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When downloading resident keys from a FIDO token, pass back the
user ID that was used when the key was created and append it to
the filename the key is written to (if it is not the default).

Avoids keys being clobbered if the user created multiple
resident keys with the same application string but different
user IDs.

feedback Pedro Martelletto; ok markus

NB. increments SSH_SK_VERSION_MAJOR

/* $OpenBSD: ssh-sk-helper.c,v 1.12 2021/10/28 02:54:18 djm Exp $ */
/*
 * Copyright (c) 2019 Google LLC
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

/*
 * This is a tiny program used to isolate the address space used for
 * security key middleware signing operations from ssh-agent. It is similar
 * to ssh-pkcs11-helper.c but considerably simpler as the operations for
 * security keys are stateless.
 *
 * Please crank SSH_SK_HELPER_VERSION in sshkey.h for any incompatible
 * protocol changes.
 */

#include <limits.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#include "xmalloc.h"
#include "log.h"
#include "sshkey.h"
#include "authfd.h"
#include "misc.h"
#include "sshbuf.h"
#include "msg.h"
#include "uidswap.h"
#include "sshkey.h"
#include "ssherr.h"
#include "ssh-sk.h"

extern char *__progname;

static struct sshbuf *reply_error(int r, char *fmt, ...)
    __attribute__((__format__ (printf, 2, 3)));

static struct sshbuf *
reply_error(int r, char *fmt, ...)
{
	char *msg;
	va_list ap;
	struct sshbuf *resp;

	va_start(ap, fmt);
	xvasprintf(&msg, fmt, ap);
	va_end(ap);
	debug("%s: %s", __progname, msg);
	free(msg);

	if (r >= 0)
		fatal_f("invalid error code %d", r);

	if ((resp = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);
	if (sshbuf_put_u32(resp, SSH_SK_HELPER_ERROR) != 0 ||
	    sshbuf_put_u32(resp, (u_int)-r) != 0)
		fatal("%s: buffer error", __progname);
	return resp;
}

/* If the specified string is zero length, then free it and replace with NULL */
static void
null_empty(char **s)
{
	if (s == NULL || *s == NULL || **s != '\0')
		return;

	free(*s);
	*s = NULL;
}

static struct sshbuf *
process_sign(struct sshbuf *req)
{
	int r = SSH_ERR_INTERNAL_ERROR;
	struct sshbuf *resp, *kbuf;
	struct sshkey *key = NULL;
	uint32_t compat;
	const u_char *message;
	u_char *sig = NULL;
	size_t msglen, siglen = 0;
	char *provider = NULL, *pin = NULL;

	if ((r = sshbuf_froms(req, &kbuf)) != 0 ||
	    (r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
	    (r = sshbuf_get_string_direct(req, &message, &msglen)) != 0 ||
	    (r = sshbuf_get_cstring(req, NULL, NULL)) != 0 || /* alg */
	    (r = sshbuf_get_u32(req, &compat)) != 0 ||
	    (r = sshbuf_get_cstring(req, &pin, NULL)) != 0)
		fatal_r(r, "%s: parse", __progname);
	if (sshbuf_len(req) != 0)
		fatal("%s: trailing data in request", __progname);

	if ((r = sshkey_private_deserialize(kbuf, &key)) != 0)
		fatal_r(r, "%s: Unable to parse private key", __progname);
	if (!sshkey_is_sk(key)) {
		fatal("%s: Unsupported key type %s",
		    __progname, sshkey_ssh_name(key));
	}

	debug_f("ready to sign with key %s, provider %s: "
	    "msg len %zu, compat 0x%lx", sshkey_type(key),
	    provider, msglen, (u_long)compat);

	null_empty(&pin);

	if ((r = sshsk_sign(provider, key, &sig, &siglen,
	    message, msglen, compat, pin)) != 0) {
		resp = reply_error(r, "Signing failed: %s", ssh_err(r));
		goto out;
	}

	if ((resp = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);

