upstream: ssh-add side of destination constraints

Have ssh-add accept a list of "destination constraints" that allow
restricting where keys may be used in conjunction with a ssh-agent/ssh
that supports session ID/hostkey binding.

Constraints are specified as either "[user@]host-pattern" or
"host-pattern>[user@]host-pattern".

The first form permits a key to be used to authenticate as the
specified user to the specified host.

The second form permits a key that has previously been permitted
for use at a host to be available via a forwarded agent to an
additional host.

For example, constraining a key with "user1@host_a" and
"host_a>host_b". Would permit authentication as "user1" at
"host_a", and allow the key to be available on an agent forwarded
to "host_a" only for authentication to "host_b". The key would not
be visible on agent forwarded to other hosts or usable for
authentication there.

Internally, destination constraints use host keys to identify hosts.
The host patterns are used to obtain lists of host keys for that
destination that are communicated to the agent. The user/hostkeys are
encoded using a new restrict-destination-v00@openssh.com key
constraint.

host keys are looked up in the default client user/system known_hosts
files. It is possible to override this set on the command-line.

feedback Jann Horn & markus@
ok markus@

OpenBSD-Commit-ID: ef47fa9ec0e3c2a82e30d37ef616e245df73163e

authfd.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: authfd.c,v 1.128 2021/12/19 22:08:48 djm Exp $ */
+/* $OpenBSD: authfd.c,v 1.129 2021/12/19 22:10:24 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -454,12 +454,63 @@ ssh_agent_sign(int sock, const struct sshkey *key,
 
 /* Encode key for a message to the agent. */
 
+static int
+encode_dest_constraint_hop(struct sshbuf *m,
+    const struct dest_constraint_hop *dch)
+{
+	struct sshbuf *b;
+	u_int i;
+	int r;
+
+	if ((b = sshbuf_new()) == NULL)
+		return SSH_ERR_ALLOC_FAIL;
+	if ((r = sshbuf_put_cstring(b, dch->user)) != 0 ||
+	    (r = sshbuf_put_cstring(b, dch->hostname)) != 0 ||
+	    (r = sshbuf_put_string(b, NULL, 0)) != 0) /* reserved */
+		goto out;
+	for (i = 0; i < dch->nkeys; i++) {
+		if ((r = sshkey_puts(dch->keys[i], b)) != 0 ||
+		    (r = sshbuf_put_u8(b, dch->key_is_ca[i] != 0)) != 0)
+			goto out;
+	}
+	if ((r = sshbuf_put_stringb(m, b)) != 0)
+		goto out;
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(b);
+	return r;
+}
+
+static int
+encode_dest_constraint(struct sshbuf *m, const struct dest_constraint *dc)
+{
+	struct sshbuf *b;
+	int r;
+
+	if ((b = sshbuf_new()) == NULL)
+		return SSH_ERR_ALLOC_FAIL;
+	if ((r = encode_dest_constraint_hop(b, &dc->from) != 0) ||
+	    (r = encode_dest_constraint_hop(b, &dc->to) != 0) ||
+	    (r = sshbuf_put_string(b, NULL, 0)) != 0) /* reserved */
+		goto out;
+	if ((r = sshbuf_put_stringb(m, b)) != 0)
+		goto out;
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(b);
+	return r;
+}
 
 static int
 encode_constraints(struct sshbuf *m, u_int life, u_int confirm, u_int maxsign,
-    const char *provider)
+    const char *provider, struct dest_constraint **dest_constraints,
+    size_t ndest_constraints)
 {
 	int r;
+	struct sshbuf *b = NULL;
+	size_t i;
 
