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ssh-sk-client.c revision 1.8 
      1 /*	$NetBSD: ssh-sk-client.c,v 1.8 2025/04/09 15:49:33 christos Exp $	*/
      2 /* $OpenBSD: ssh-sk-client.c,v 1.13 2025/02/18 08:02:48 djm Exp $ */
      3 /*
      4  * Copyright (c) 2019 Google LLC
      5  *
      6  * Permission to use, copy, modify, and distribute this software for any
      7  * purpose with or without fee is hereby granted, provided that the above
      8  * copyright notice and this permission notice appear in all copies.
      9  *
     10  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
     11  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
     12  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
     13  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
     14  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
     15  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
     16  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
     17  */
     18 #include "includes.h"
     19 __RCSID("$NetBSD: ssh-sk-client.c,v 1.8 2025/04/09 15:49:33 christos Exp $");
     20 
     21 #include <sys/types.h>
     22 #include <sys/socket.h>
     23 #include <sys/wait.h>
     24 
     25 #include <fcntl.h>
     26 #include <limits.h>
     27 #include <errno.h>
     28 #include <signal.h>
     29 #include <stdarg.h>
     30 #include <stdio.h>
     31 #include <stdlib.h>
     32 #include <string.h>
     33 #include <unistd.h>
     34 
     35 #include "log.h"
     36 #include "ssherr.h"
     37 #include "sshbuf.h"
     38 #include "sshkey.h"
     39 #include "msg.h"
     40 #include "digest.h"
     41 #include "pathnames.h"
     42 #include "ssh-sk.h"
     43 #include "misc.h"
     44 
     45 /* #define DEBUG_SK 1 */
     46 
     47 static int
     48 start_helper(int *fdp, pid_t *pidp, void (**osigchldp)(int))
     49 {
     50 	void (*osigchld)(int);
     51 	int oerrno, pair[2];
     52 	pid_t pid;
     53 	const char *helper, *verbosity = NULL;
     54 
     55 	*fdp = -1;
     56 	*pidp = 0;
     57 	*osigchldp = SIG_DFL;
     58 
     59 	helper = getenv("SSH_SK_HELPER");
     60 	if (helper == NULL || strlen(helper) == 0)
     61 		helper = _PATH_SSH_SK_HELPER;
     62 	if (access(helper, X_OK) != 0) {
     63 		oerrno = errno;
     64 		error_f("helper \"%s\" unusable: %s", helper, strerror(errno));
     65 		errno = oerrno;
     66 		return SSH_ERR_SYSTEM_ERROR;
     67 	}
     68 #ifdef DEBUG_SK
     69 	verbosity = "-vvv";
     70 #endif
     71 
     72 	/* Start helper */
     73 	if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair) == -1) {
     74 		error("socketpair: %s", strerror(errno));
     75 		return SSH_ERR_SYSTEM_ERROR;
     76 	}
     77 	osigchld = ssh_signal(SIGCHLD, SIG_DFL);
     78 	if ((pid = fork()) == -1) {
     79 		oerrno = errno;
     80 		error("fork: %s", strerror(errno));
     81 		close(pair[0]);
     82 		close(pair[1]);
     83 		ssh_signal(SIGCHLD, osigchld);
     84 		errno = oerrno;
     85 		return SSH_ERR_SYSTEM_ERROR;
     86 	}
