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Make it possible to load certs from PKCS#11 tokens

Adds a protocol extension to allow grafting certificates supplied by
ssh-add to keys loaded from PKCS#11 tokens in the agent.

feedback/ok markus@

/* $OpenBSD: ssh-pkcs11-client.c,v 1.19 2023/12/18 14:46:56 djm Exp $ */
/*
 * Copyright (c) 2010 Markus Friedl.  All rights reserved.
 * Copyright (c) 2014 Pedro Martelletto. All rights reserved.
 *
 * 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.
 */

#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>

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

#include <openssl/ecdsa.h>
#include <openssl/rsa.h>

#include "pathnames.h"
#include "xmalloc.h"
#include "sshbuf.h"
#include "log.h"
#include "misc.h"
#include "sshkey.h"
#include "authfd.h"
#include "atomicio.h"
#include "ssh-pkcs11.h"
#include "ssherr.h"

/* borrows code from sftp-server and ssh-agent */

/*
 * Maintain a list of ssh-pkcs11-helper subprocesses. These may be looked up
 * by provider path or their unique EC/RSA METHOD pointers.
 */
struct helper {
	char *path;
	pid_t pid;
	int fd;
	RSA_METHOD *rsa_meth;
	EC_KEY_METHOD *ec_meth;
	int (*rsa_finish)(RSA *rsa);
	void (*ec_finish)(EC_KEY *key);
	size_t nrsa, nec; /* number of active keys of each type */
};
static struct helper **helpers;
static size_t nhelpers;

static struct helper *
helper_by_provider(const char *path)
{
	size_t i;

	for (i = 0; i < nhelpers; i++) {
		if (helpers[i] == NULL || helpers[i]->path == NULL ||
		    helpers[i]->fd == -1)
			continue;
		if (strcmp(helpers[i]->path, path) == 0)
			return helpers[i];
	}
	return NULL;
}

static struct helper *
helper_by_rsa(const RSA *rsa)
{
	size_t i;
	const RSA_METHOD *meth;

	if ((meth = RSA_get_method(rsa)) == NULL)
		return NULL;
	for (i = 0; i < nhelpers; i++) {
		if (helpers[i] != NULL && helpers[i]->rsa_meth == meth)
			return helpers[i];
	}
	return NULL;

}

static struct helper *
helper_by_ec(const EC_KEY *ec)
{
	size_t i;
	const EC_KEY_METHOD *meth;

	if ((meth = EC_KEY_get_method(ec)) == NULL)
		return NULL;
	for (i = 0; i < nhelpers; i++) {
		if (helpers[i] != NULL && helpers[i]->ec_meth == meth)
			return helpers[i];
	}
	return NULL;

}

static void
helper_free(struct helper *helper)
{
	size_t i;
	int found = 0;

	if (helper == NULL)
		return;
	if (helper->path == NULL || helper->ec_meth == NULL ||
	    helper->rsa_meth == NULL)
		fatal_f("inconsistent helper");
	debug3_f("free helper for provider %s", helper->path);
	for (i = 0; i < nhelpers; i++) {
		if (helpers[i] == helper) {
			if (found)
				fatal_f("helper recorded more than once");
			found = 1;
		}
		else if (found)
			helpers[i - 1] = helpers[i];
	}
	if (found) {
		helpers = xrecallocarray(helpers, nhelpers,
		    nhelpers - 1, sizeof(*helpers));
		nhelpers--;
	}
	free(helper->path);
	EC_KEY_METHOD_free(helper->ec_meth);
	RSA_meth_free(helper->rsa_meth);
	free(helper);
}

static void
helper_terminate(struct helper *helper)
{
	if (helper == NULL) {
		return;
	} else if (helper->fd == -1) {
		debug3_f("already terminated");
	} else {
		debug3_f("terminating helper for %s; "
		    "remaining %zu RSA %zu ECDSA",
		    helper->path, helper->nrsa, helper->nec);
		close(helper->fd);
		/* XXX waitpid() */
		helper->fd = -1;
		helper->pid = -1;
	}
	/*
	 * Don't delete the helper entry until there are no remaining keys
	 * that reference it. Otherwise, any signing operation would call
	 * a free'd METHOD pointer and that would be bad.
	 */
	if (helper->nrsa == 0 && helper->nec == 0)
		helper_free(helper);
}