	if ((r = sshbuf_put_u32(resp, SSH_SK_HELPER_SIGN)) != 0 ||
	    (r = sshbuf_put_string(resp, sig, siglen)) != 0)
		fatal_r(r, "%s: compose", __progname);
 out:
	sshkey_free(key);
	sshbuf_free(kbuf);
	free(provider);
	if (sig != NULL)
		freezero(sig, siglen);
	if (pin != NULL)
		freezero(pin, strlen(pin));
	return resp;
}

static struct sshbuf *
process_enroll(struct sshbuf *req)
{
	int r;
	u_int type;
	char *provider, *application, *pin, *device, *userid;
	uint8_t flags;
	struct sshbuf *challenge, *attest, *kbuf, *resp;
	struct sshkey *key;

	if ((attest = sshbuf_new()) == NULL ||
	    (kbuf = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);

	if ((r = sshbuf_get_u32(req, &type)) != 0 ||
	    (r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
	    (r = sshbuf_get_cstring(req, &device, NULL)) != 0 ||
	    (r = sshbuf_get_cstring(req, &application, NULL)) != 0 ||
	    (r = sshbuf_get_cstring(req, &userid, NULL)) != 0 ||
	    (r = sshbuf_get_u8(req, &flags)) != 0 ||
	    (r = sshbuf_get_cstring(req, &pin, NULL)) != 0 ||
	    (r = sshbuf_froms(req, &challenge)) != 0)
		fatal_r(r, "%s: parse", __progname);
	if (sshbuf_len(req) != 0)
		fatal("%s: trailing data in request", __progname);

	if (type > INT_MAX)
		fatal("%s: bad type %u", __progname, type);
	if (sshbuf_len(challenge) == 0) {
		sshbuf_free(challenge);
		challenge = NULL;
	}
	null_empty(&device);
	null_empty(&userid);
	null_empty(&pin);

	if ((r = sshsk_enroll((int)type, provider, device, application, userid,
	    flags, pin, challenge, &key, attest)) != 0) {
		resp = reply_error(r, "Enrollment failed: %s", ssh_err(r));
		goto out;
	}

	if ((resp = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);
	if ((r = sshkey_private_serialize(key, kbuf)) != 0)
		fatal_r(r, "%s: encode key", __progname);
	if ((r = sshbuf_put_u32(resp, SSH_SK_HELPER_ENROLL)) != 0 ||
	    (r = sshbuf_put_stringb(resp, kbuf)) != 0 ||
	    (r = sshbuf_put_stringb(resp, attest)) != 0)
		fatal_r(r, "%s: compose", __progname);

 out:
	sshkey_free(key);
	sshbuf_free(kbuf);
	sshbuf_free(attest);
	sshbuf_free(challenge);
	free(provider);
	free(application);
	if (pin != NULL)
		freezero(pin, strlen(pin));

	return resp;
}

static struct sshbuf *
process_load_resident(struct sshbuf *req)
{
	int r;
	char *provider, *pin, *device;
	struct sshbuf *kbuf, *resp;
	struct sshsk_resident_key **srks = NULL;
	size_t nsrks = 0, i;
	u_int flags;

	if ((kbuf = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);

	if ((r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
	    (r = sshbuf_get_cstring(req, &device, NULL)) != 0 ||
	    (r = sshbuf_get_cstring(req, &pin, NULL)) != 0 ||
	    (r = sshbuf_get_u32(req, &flags)) != 0)
		fatal_r(r, "%s: parse", __progname);
	if (sshbuf_len(req) != 0)
		fatal("%s: trailing data in request", __progname);

	null_empty(&device);
	null_empty(&pin);

	if ((r = sshsk_load_resident(provider, device, pin, flags,
	    &srks, &nsrks)) != 0) {
		resp = reply_error(r, "sshsk_load_resident failed: %s",
		    ssh_err(r));
		goto out;
	}

	if ((resp = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);

	if ((r = sshbuf_put_u32(resp, SSH_SK_HELPER_LOAD_RESIDENT)) != 0)
		fatal_r(r, "%s: compose", __progname);

