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

1 files changed, 171 insertions, 16 deletions

diff --git a/ssh-add.c b/ssh-add.c
index 1b41cc18e..5090aca47 100644
--- a/ssh-add.c
+++ b/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) {
-			error("Unable to add key %s %s", sshkey_type(key), fp);
+		    lifetime, confirm, maxsign, skprovider,
+		    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;
 		}
 	}