static void
send_msg(int fd, struct sshbuf *m)
{
	u_char buf[4];
	size_t mlen = sshbuf_len(m);
	int r;

	if (fd == -1)
		return;
	POKE_U32(buf, mlen);
	if (atomicio(vwrite, fd, buf, 4) != 4 ||
	    atomicio(vwrite, fd, sshbuf_mutable_ptr(m),
	    sshbuf_len(m)) != sshbuf_len(m))
		error("write to helper failed");
	if ((r = sshbuf_consume(m, mlen)) != 0)
		fatal_fr(r, "consume");
}

static int
recv_msg(int fd, struct sshbuf *m)
{
	u_int l, len;
	u_char c, buf[1024];
	int r;

	sshbuf_reset(m);
	if (fd == -1)
		return 0; /* XXX */
	if ((len = atomicio(read, fd, buf, 4)) != 4) {
		error("read from helper failed: %u", len);
		return (0); /* XXX */
	}
	len = PEEK_U32(buf);
	if (len > 256 * 1024)
		fatal("response too long: %u", len);
	/* read len bytes into m */
	while (len > 0) {
		l = len;
		if (l > sizeof(buf))
			l = sizeof(buf);
		if (atomicio(read, fd, buf, l) != l) {
			error("response from helper failed.");
			return (0); /* XXX */
		}
		if ((r = sshbuf_put(m, buf, l)) != 0)
			fatal_fr(r, "sshbuf_put");
		len -= l;
	}
	if ((r = sshbuf_get_u8(m, &c)) != 0)
		fatal_fr(r, "parse type");
	return c;
}

int
pkcs11_init(int interactive)
{
	return 0;
}

void
pkcs11_terminate(void)
{
	size_t i;

	debug3_f("terminating %zu helpers", nhelpers);
	for (i = 0; i < nhelpers; i++)
		helper_terminate(helpers[i]);
}

static int
rsa_encrypt(int flen, const u_char *from, u_char *to, RSA *rsa, int padding)
{
	struct sshkey *key = NULL;
	struct sshbuf *msg = NULL;
	u_char *blob = NULL, *signature = NULL;
	size_t blen, slen = 0;
	int r, ret = -1;
	struct helper *helper;

	if ((helper = helper_by_rsa(rsa)) == NULL || helper->fd == -1)
		fatal_f("no helper for PKCS11 key");
	debug3_f("signing with PKCS11 provider %s", helper->path);
	if (padding != RSA_PKCS1_PADDING)
		goto fail;
	key = sshkey_new(KEY_UNSPEC);
	if (key == NULL) {
		error_f("sshkey_new failed");
		goto fail;
	}
	key->type = KEY_RSA;
	RSA_up_ref(rsa);
	key->rsa = rsa;
	if ((r = sshkey_to_blob(key, &blob, &blen)) != 0) {
		error_fr(r, "encode key");
		goto fail;
	}
	if ((msg = sshbuf_new()) == NULL)
		fatal_f("sshbuf_new failed");
	if ((r = sshbuf_put_u8(msg, SSH2_AGENTC_SIGN_REQUEST)) != 0 ||
	    (r = sshbuf_put_string(msg, blob, blen)) != 0 ||
	    (r = sshbuf_put_string(msg, from, flen)) != 0 ||
	    (r = sshbuf_put_u32(msg, 0)) != 0)
		fatal_fr(r, "compose");
	send_msg(helper->fd, msg);
	sshbuf_reset(msg);