 	if (life != 0) {
 		if ((r = sshbuf_put_u8(m, SSH_AGENT_CONSTRAIN_LIFETIME)) != 0 ||
@@ -483,8 +534,26 @@ encode_constraints(struct sshbuf *m, u_int life, u_int confirm, u_int maxsign,
 		    (r = sshbuf_put_cstring(m, provider)) != 0)
 			goto out;
 	}
+	if (dest_constraints != NULL && ndest_constraints > 0) {
+		if ((b = sshbuf_new()) == NULL) {
+			r = SSH_ERR_ALLOC_FAIL;
+			goto out;
+		}
+		for (i = 0; i < ndest_constraints; i++) {
+			if ((r = encode_dest_constraint(b,
+			    dest_constraints[i])) != 0)
+				goto out;
+		}
+		if ((r = sshbuf_put_u8(m,
+		    SSH_AGENT_CONSTRAIN_EXTENSION)) != 0 ||
+		    (r = sshbuf_put_cstring(m,
+		    "restrict-destination-v00@openssh.com")) != 0 ||
+		    (r = sshbuf_put_stringb(m, b)) != 0)
+			goto out;
+	}
 	r = 0;
  out:
+	sshbuf_free(b);
 	return r;
 }
 
@@ -495,10 +564,12 @@ encode_constraints(struct sshbuf *m, u_int life, u_int confirm, u_int maxsign,
 int
 ssh_add_identity_constrained(int sock, struct sshkey *key,
     const char *comment, u_int life, u_int confirm, u_int maxsign,
-    const char *provider)
+    const char *provider, struct dest_constraint **dest_constraints,
+    size_t ndest_constraints)
 {
 	struct sshbuf *msg;
-	int r, constrained = (life || confirm || maxsign || provider);
+	int r, constrained = (life || confirm || maxsign ||
+	    provider || dest_constraints);
 	u_char type;
 
 	if ((msg = sshbuf_new()) == NULL)
@@ -536,7 +607,7 @@ ssh_add_identity_constrained(int sock, struct sshkey *key,
 	}
 	if (constrained &&
 	    (r = encode_constraints(msg, life, confirm, maxsign,
-	    provider)) != 0)
+	    provider, dest_constraints, ndest_constraints)) != 0)
 		goto out;
 	if ((r = ssh_request_reply_decode(sock, msg)) != 0)
 		goto out;
@@ -590,7 +661,8 @@ ssh_remove_identity(int sock, const struct sshkey *key)
  */
 int
 ssh_update_card(int sock, int add, const char *reader_id, const char *pin,
-    u_int life, u_int confirm)
+    u_int life, u_int confirm,
+    struct dest_constraint **dest_constraints, size_t ndest_constraints)
 {
 	struct sshbuf *msg;
 	int r, constrained = (life || confirm);
@@ -610,7 +682,8 @@ ssh_update_card(int sock, int add, const char *reader_id, const char *pin,
 	    (r = sshbuf_put_cstring(msg, pin)) != 0)
 		goto out;
 	if (constrained &&
-	    (r = encode_constraints(msg, life, confirm, 0, NULL)) != 0)
+	    (r = encode_constraints(msg, life, confirm, 0, NULL,
+	    dest_constraints, ndest_constraints)) != 0)
 		goto out;
 	if ((r = ssh_request_reply_decode(sock, msg)) != 0)
 		goto out;

authfd.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: authfd.h,v 1.50 2021/12/19 22:08:48 djm Exp $ */
+/* $OpenBSD: authfd.h,v 1.51 2021/12/19 22:10:24 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -17,6 +17,7 @@
 #define AUTHFD_H
 
 struct sshbuf;
+struct sshkey;
 
 /* List of identities returned by ssh_fetch_identitylist() */
 struct ssh_identitylist {
@@ -25,6 +26,20 @@ struct ssh_identitylist {
 	char **comments;
 };
 
