add CASignatureAlgorithms option for the client, allowing it to specify

which signature algorithms may be used by CAs when signing certificates.
Useful if you want to ban RSA/SHA1; ok markus@

4 files changed, 42 insertions, 10 deletions

diff --git a/usr.bin/ssh/readconf.c b/usr.bin/ssh/readconf.c
index b2103575206..c4f8e19486b 100644
--- a/usr.bin/ssh/readconf.c
+++ b/usr.bin/ssh/readconf.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.297 2018/08/12 20:19:13 djm Exp $ */
+/* $OpenBSD: readconf.c,v 1.298 2018/09/20 03:30:44 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -157,7 +157,7 @@ typedef enum {
 	oCanonicalizeFallbackLocal, oCanonicalizePermittedCNAMEs,
 	oStreamLocalBindMask, oStreamLocalBindUnlink, oRevokedHostKeys,
 	oFingerprintHash, oUpdateHostkeys, oHostbasedKeyTypes,
-	oPubkeyAcceptedKeyTypes, oProxyJump,
+	oPubkeyAcceptedKeyTypes, oCASignatureAlgorithms, oProxyJump,
 	oIgnore, oIgnoredUnknownOption, oDeprecated, oUnsupported
 } OpCodes;
 
@@ -251,6 +251,7 @@ static struct {
 	{ "dynamicforward", oDynamicForward },
 	{ "preferredauthentications", oPreferredAuthentications },
 	{ "hostkeyalgorithms", oHostKeyAlgorithms },
+	{ "casignaturealgorithms", oCASignatureAlgorithms },
 	{ "bindaddress", oBindAddress },
 	{ "bindinterface", oBindInterface },
 	{ "clearallforwardings", oClearAllForwardings },
@@ -1206,6 +1207,10 @@ parse_keytypes:
 			*charptr = xstrdup(arg);
 		break;
 
+	case oCASignatureAlgorithms:
+		charptr = &options->ca_sign_algorithms;
+		goto parse_keytypes;
+
 	case oLogLevel:
 		log_level_ptr = &options->log_level;
 		arg = strdelim(&s);
@@ -1821,6 +1826,7 @@ initialize_options(Options * options)
 	options->macs = NULL;
 	options->kex_algorithms = NULL;
 	options->hostkeyalgorithms = NULL;
+	options->ca_sign_algorithms = NULL;
 	options->num_identity_files = 0;
 	options->num_certificate_files = 0;
 	options->hostname = NULL;
@@ -1909,7 +1915,7 @@ fill_default_options_for_canonicalization(Options *options)
 void
 fill_default_options(Options * options)
 {
-	char *all_cipher, *all_mac, *all_kex, *all_key;
+	char *all_cipher, *all_mac, *all_kex, *all_key, *all_sig;
 	int r;
 
 	if (options->forward_agent == -1)
@@ -2060,6 +2066,7 @@ fill_default_options(Options * options)
 	all_mac = mac_alg_list(',');
 	all_kex = kex_alg_list(',');
 	all_key = sshkey_alg_list(0, 0, 1, ',');
+	all_sig = sshkey_alg_list(0, 1, 1, ',');
 #define ASSEMBLE(what, defaults, all) \
 	do { \
 		if ((r = kex_assemble_names(&options->what, \
@@ -2071,11 +2078,13 @@ fill_default_options(Options * options)
 	ASSEMBLE(kex_algorithms, KEX_SERVER_KEX, all_kex);
 	ASSEMBLE(hostbased_key_types, KEX_DEFAULT_PK_ALG, all_key);
 	ASSEMBLE(pubkey_key_types, KEX_DEFAULT_PK_ALG, all_key);
+	ASSEMBLE(ca_sign_algorithms, SSH_ALLOWED_CA_SIGALGS, all_sig);
 #undef ASSEMBLE
 	free(all_cipher);
 	free(all_mac);
 	free(all_kex);
 	free(all_key);
+	free(all_sig);
 