     87 	if (pid == 0) {
     88 		if ((dup2(pair[1], STDIN_FILENO) == -1) ||
     89 		    (dup2(pair[1], STDOUT_FILENO) == -1)) {
     90 			error_f("dup2: %s", strerror(errno));
     91 			_exit(1);
     92 		}
     93 		close(pair[0]);
     94 		close(pair[1]);
     95 		closefrom(STDERR_FILENO + 1);
     96 		debug_f("starting %s %s", helper,
     97 		    verbosity == NULL ? "" : verbosity);
     98 		execlp(helper, helper, verbosity, (char *)NULL);
     99 		error_f("execlp: %s", strerror(errno));
    100 		_exit(1);
    101 	}
    102 	close(pair[1]);
    103 
    104 	/* success */
    105 	debug3_f("started pid=%ld", (long)pid);
    106 	*fdp = pair[0];
    107 	*pidp = pid;
    108 	*osigchldp = osigchld;
    109 	return 0;
    110 }
    111 
    112 static int
    113 reap_helper(pid_t pid)
    114 {
    115 	int status, oerrno;
    116 
    117 	debug3_f("pid=%ld", (long)pid);
    118 
    119 	errno = 0;
    120 	while (waitpid(pid, &status, 0) == -1) {
    121 		if (errno == EINTR) {
    122 			errno = 0;
    123 			continue;
    124 		}
    125 		oerrno = errno;
    126 		error_f("waitpid: %s", strerror(errno));
    127 		errno = oerrno;
    128 		return SSH_ERR_SYSTEM_ERROR;
    129 	}
    130 	if (!WIFEXITED(status)) {
    131 		error_f("helper exited abnormally");
    132 		return SSH_ERR_AGENT_FAILURE;
    133 	} else if (WEXITSTATUS(status) != 0) {
    134 		error_f("helper exited with non-zero exit status");
    135 		return SSH_ERR_AGENT_FAILURE;
    136 	}
    137 	return 0;
    138 }
    139 
    140 static int
    141 client_converse(struct sshbuf *msg, struct sshbuf **respp, u_int type)
    142 {
    143 	int oerrno, fd, r2, ll, r = SSH_ERR_INTERNAL_ERROR;
    144 	u_int rtype, rerr;
    145 	pid_t pid;
    146 	u_char version;
    147 	void (*osigchld)(int);
    148 	struct sshbuf *req = NULL, *resp = NULL;
    149 	*respp = NULL;
    150 
    151 	if ((r = start_helper(&fd, &pid, &osigchld)) != 0)
    152 		return r;
    153 
    154 	if ((req = sshbuf_new()) == NULL || (resp = sshbuf_new()) == NULL) {
    155 		r = SSH_ERR_ALLOC_FAIL;
    156 		goto out;
    157 	}
    158 	/* Request preamble: type, log_on_stderr, log_level */
    159 	ll = log_level_get();
    160 	if ((r = sshbuf_put_u32(req, type)) != 0 ||
    161 	    (r = sshbuf_put_u8(req, log_is_on_stderr() != 0)) != 0 ||
    162 	    (r = sshbuf_put_u32(req, (uint32_t)(ll < 0 ? 0 : ll))) != 0 ||
    163 	    (r = sshbuf_putb(req, msg)) != 0) {
    164 		error_fr(r, "compose");
    165 		goto out;
    166 	}
    167 	if ((r = ssh_msg_send(fd, SSH_SK_HELPER_VERSION, req)) != 0) {
    168 		error_fr(r, "send");
    169 		goto out;
    170 	}
    171 	if ((r = ssh_msg_recv(fd, resp)) != 0) {
    172 		error_fr(r, "receive");
    173 		goto out;
    174 	}
    175 	if ((r = sshbuf_get_u8(resp, &version)) != 0) {
    176 		error_fr(r, "parse version");
    177 		goto out;
    178 	}
    179 	if (version != SSH_SK_HELPER_VERSION) {
    180 		error_f("unsupported version: got %u, expected %u",
    181 		    version, SSH_SK_HELPER_VERSION);
