From 82d651daaf1af2272ec444693bdc4e32d3a16184 Mon Sep 17 00:00:00 2001 From: djm Date: Sun, 19 Dec 2021 22:10:24 +0000 Subject: [PATCH] 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@ --- usr.bin/ssh/authfd.c | 87 ++++++++++++++++-- usr.bin/ssh/authfd.h | 26 +++++- usr.bin/ssh/ssh-add.c | 186 +++++++++++++++++++++++++++++++++++---- usr.bin/ssh/sshconnect.c | 4 +- 4 files changed, 274 insertions(+), 29 deletions(-) diff --git a/usr.bin/ssh/authfd.c b/usr.bin/ssh/authfd.c index 18fa1f19c2b..c358581b939 100644 --- a/usr.bin/ssh/authfd.c +++ b/usr.bin/ssh/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 * Copyright (c) 1995 Tatu Ylonen , Espoo, Finland @@ -453,12 +453,63 @@ 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 || @@ -482,8 +533,26 @@ 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; } @@ -494,10 +563,12 @@ 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) @@ -535,7 +606,7 @@ 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; @@ -589,7 +660,8 @@ 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); @@ -609,7 +681,8 @@ 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; diff --git a/usr.bin/ssh/authfd.h b/usr.bin/ssh/authfd.h index fe1ed8a1abc..7a1c0ddff98 100644 --- a/usr.bin/ssh/authfd.h +++ b/usr.bin/ssh/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 @@ -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 *provider); + const char *comment, u_int life, u_int confirm, u_int maxsign, + 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, diff --git a/usr.bin/ssh/ssh-add.c b/usr.bin/ssh/ssh-add.c index 116b5a87cdc..970d1163795 100644 --- a/usr.bin/ssh/ssh-add.c +++ b/usr.bin/ssh/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 * Copyright (c) 1995 Tatu Ylonen , Espoo, Finland @@ -65,6 +65,7 @@ #include "digest.h" #include "ssh-sk.h" #include "sk-api.h" +#include "hostfile.h" /* argv0 */ extern char *__progname; @@ -224,7 +225,8 @@ 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; @@ -358,7 +360,8 @@ 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", @@ -410,7 +413,8 @@ 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; @@ -438,7 +442,8 @@ 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; @@ -450,7 +455,7 @@ 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", @@ -571,7 +576,8 @@ 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; @@ -591,8 +597,10 @@ 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; @@ -621,23 +629,144 @@ 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 @@ -655,10 +784,13 @@ 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(); @@ -685,7 +817,7 @@ 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) @@ -698,6 +830,12 @@ 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; @@ -791,6 +929,19 @@ main(int argc, char **argv) if (skprovider == NULL) skprovider = "internal"; + 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; @@ -804,14 +955,15 @@ 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; } @@ -834,7 +986,8 @@ 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++; @@ -844,7 +997,8 @@ 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; } } diff --git a/usr.bin/ssh/sshconnect.c b/usr.bin/ssh/sshconnect.c index 465c4d37339..52daead44d6 100644 --- a/usr.bin/ssh/sshconnect.c +++ b/usr.bin/ssh/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 * Copyright (c) 1995 Tatu Ylonen , Espoo, Finland @@ -1662,7 +1662,7 @@ 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); -- 2.20.1