	for (i = 0; i < nsrks; i++) {
		debug_f("key %zu %s %s uidlen %zu", i,
		    sshkey_type(srks[i]->key), srks[i]->key->sk_application,
		    srks[i]->user_id_len);
		sshbuf_reset(kbuf);
		if ((r = sshkey_private_serialize(srks[i]->key, kbuf)) != 0)
			fatal_r(r, "%s: encode key", __progname);
		if ((r = sshbuf_put_stringb(resp, kbuf)) != 0 ||
		    (r = sshbuf_put_cstring(resp, "")) != 0 || /* comment */
		    (r = sshbuf_put_string(resp, srks[i]->user_id,
		    srks[i]->user_id_len)) != 0)
			fatal_r(r, "%s: compose key", __progname);
	}

 out:
	sshsk_free_resident_keys(srks, nsrks);
	sshbuf_free(kbuf);
	free(provider);
	if (pin != NULL)
		freezero(pin, strlen(pin));
	return resp;
}

int
main(int argc, char **argv)
{
	SyslogFacility log_facility = SYSLOG_FACILITY_AUTH;
	LogLevel log_level = SYSLOG_LEVEL_ERROR;
	struct sshbuf *req, *resp;
	int in, out, ch, r, vflag = 0;
	u_int rtype, ll = 0;
	uint8_t version, log_stderr = 0;

	sanitise_stdfd();
	log_init(__progname, log_level, log_facility, log_stderr);

	while ((ch = getopt(argc, argv, "v")) != -1) {
		switch (ch) {
		case 'v':
			vflag = 1;
			if (log_level == SYSLOG_LEVEL_ERROR)
				log_level = SYSLOG_LEVEL_DEBUG1;
			else if (log_level < SYSLOG_LEVEL_DEBUG3)
				log_level++;
			break;
		default:
			fprintf(stderr, "usage: %s [-v]\n", __progname);
			exit(1);
		}
	}
	log_init(__progname, log_level, log_facility, vflag);

	/*
	 * Rearrange our file descriptors a little; we don't trust the
	 * providers not to fiddle with stdin/out.
	 */
	closefrom(STDERR_FILENO + 1);
	if ((in = dup(STDIN_FILENO)) == -1 || (out = dup(STDOUT_FILENO)) == -1)
		fatal("%s: dup: %s", __progname, strerror(errno));
	close(STDIN_FILENO);
	close(STDOUT_FILENO);
	sanitise_stdfd(); /* resets to /dev/null */

	if ((req = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __progname);
	if (ssh_msg_recv(in, req) < 0)
		fatal("ssh_msg_recv failed");
	close(in);
	debug_f("received message len %zu", sshbuf_len(req));

	if ((r = sshbuf_get_u8(req, &version)) != 0)
		fatal_r(r, "%s: parse version", __progname);
	if (version != SSH_SK_HELPER_VERSION) {
		fatal("unsupported version: received %d, expected %d",
		    version, SSH_SK_HELPER_VERSION);
	}

	if ((r = sshbuf_get_u32(req, &rtype)) != 0 ||
	    (r = sshbuf_get_u8(req, &log_stderr)) != 0 ||
	    (r = sshbuf_get_u32(req, &ll)) != 0)
		fatal_r(r, "%s: parse", __progname);

	if (!vflag && log_level_name((LogLevel)ll) != NULL)
		log_init(__progname, (LogLevel)ll, log_facility, log_stderr);

	switch (rtype) {
	case SSH_SK_HELPER_SIGN:
		resp = process_sign(req);
		break;
	case SSH_SK_HELPER_ENROLL:
		resp = process_enroll(req);
		break;
	case SSH_SK_HELPER_LOAD_RESIDENT:
		resp = process_load_resident(req);
		break;
	default:
		fatal("%s: unsupported request type %u", __progname, rtype);
	}
	sshbuf_free(req);
	debug_f("reply len %zu", sshbuf_len(resp));

	if (ssh_msg_send(out, SSH_SK_HELPER_VERSION, resp) == -1)
		fatal("ssh_msg_send failed");
	sshbuf_free(resp);
	close(out);

	return (0);
}