    182 		r = SSH_ERR_INVALID_FORMAT;
    183 		goto out;
    184 	}
    185 	if ((r = sshbuf_get_u32(resp, &rtype)) != 0) {
    186 		error_fr(r, "parse message type");
    187 		goto out;
    188 	}
    189 	if (rtype == SSH_SK_HELPER_ERROR) {
    190 		if ((r = sshbuf_get_u32(resp, &rerr)) != 0) {
    191 			error_fr(r, "parse");
    192 			goto out;
    193 		}
    194 		debug_f("helper returned error -%u", rerr);
    195 		/* OpenSSH error values are negative; encoded as -err on wire */
    196 		if (rerr == 0 || rerr >= INT_MAX)
    197 			r = SSH_ERR_INTERNAL_ERROR;
    198 		else
    199 			r = -(int)rerr;
    200 		goto out;
    201 	} else if (rtype != type) {
    202 		error_f("helper returned incorrect message type %u, "
    203 		    "expecting %u", rtype, type);
    204 		r = SSH_ERR_INTERNAL_ERROR;
    205 		goto out;
    206 	}
    207 	/* success */
    208 	r = 0;
    209  out:
    210 	oerrno = errno;
    211 	close(fd);
    212 	if ((r2 = reap_helper(pid)) != 0) {
    213 		if (r == 0) {
    214 			r = r2;
    215 			oerrno = errno;
    216 		}
    217 	}
    218 	if (r == 0) {
    219 		*respp = resp;
    220 		resp = NULL;
    221 	}
    222 	sshbuf_free(req);
    223 	sshbuf_free(resp);
    224 	ssh_signal(SIGCHLD, osigchld);
    225 	errno = oerrno;
    226 	return r;
    227 
    228 }
    229 
    230 int
    231 sshsk_sign(const char *provider, struct sshkey *key,
    232     u_char **sigp, size_t *lenp, const u_char *data, size_t datalen,
    233     u_int compat, const char *pin)
    234 {
    235 	int oerrno, r = SSH_ERR_INTERNAL_ERROR;
    236 	struct sshbuf *kbuf = NULL, *req = NULL, *resp = NULL;
    237 
    238 	*sigp = NULL;
    239 	*lenp = 0;
    240 
    241 	if ((kbuf = sshbuf_new()) == NULL ||
    242 	    (req = sshbuf_new()) == NULL) {
    243 		r = SSH_ERR_ALLOC_FAIL;
    244 		goto out;
    245 	}
    246 
    247 	if ((r = sshkey_private_serialize(key, kbuf)) != 0) {
    248 		error_fr(r, "encode key");
    249 		goto out;
    250 	}
    251 	if ((r = sshbuf_put_stringb(req, kbuf)) != 0 ||
    252 	    (r = sshbuf_put_cstring(req, provider)) != 0 ||
    253 	    (r = sshbuf_put_string(req, data, datalen)) != 0 ||
    254 	    (r = sshbuf_put_cstring(req, NULL)) != 0 || /* alg */
    255 	    (r = sshbuf_put_u32(req, compat)) != 0 ||
    256 	    (r = sshbuf_put_cstring(req, pin)) != 0) {
    257 		error_fr(r, "compose");
    258 		goto out;
    259 	}
    260 
    261 	if ((r = client_converse(req, &resp, SSH_SK_HELPER_SIGN)) != 0)
    262 		goto out;
    263 
    264 	if ((r = sshbuf_get_string(resp, sigp, lenp)) != 0) {
    265 		error_fr(r, "parse signature");
    266 		r = SSH_ERR_INVALID_FORMAT;
    267 		goto out;
    268 	}
    269 	if (sshbuf_len(resp) != 0) {
    270 		error_f("trailing data in response");
    271 		r = SSH_ERR_INVALID_FORMAT;
    272 		goto out;
    273 	}
    274 	/* success */
    275 	r = 0;
    276  out:
    277 	oerrno = errno;
    278 	if (r != 0) {
    279 		freezero(*sigp, *lenp);
    280 		*sigp = NULL;
    281 		*lenp = 0;
    282 	}