+/* Key destination restrictions */
+struct dest_constraint_hop {
+	char *user;	/* wildcards allowed */
+	char *hostname; /* used to matching cert principals and for display */
+	int is_ca;
+	u_int nkeys;	/* number of entries in *both* 'keys' and 'key_is_ca' */
+	struct sshkey **keys;
+	int *key_is_ca;
+};
+struct dest_constraint {
+	struct dest_constraint_hop from;
+	struct dest_constraint_hop to;
+};
+
 int	ssh_get_authentication_socket(int *fdp);
 int	ssh_get_authentication_socket_path(const char *authsocket, int *fdp);
 void	ssh_close_authentication_socket(int sock);
@@ -33,12 +48,15 @@ int	ssh_lock_agent(int sock, int lock, const char *password);
 int	ssh_fetch_identitylist(int sock, struct ssh_identitylist **idlp);
 void	ssh_free_identitylist(struct ssh_identitylist *idl);
 int	ssh_add_identity_constrained(int sock, struct sshkey *key,
-	    const char *comment, u_int life, u_int confirm, u_int maxsign,
+    const char *comment, u_int life, u_int confirm, u_int maxsign,
-	    const char *provider);
+    const char *provider, struct dest_constraint **dest_constraints,
+    size_t ndest_constraints);
 int	ssh_agent_has_key(int sock, const struct sshkey *key);
 int	ssh_remove_identity(int sock, const struct sshkey *key);
 int	ssh_update_card(int sock, int add, const char *reader_id,
-	    const char *pin, u_int life, u_int confirm);
+	    const char *pin, u_int life, u_int confirm,
+	    struct dest_constraint **dest_constraints,
+	    size_t ndest_constraints);
 int	ssh_remove_all_identities(int sock, int version);
 
 int	ssh_agent_sign(int sock, const struct sshkey *key,

ssh-add.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-add.c,v 1.161 2021/10/28 02:54:18 djm Exp $ */
+/* $OpenBSD: ssh-add.c,v 1.162 2021/12/19 22:10:24 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -68,6 +68,7 @@
 #include "digest.h"
 #include "ssh-sk.h"
 #include "sk-api.h"
+#include "hostfile.h"
 
 /* argv0 */
 extern char *__progname;
@@ -231,7 +232,8 @@ delete_all(int agent_fd, int qflag)
 
 static int
 add_file(int agent_fd, const char *filename, int key_only, int qflag,
-    const char *skprovider)
+    const char *skprovider, struct dest_constraint **dest_constraints,
+    size_t ndest_constraints)
 {
 	struct sshkey *private, *cert;
 	char *comment = NULL;
@@ -365,7 +367,8 @@ add_file(int agent_fd, const char *filename, int key_only, int qflag,
 	}
 
 	if ((r = ssh_add_identity_constrained(agent_fd, private, comment,
-	    lifetime, confirm, maxsign, skprovider)) == 0) {
+	    lifetime, confirm, maxsign, skprovider,
+	    dest_constraints, ndest_constraints)) == 0) {
 		ret = 0;
 		if (!qflag) {
 			fprintf(stderr, "Identity added: %s (%s)\n",
@@ -417,7 +420,8 @@ add_file(int agent_fd, const char *filename, int key_only, int qflag,
 	sshkey_free(cert);
 
 	if ((r = ssh_add_identity_constrained(agent_fd, private, comment,
-	    lifetime, confirm, maxsign, skprovider)) != 0) {
+	    lifetime, confirm, maxsign, skprovider,
+	    dest_constraints, ndest_constraints)) != 0) {
 		error_r(r, "Certificate %s (%s) add failed", certpath,
 		    private->cert->key_id);
 		goto out;
@@ -445,7 +449,8 @@ add_file(int agent_fd, const char *filename, int key_only, int qflag,
 }
 
 static int
-update_card(int agent_fd, int add, const char *id, int qflag)
+update_card(int agent_fd, int add, const char *id, int qflag,
+    struct dest_constraint **dest_constraints, size_t ndest_constraints)
 {
 	char *pin = NULL;
 	int r, ret = -1;
@@ -457,7 +462,7 @@ update_card(int agent_fd, int add, const char *id, int qflag)
 	}
 
 	if ((r = ssh_update_card(agent_fd, add, id, pin == NULL ? "" : pin,
-	    lifetime, confirm)) == 0) {
+	    lifetime, confirm, dest_constraints, ndest_constraints)) == 0) {
 		ret = 0;
 		if (!qflag) {
 			fprintf(stderr, "Card %s: %s\n",
@@ -578,7 +583,8 @@ lock_agent(int agent_fd, int lock)
 }
 
