenBSD: servconf.c,v 1.386 2022/09/17 10:34:29 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -195,6 +195,7 @@ initialize_server_options(ServerOptions *options)
 	options->fingerprint_hash = -1;
 	options->disable_forwarding = -1;
 	options->expose_userauth_info = -1;
+	options->required_rsa_size = -1;
 }
 
 /* Returns 1 if a string option is unset or set to "none" or 0 otherwise. */
@@ -441,6 +442,8 @@ fill_default_server_options(ServerOptions *options)
 		options->expose_userauth_info = 0;
 	if (options->sk_provider == NULL)
 		options->sk_provider = xstrdup("internal");
+	if (options->required_rsa_size == -1)
+		options->required_rsa_size = SSH_RSA_MINIMUM_MODULUS_SIZE;
 
 	assemble_algorithms(options);
 
@@ -517,6 +520,7 @@ typedef enum {
 	sStreamLocalBindMask, sStreamLocalBindUnlink,
 	sAllowStreamLocalForwarding, sFingerprintHash, sDisableForwarding,
 	sExposeAuthInfo, sRDomain, sPubkeyAuthOptions, sSecurityKeyProvider,
+	sRequiredRSASize,
 	sDeprecated, sIgnore, sUnsupported
 } ServerOpCodes;
 
@@ -676,6 +680,7 @@ static struct {
 	{ "rdomain", sRDomain, SSHCFG_ALL },
 	{ "casignaturealgorithms", sCASignatureAlgorithms, SSHCFG_ALL },
 	{ "securitykeyprovider", sSecurityKeyProvider, SSHCFG_GLOBAL },
+	{ "requiredrsasize", sRequiredRSASize, SSHCFG_ALL },
 	{ NULL, sBadOption, 0 }
 };
 
@@ -2438,6 +2443,10 @@ process_server_config_line_depth(ServerOptions *options, char *line,
 			*charptr = xstrdup(arg);
 		break;
 
+	case sRequiredRSASize:
+		intptr = &options->required_rsa_size;
+		goto parse_int;
+
 	case sDeprecated:
 	case sIgnore:
 	case sUnsupported:
@@ -2610,6 +2619,7 @@ copy_set_server_options(ServerOptions *dst, ServerOptions *src, int preauth)
 	M_CP_INTOPT(rekey_limit);
 	M_CP_INTOPT(rekey_interval);
 	M_CP_INTOPT(log_level);
+	M_CP_INTOPT(required_rsa_size);
 
 	/*
 	 * The bind_mask is a mode_t that may be unsigned, so we can't use
@@ -2874,6 +2884,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_int(sMaxSessions, o->max_sessions);
 	dump_cfg_int(sClientAliveInterval, o->client_alive_interval);
 	dump_cfg_int(sClientAliveCountMax, o->client_alive_count_max);
+	dump_cfg_int(sRequiredRSASize, o->required_rsa_size);
 	dump_cfg_oct(sStreamLocalBindMask, o->fwd_opts.streamlocal_bind_mask);
 
 	/* formatted integer arguments */

servconf.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.156 2022/03/18 04:04:11 djm Exp $ */
+/* $OpenBSD: servconf.h,v 1.157 2022/09/17 10:34:29 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -229,6 +229,7 @@ typedef struct {
 	int	expose_userauth_info;
 	u_int64_t timing_secret;
 	char   *sk_provider;
+	int	required_rsa_size;	/* minimum size of RSA keys */
 }       ServerOptions;
 
 /* Information about the incoming connection as used by Match */

sshd.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: sshd.c,v 1.590 2022/07/01 05:08:23 dtucker Exp $ */
+/* $OpenBSD: sshd.c,v 1.591 2022/09/17 10:34:29 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -1870,6 +1870,13 @@ main(int ac, char **av)
 				fatal_r(r, "Could not demote key: \"%s\"",
 				    options.host_key_files[i]);
 		}
+		if (pubkey != NULL && (r = sshkey_check_rsa_length(pubkey,
+		    options.required_rsa_size)) != 0) {
+			error_fr(r, "Host key %s", options.host_key_files[i]);
+			sshkey_free(pubkey);
+			sshkey_free(key);
+			continue;
+		}
 		sensitive_data.host_keys[i] = key;
 		sensitive_data.host_pubkeys[i] = pubkey;
 

sshd_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: sshd_config.5,v 1.342 2022/06/24 04:27:14 djm Exp $
-.Dd $Mdocdate: June 24 2022 $
+.\" $OpenBSD: sshd_config.5,v 1.343 2022/09/17 10:34:29 djm Exp $
+.Dd $Mdocdate: September 17 2022 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -1596,6 +1596,16 @@ is
 .Cm default none ,
 which means that rekeying is performed after the cipher's default amount
 of data has been sent or received and no time based rekeying is done.
+.It Cm RequiredRSASize
+Specifies the minimum RSA key size (in bits) that
+.Xr sshd 8
+will accept.
+User and host-based authentication keys smaller than this limit will be
+refused.
+The default is
+.Cm 1024
+bits.
+Note that this limit may only be raised from the default.
 .It Cm RevokedKeys
 Specifies revoked public keys file, or
 .Cm none