 #define CLEAR_ON_NONE(v) \
 	do { \
@@ -2597,6 +2606,7 @@ dump_client_config(Options *o, const char *host)
 	dump_cfg_string(oIgnoreUnknown, o->ignored_unknown);
 	dump_cfg_string(oKbdInteractiveDevices, o->kbd_interactive_devices);
 	dump_cfg_string(oKexAlgorithms, o->kex_algorithms ? o->kex_algorithms : KEX_CLIENT_KEX);
+	dump_cfg_string(oCASignatureAlgorithms, o->ca_sign_algorithms ? o->ca_sign_algorithms : SSH_ALLOWED_CA_SIGALGS);
 	dump_cfg_string(oLocalCommand, o->local_command);
 	dump_cfg_string(oRemoteCommand, o->remote_command);
 	dump_cfg_string(oLogLevel, log_level_name(o->log_level));
diff --git a/usr.bin/ssh/readconf.h b/usr.bin/ssh/readconf.h
index c56887816d1..fc7e3825101 100644
--- a/usr.bin/ssh/readconf.h
+++ b/usr.bin/ssh/readconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.h,v 1.127 2018/07/19 10:28:47 dtucker Exp $ */
+/* $OpenBSD: readconf.h,v 1.128 2018/09/20 03:30:44 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -67,6 +67,7 @@ typedef struct {
 	char   *macs;		/* SSH2 macs in order of preference. */
 	char   *hostkeyalgorithms;	/* SSH2 server key types in order of preference. */
 	char   *kex_algorithms;	/* SSH2 kex methods in order of preference. */
+	char   *ca_sign_algorithms;	/* Allowed CA signature algorithms */
 	char   *hostname;	/* Real host to connect. */
 	char   *host_key_alias;	/* hostname alias for .ssh/known_hosts */
 	char   *proxy_command;	/* Proxy command for connecting the host. */
diff --git a/usr.bin/ssh/ssh_config.5 b/usr.bin/ssh/ssh_config.5
index fe52578f46c..38f177fa43a 100644
--- a/usr.bin/ssh/ssh_config.5
+++ b/usr.bin/ssh/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 1.281 2018/07/23 19:02:49 kn Exp $
-.Dd $Mdocdate: July 23 2018 $
+.\" $OpenBSD: ssh_config.5,v 1.282 2018/09/20 03:30:44 djm Exp $
+.Dd $Mdocdate: September 20 2018 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -261,6 +261,18 @@ Only useful on systems with more than one address.
 .It Cm BindInterface
 Use the address of the specified interface on the local machine as the
 source address of the connection.
+.It Cm CASignatureAlgorithms
+Specifies which algorithms are allowed for signing of certificates
+by certificate authorities (CAs).
+The default is:
+.Bd -literal -offset indent
+ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
+ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
+.Ed
+.Pp
+.Xr ssh 1
+will not accept host certificates signed using algorithms other than those
+specified.
 .It Cm CanonicalDomains
 When
 .Cm CanonicalizeHostname
diff --git a/usr.bin/ssh/sshconnect.c b/usr.bin/ssh/sshconnect.c
index 359264edc6a..47b261f74ea 100644
--- a/usr.bin/ssh/sshconnect.c
+++ b/usr.bin/ssh/sshconnect.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.304 2018/07/27 05:34:42 dtucker Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.305 2018/09/20 03:30:44 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -711,19 +711,28 @@ confirm(const char *prompt)
 }
 
 static int
-check_host_cert(const char *host, const struct sshkey *host_key)
+check_host_cert(const char *host, const struct sshkey *key)
 {
 	const char *reason;
+	int r;
 
-	if (sshkey_cert_check_authority(host_key, 1, 0, host, &reason) != 0) {
+	if (sshkey_cert_check_authority(key, 1, 0, host, &reason) != 0) {
 		error("%s", reason);
 		return 0;
 	}
-	if (sshbuf_len(host_key->cert->critical) != 0) {
+	if (sshbuf_len(key->cert->critical) != 0) {
 		error("Certificate for %s contains unsupported "
 		    "critical options(s)", host);
 		return 0;
 	}
+	if ((r = sshkey_check_cert_sigtype(key,
+	    options.ca_sign_algorithms)) != 0) {
+		logit("%s: certificate signature algorithm %s: %s", __func__,
+		    (key->cert == NULL || key->cert->signature_type == NULL) ?
+		    "(null)" : key->cert->signature_type, ssh_err(r));
+		return 0;
+	}
+
 	return 1;
 }