 static int
-load_resident_keys(int agent_fd, const char *skprovider, int qflag)
+load_resident_keys(int agent_fd, const char *skprovider, int qflag,
+    struct dest_constraint **dest_constraints, size_t ndest_constraints)
 {
 	struct sshsk_resident_key **srks;
 	size_t nsrks, i;
@@ -598,8 +604,10 @@ load_resident_keys(int agent_fd, const char *skprovider, int qflag)
 		    fingerprint_hash, SSH_FP_DEFAULT)) == NULL)
 			fatal_f("sshkey_fingerprint failed");
 		if ((r = ssh_add_identity_constrained(agent_fd, key, "",
-		    lifetime, confirm, maxsign, skprovider)) != 0) {
+		    lifetime, confirm, maxsign, skprovider,
-			error("Unable to add key %s %s", sshkey_type(key), fp);
+		    dest_constraints, ndest_constraints)) != 0) {
+			error("Unable to add key %s %s",
+			    sshkey_type(key), fp);
 			free(fp);
 			ok = r;
 			continue;
@@ -628,23 +636,144 @@ load_resident_keys(int agent_fd, const char *skprovider, int qflag)
 
 static int
 do_file(int agent_fd, int deleting, int key_only, char *file, int qflag,
-    const char *skprovider)
+    const char *skprovider, struct dest_constraint **dest_constraints,
+    size_t ndest_constraints)
 {
 	if (deleting) {
 		if (delete_file(agent_fd, file, key_only, qflag) == -1)
 			return -1;
 	} else {
-		if (add_file(agent_fd, file, key_only, qflag, skprovider) == -1)
+		if (add_file(agent_fd, file, key_only, qflag, skprovider,
+		    dest_constraints, ndest_constraints) == -1)
 			return -1;
 	}
 	return 0;
 }
 