	if (recv_msg(helper->fd, msg) == SSH2_AGENT_SIGN_RESPONSE) {
		if ((r = sshbuf_get_string(msg, &signature, &slen)) != 0)
			fatal_fr(r, "parse");
		if (slen <= (size_t)RSA_size(rsa)) {
			memcpy(to, signature, slen);
			ret = slen;
		}
		free(signature);
	}
 fail:
	free(blob);
	sshkey_free(key);
	sshbuf_free(msg);
	return (ret);
}

static int
rsa_finish(RSA *rsa)
{
	struct helper *helper;

	if ((helper = helper_by_rsa(rsa)) == NULL)
		fatal_f("no helper for PKCS11 key");
	debug3_f("free PKCS11 RSA key for provider %s", helper->path);
	if (helper->rsa_finish != NULL)
		helper->rsa_finish(rsa);
	if (helper->nrsa == 0)
		fatal_f("RSA refcount error");
	helper->nrsa--;
	debug3_f("provider %s remaining keys: %zu RSA %zu ECDSA",
	    helper->path, helper->nrsa, helper->nec);
	if (helper->nrsa == 0 && helper->nec == 0)
		helper_terminate(helper);
	return 1;
}

static ECDSA_SIG *
ecdsa_do_sign(const unsigned char *dgst, int dgst_len, const BIGNUM *inv,
    const BIGNUM *rp, EC_KEY *ec)
{
	struct sshkey *key = NULL;
	struct sshbuf *msg = NULL;
	ECDSA_SIG *ret = NULL;
	const u_char *cp;
	u_char *blob = NULL, *signature = NULL;
	size_t blen, slen = 0;
	int r, nid;
	struct helper *helper;

	if ((helper = helper_by_ec(ec)) == NULL || helper->fd == -1)
		fatal_f("no helper for PKCS11 key");
	debug3_f("signing with PKCS11 provider %s", helper->path);
	nid = sshkey_ecdsa_key_to_nid(ec);
	if (nid < 0) {
		error_f("couldn't get curve nid");
		goto fail;
	}

	key = sshkey_new(KEY_UNSPEC);
	if (key == NULL) {
		error_f("sshkey_new failed");
		goto fail;
	}
	key->ecdsa = ec;
	key->ecdsa_nid = nid;
	key->type = KEY_ECDSA;
	EC_KEY_up_ref(ec);

	if ((r = sshkey_to_blob(key, &blob, &blen)) != 0) {
		error_fr(r, "encode key");
		goto fail;
	}
	if ((msg = sshbuf_new()) == NULL)
		fatal_f("sshbuf_new failed");
	if ((r = sshbuf_put_u8(msg, SSH2_AGENTC_SIGN_REQUEST)) != 0 ||
	    (r = sshbuf_put_string(msg, blob, blen)) != 0 ||
	    (r = sshbuf_put_string(msg, dgst, dgst_len)) != 0 ||
	    (r = sshbuf_put_u32(msg, 0)) != 0)
		fatal_fr(r, "compose");
	send_msg(helper->fd, msg);
	sshbuf_reset(msg);

	if (recv_msg(helper->fd, msg) == SSH2_AGENT_SIGN_RESPONSE) {
		if ((r = sshbuf_get_string(msg, &signature, &slen)) != 0)
			fatal_fr(r, "parse");
		cp = signature;
		ret = d2i_ECDSA_SIG(NULL, &cp, slen);
		free(signature);
	}

 fail:
	free(blob);
	sshkey_free(key);
	sshbuf_free(msg);
	return (ret);
}

static void
ecdsa_do_finish(EC_KEY *ec)
{
	struct helper *helper;

	if ((helper = helper_by_ec(ec)) == NULL)
		fatal_f("no helper for PKCS11 key");
	debug3_f("free PKCS11 ECDSA key for provider %s", helper->path);
	if (helper->ec_finish != NULL)
		helper->ec_finish(ec);
	if (helper->nec == 0)
		fatal_f("ECDSA refcount error");
	helper->nec--;
	debug3_f("provider %s remaining keys: %zu RSA %zu ECDSA",
	    helper->path, helper->nrsa, helper->nec);
	if (helper->nrsa == 0 && helper->nec == 0)
		helper_terminate(helper);
}