    283 	sshbuf_free(kbuf);
    284 	sshbuf_free(req);
    285 	sshbuf_free(resp);
    286 	errno = oerrno;
    287 	return r;
    288 }
    289 
    290 int
    291 sshsk_enroll(int type, const char *provider_path, const char *device,
    292     const char *application, const char *userid, uint8_t flags,
    293     const char *pin, struct sshbuf *challenge_buf,
    294     struct sshkey **keyp, struct sshbuf *attest)
    295 {
    296 	int oerrno, r = SSH_ERR_INTERNAL_ERROR;
    297 	struct sshbuf *kbuf = NULL, *abuf = NULL, *req = NULL, *resp = NULL;
    298 	struct sshkey *key = NULL;
    299 
    300 	*keyp = NULL;
    301 	if (attest != NULL)
    302 		sshbuf_reset(attest);
    303 
    304 	if (type < 0)
    305 		return SSH_ERR_INVALID_ARGUMENT;
    306 
    307 	if ((abuf = sshbuf_new()) == NULL ||
    308 	    (kbuf = sshbuf_new()) == NULL ||
    309 	    (req = sshbuf_new()) == NULL) {
    310 		r = SSH_ERR_ALLOC_FAIL;
    311 		goto out;
    312 	}
    313 
    314 	if ((r = sshbuf_put_u32(req, (u_int)type)) != 0 ||
    315 	    (r = sshbuf_put_cstring(req, provider_path)) != 0 ||
    316 	    (r = sshbuf_put_cstring(req, device)) != 0 ||
    317 	    (r = sshbuf_put_cstring(req, application)) != 0 ||
    318 	    (r = sshbuf_put_cstring(req, userid)) != 0 ||
    319 	    (r = sshbuf_put_u8(req, flags)) != 0 ||
    320 	    (r = sshbuf_put_cstring(req, pin)) != 0 ||
    321 	    (r = sshbuf_put_stringb(req, challenge_buf)) != 0) {
    322 		error_fr(r, "compose");
    323 		goto out;
    324 	}
    325 
    326 	if ((r = client_converse(req, &resp, SSH_SK_HELPER_ENROLL)) != 0)
    327 		goto out;
    328 
    329 	if ((r = sshbuf_get_stringb(resp, kbuf)) != 0 ||
    330 	    (r = sshbuf_get_stringb(resp, abuf)) != 0) {
    331 		error_fr(r, "parse");
    332 		r = SSH_ERR_INVALID_FORMAT;
    333 		goto out;
    334 	}
    335 	if (sshbuf_len(resp) != 0) {
    336 		error_f("trailing data in response");
    337 		r = SSH_ERR_INVALID_FORMAT;
    338 		goto out;
    339 	}
    340 	if ((r = sshkey_private_deserialize(kbuf, &key)) != 0) {
    341 		error_fr(r, "encode");
    342 		goto out;
    343 	}
    344 	if (attest != NULL && (r = sshbuf_putb(attest, abuf)) != 0) {
    345 		error_fr(r, "encode attestation information");
    346 		goto out;
    347 	}
    348 
    349 	/* success */
    350 	r = 0;
    351 	*keyp = key;
    352 	key = NULL;
    353  out:
    354 	oerrno = errno;
    355 	sshkey_free(key);
    356 	sshbuf_free(kbuf);
    357 	sshbuf_free(abuf);
    358 	sshbuf_free(req);
    359 	sshbuf_free(resp);
    360 	errno = oerrno;
    361 	return r;
    362 }
    363 
    364 static void
    365 sshsk_free_resident_key(struct sshsk_resident_key *srk)
    366 {
    367 	if (srk == NULL)
    368 		return;
    369 	sshkey_free(srk->key);
    370 	freezero(srk->user_id, srk->user_id_len);
    371 	free(srk);
    372 }
    373 
    374 
    375 void
    376 sshsk_free_resident_keys(struct sshsk_resident_key **srks, size_t nsrks)
    377 {
    378 	size_t i;
    379 
    380 	if (srks == NULL || nsrks == 0)