+/* Append string 's' to a NULL-terminated array of strings */
+static void
+stringlist_append(char ***listp, const char *s)
+{
+	size_t i = 0;
+
+	if (*listp == NULL)
+		*listp = xcalloc(2, sizeof(**listp));
+	else {
+		for (i = 0; (*listp)[i] != NULL; i++)
+			; /* count */
+		*listp = xrecallocarray(*listp, i + 1, i + 2, sizeof(**listp));
+	}
+	(*listp)[i] = xstrdup(s);
+}
+
+static void
+parse_dest_constraint_hop(const char *s, struct dest_constraint_hop *dch,
+    char **hostkey_files)
+{
+	char *user = NULL, *host, *os, *path;
+	size_t i;
+	struct hostkeys *hostkeys;
+	const struct hostkey_entry *hke;
+	int r, want_ca;
+
+	memset(dch, '\0', sizeof(*dch));
+	os = xstrdup(s);
+	if ((host = strchr(os, '@')) == NULL)
+		host = os;
+	else {
+		*host++ = '\0';
+		user = os;
+	}
+	cleanhostname(host);
+	/* Trivial case: username@ (all hosts) */
+	if (*host == '\0') {
+		if (user == NULL) {
+			fatal("Invalid key destination constraint \"%s\": "
+			    "does not specify user or host", s);
+		}
+		dch->user = xstrdup(user);
+		/* other fields left blank */
+		free(os);
+		return;
+	}
+	if (hostkey_files == NULL)
+		fatal_f("no hostkey files");
+	/* Otherwise we need to look up the keys for this hostname */
+	hostkeys = init_hostkeys();
+	for (i = 0; hostkey_files[i]; i++) {
+		path = tilde_expand_filename(hostkey_files[i], getuid());
+		debug2_f("looking up host keys for \"%s\" in %s", host, path);
+                load_hostkeys(hostkeys, host, path, 0);
+		free(path);
+	}
+	dch->user = user == NULL ? NULL : xstrdup(user);
+	dch->hostname = xstrdup(host);
+	for (i = 0; i < hostkeys->num_entries; i++) {
+		hke = hostkeys->entries + i;
+		want_ca = hke->marker == MRK_CA;
+		if (hke->marker != MRK_NONE && !want_ca)
+			continue;
+		debug3_f("%s%s%s: adding %s %skey from %s:%lu as key %u",
+		    user == NULL ? "": user, user == NULL ? "" : "@",
+		    host, sshkey_type(hke->key), want_ca ? "CA " : "",
+		    hke->file, hke->line, dch->nkeys);
+		dch->keys = xrecallocarray(dch->keys, dch->nkeys,
+		    dch->nkeys + 1, sizeof(*dch->keys));
+		dch->key_is_ca = xrecallocarray(dch->key_is_ca, dch->nkeys,
+		    dch->nkeys + 1, sizeof(*dch->key_is_ca));
+		if ((r = sshkey_from_private(hke->key,
+		    &(dch->keys[dch->nkeys]))) != 0)
+			fatal_fr(r, "sshkey_from_private");
+		dch->key_is_ca[dch->nkeys] = want_ca;
+		dch->nkeys++;
+	}
+	if (dch->nkeys == 0)
+		fatal("No host keys found for destination \"%s\"", host);
+	free_hostkeys(hostkeys);
+	free(os);
+	return;
+}
+
+static void
+parse_dest_constraint(const char *s, struct dest_constraint ***dcp,
+    size_t *ndcp, char **hostkey_files)
+{
+	struct dest_constraint *dc;
+	char *os, *cp;
+
+	dc = xcalloc(1, sizeof(*dc));
+	os = xstrdup(s);
+	if ((cp = strchr(os, '>')) == NULL) {
+		/* initial hop; no 'from' hop specified */
+		parse_dest_constraint_hop(os, &dc->to, hostkey_files);
+	} else {
+		/* two hops specified */
+		*(cp++) = '\0';
+		parse_dest_constraint_hop(os, &dc->from, hostkey_files);
+		parse_dest_constraint_hop(cp, &dc->to, hostkey_files);
+		if (dc->from.user != NULL) {
+			fatal("Invalid key constraint %s: cannot specify "
+			    "user on 'from' host", os);
+		}
+	}
+	/* XXX eliminate or error on duplicates */
+	debug2_f("constraint %zu: %s%s%s (%u keys) > %s%s%s (%u keys)", *ndcp,
+	    dc->from.user ? dc->from.user : "", dc->from.user ? "@" : "",
+	    dc->from.hostname ? dc->from.hostname : "(ORIGIN)", dc->from.nkeys,
+	    dc->to.user ? dc->to.user : "", dc->to.user ? "@" : "",
+	    dc->to.hostname ? dc->to.hostname : "(ANY)", dc->to.nkeys);
+	*dcp = xrecallocarray(*dcp, *ndcp, *ndcp + 1, sizeof(**dcp));
+	(*dcp)[(*ndcp)++] = dc;
+	free(os);
+}
+
+
 static void
 usage(void)
 {
 	fprintf(stderr,
 "usage: ssh-add [-cDdKkLlqvXx] [-E fingerprint_hash] [-S provider] [-t life]\n"
+"               [-H hostkey_file] [-h destination]\n"
 #ifdef WITH_XMSS
 "               [-M maxsign] [-m minleft]\n"
 #endif
@@ -662,10 +791,13 @@ main(int argc, char **argv)
 	extern int optind;
 	int agent_fd;
 	char *pkcs11provider = NULL, *skprovider = NULL;
+	char **dest_constraint_strings = NULL, **hostkey_files = NULL;
 	int r, i, ch, deleting = 0, ret = 0, key_only = 0, do_download = 0;
 	int xflag = 0, lflag = 0, Dflag = 0, qflag = 0, Tflag = 0;
 	SyslogFacility log_facility = SYSLOG_FACILITY_AUTH;
 	LogLevel log_level = SYSLOG_LEVEL_INFO;
+	struct dest_constraint **dest_constraints = NULL;
+	size_t ndest_constraints = 0;
 
 	/* Ensure that fds 0, 1 and 2 are open or directed to /dev/null */
 	sanitise_stdfd();
@@ -692,7 +824,7 @@ main(int argc, char **argv)
 
 	skprovider = getenv("SSH_SK_PROVIDER");
 