    381 		return;
    382 
    383 	for (i = 0; i < nsrks; i++)
    384 		sshsk_free_resident_key(srks[i]);
    385 	free(srks);
    386 }
    387 
    388 int
    389 sshsk_load_resident(const char *provider_path, const char *device,
    390     const char *pin, u_int flags, struct sshsk_resident_key ***srksp,
    391     size_t *nsrksp)
    392 {
    393 	int oerrno, r = SSH_ERR_INTERNAL_ERROR;
    394 	struct sshbuf *kbuf = NULL, *req = NULL, *resp = NULL;
    395 	struct sshkey *key = NULL;
    396 	struct sshsk_resident_key *srk = NULL, **srks = NULL, **tmp;
    397 	u_char *userid = NULL;
    398 	size_t userid_len = 0, nsrks = 0;
    399 
    400 	*srksp = NULL;
    401 	*nsrksp = 0;
    402 
    403 	if ((kbuf = sshbuf_new()) == NULL ||
    404 	    (req = sshbuf_new()) == NULL) {
    405 		r = SSH_ERR_ALLOC_FAIL;
    406 		goto out;
    407 	}
    408 
    409 	if ((r = sshbuf_put_cstring(req, provider_path)) != 0 ||
    410 	    (r = sshbuf_put_cstring(req, device)) != 0 ||
    411 	    (r = sshbuf_put_cstring(req, pin)) != 0 ||
    412 	    (r = sshbuf_put_u32(req, flags)) != 0) {
    413 		error_fr(r, "compose");
    414 		goto out;
    415 	}
    416 
    417 	if ((r = client_converse(req, &resp, SSH_SK_HELPER_LOAD_RESIDENT)) != 0)
    418 		goto out;
    419 
    420 	while (sshbuf_len(resp) != 0) {
    421 		/* key, comment, user_id */
    422 		if ((r = sshbuf_get_stringb(resp, kbuf)) != 0 ||
    423 		    (r = sshbuf_get_cstring(resp, NULL, NULL)) != 0 ||
    424 		    (r = sshbuf_get_string(resp, &userid, &userid_len)) != 0) {
    425 			error_fr(r, "parse");
    426 			r = SSH_ERR_INVALID_FORMAT;
    427 			goto out;
    428 		}
    429 		if ((r = sshkey_private_deserialize(kbuf, &key)) != 0) {
    430 			error_fr(r, "decode key");
    431 			goto out;
    432 		}
    433 		if ((srk = calloc(1, sizeof(*srk))) == NULL) {
    434 			error_f("calloc failed");
    435 			r = SSH_ERR_ALLOC_FAIL;
    436 			goto out;
    437 		}
    438 		srk->key = key;
    439 		key = NULL;
    440 		srk->user_id = userid;
    441 		srk->user_id_len = userid_len;
    442 		userid = NULL;
    443 		userid_len = 0;
    444 		if ((tmp = recallocarray(srks, nsrks, nsrks + 1,
    445 		    sizeof(*srks))) == NULL) {
    446 			error_f("recallocarray keys failed");
    447 			r = SSH_ERR_ALLOC_FAIL;
    448 			goto out;
    449 		}
    450 		debug_f("srks[%zu]: %s %s uidlen %zu", nsrks,
    451 		    sshkey_type(srk->key), srk->key->sk_application,
    452 		    srk->user_id_len);
    453 		srks = tmp;
    454 		srks[nsrks++] = srk;
    455 		srk = NULL;
    456 	}
    457 
    458 	/* success */
    459 	r = 0;
    460 	*srksp = srks;
    461 	*nsrksp = nsrks;
    462 	srks = NULL;
    463 	nsrks = 0;
    464  out:
    465 	oerrno = errno;
    466 	sshsk_free_resident_key(srk);
    467 	sshsk_free_resident_keys(srks, nsrks);
    468 	freezero(userid, userid_len);
    469 	sshkey_free(key);
    470 	sshbuf_free(kbuf);
    471 	sshbuf_free(req);
    472 	sshbuf_free(resp);
    473 	errno = oerrno;
    474 	return r;
    475 }
    476