-	while ((ch = getopt(argc, argv, "vkKlLcdDTxXE:e:M:m:qs:S:t:")) != -1) {
+	while ((ch = getopt(argc, argv, "vkKlLcdDTxXE:e:h:H:M:m:qs:S:t:")) != -1) {
 		switch (ch) {
 		case 'v':
 			if (log_level == SYSLOG_LEVEL_INFO)
@@ -705,6 +837,12 @@ main(int argc, char **argv)
 			if (fingerprint_hash == -1)
 				fatal("Invalid hash algorithm \"%s\"", optarg);
 			break;
+		case 'H':
+			stringlist_append(&hostkey_files, optarg);
+			break;
+		case 'h':
+			stringlist_append(&dest_constraint_strings, optarg);
+			break;
 		case 'k':
 			key_only = 1;
 			break;
@@ -801,6 +939,20 @@ main(int argc, char **argv)
 		skprovider = "internal";
 #endif
 
+	if (hostkey_files == NULL) {
+		/* use defaults from readconf.c */
+		stringlist_append(&hostkey_files, _PATH_SSH_USER_HOSTFILE);
+		stringlist_append(&hostkey_files, _PATH_SSH_USER_HOSTFILE2);
+		stringlist_append(&hostkey_files, _PATH_SSH_SYSTEM_HOSTFILE);
+		stringlist_append(&hostkey_files, _PATH_SSH_SYSTEM_HOSTFILE2);
+	}
+	if (dest_constraint_strings != NULL) {
+		for (i = 0; dest_constraint_strings[i] != NULL; i++) {
+			parse_dest_constraint(dest_constraint_strings[i],
+			  &dest_constraints, &ndest_constraints, hostkey_files);
+		}
+	}
+
 	argc -= optind;
 	argv += optind;
 	if (Tflag) {
@@ -813,14 +965,15 @@ main(int argc, char **argv)
 	}
 	if (pkcs11provider != NULL) {
 		if (update_card(agent_fd, !deleting, pkcs11provider,
-		    qflag) == -1)
+		    qflag, dest_constraints, ndest_constraints) == -1)
 			ret = 1;
 		goto done;
 	}
 	if (do_download) {
 		if (skprovider == NULL)
 			fatal("Cannot download keys without provider");
-		if (load_resident_keys(agent_fd, skprovider, qflag) != 0)
+		if (load_resident_keys(agent_fd, skprovider, qflag,
+		    dest_constraints, ndest_constraints) != 0)
 			ret = 1;
 		goto done;
 	}
@@ -843,7 +996,8 @@ main(int argc, char **argv)
 			if (stat(buf, &st) == -1)
 				continue;
 			if (do_file(agent_fd, deleting, key_only, buf,
-			    qflag, skprovider) == -1)
+			    qflag, skprovider,
+			    dest_constraints, ndest_constraints) == -1)
 				ret = 1;
 			else
 				count++;
@@ -853,7 +1007,8 @@ main(int argc, char **argv)
 	} else {
 		for (i = 0; i < argc; i++) {
 			if (do_file(agent_fd, deleting, key_only,
-			    argv[i], qflag, skprovider) == -1)
+			    argv[i], qflag, skprovider,
+			    dest_constraints, ndest_constraints) == -1)
 				ret = 1;
 		}
 	}

sshconnect.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.355 2021/07/02 05:11:21 dtucker Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.356 2021/12/19 22:10:24 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -1700,7 +1700,7 @@ maybe_add_key_to_agent(const char *authfile, struct sshkey *private,
 	if ((r = ssh_add_identity_constrained(auth_sock, private,
 	    comment == NULL ? authfile : comment,
 	    options.add_keys_to_agent_lifespan,
-	    (options.add_keys_to_agent == 3), 0, skprovider)) == 0)
+	    (options.add_keys_to_agent == 3), 0, skprovider, NULL, 0)) == 0)
 		debug("identity added to agent: %s", authfile);
 	else
 		debug("could not add identity to agent: %s (%d)", authfile, r);