/* redirect private key crypto operations to the ssh-pkcs11-helper */
static void
wrap_key(struct helper *helper, struct sshkey *k)
{
	debug3_f("wrap %s for provider %s", sshkey_type(k), helper->path);
	if (k->type == KEY_RSA) {
		RSA_set_method(k->rsa, helper->rsa_meth);
		if (helper->nrsa++ >= INT_MAX)
			fatal_f("RSA refcount error");
	} else if (k->type == KEY_ECDSA) {
		EC_KEY_set_method(k->ecdsa, helper->ec_meth);
		if (helper->nec++ >= INT_MAX)
			fatal_f("EC refcount error");
	} else
		fatal_f("unknown key type");
	k->flags |= SSHKEY_FLAG_EXT;
	debug3_f("provider %s remaining keys: %zu RSA %zu ECDSA",
	    helper->path, helper->nrsa, helper->nec);
}

/*
 * Make a private PKCS#11-backed certificate by grafting a previously-loaded
 * PKCS#11 private key and a public certificate key.
 */
int
pkcs11_make_cert(const struct sshkey *priv,
    const struct sshkey *certpub, struct sshkey **certprivp)
{
	struct helper *helper = NULL;
	struct sshkey *ret;
	int r;

	debug3_f("private key type %s cert type %s", sshkey_type(priv),
	    sshkey_type(certpub));
	*certprivp = NULL;
	if (!sshkey_is_cert(certpub) || sshkey_is_cert(priv) ||
	    !sshkey_equal_public(priv, certpub)) {
		error_f("private key %s doesn't match cert %s",
		    sshkey_type(priv), sshkey_type(certpub));
		return SSH_ERR_INVALID_ARGUMENT;
	}
	*certprivp = NULL;
	if (priv->type == KEY_RSA) {
		if ((helper = helper_by_rsa(priv->rsa)) == NULL ||
		    helper->fd == -1)
			fatal_f("no helper for PKCS11 RSA key");
		if ((r = sshkey_from_private(priv, &ret)) != 0)
			fatal_fr(r, "copy key");
		RSA_set_method(ret->rsa, helper->rsa_meth);
		if (helper->nrsa++ >= INT_MAX)
			fatal_f("RSA refcount error");
	} else if (priv->type == KEY_ECDSA) {
		if ((helper = helper_by_ec(priv->ecdsa)) == NULL ||
		    helper->fd == -1)
			fatal_f("no helper for PKCS11 EC key");
		if ((r = sshkey_from_private(priv, &ret)) != 0)
			fatal_fr(r, "copy key");
		EC_KEY_set_method(ret->ecdsa, helper->ec_meth);
		if (helper->nec++ >= INT_MAX)
			fatal_f("EC refcount error");
	} else
		fatal_f("unknown key type %s", sshkey_type(priv));

	ret->flags |= SSHKEY_FLAG_EXT;
	if ((r = sshkey_to_certified(ret)) != 0 ||
	    (r = sshkey_cert_copy(certpub, ret)) != 0)
		fatal_fr(r, "graft certificate");
	debug3_f("provider %s remaining keys: %zu RSA %zu ECDSA",
	    helper->path, helper->nrsa, helper->nec);
	/* success */
	*certprivp = ret;
	return 0;
}

static int
pkcs11_start_helper_methods(struct helper *helper)
{
	int (*ec_init)(EC_KEY *key);
	int (*ec_copy)(EC_KEY *dest, const EC_KEY *src);
	int (*ec_set_group)(EC_KEY *key, const EC_GROUP *grp);
	int (*ec_set_private)(EC_KEY *key, const BIGNUM *priv_key);
	int (*ec_set_public)(EC_KEY *key, const EC_POINT *pub_key);
	int (*ec_sign)(int, const unsigned char *, int, unsigned char *,
	    unsigned int *, const BIGNUM *, const BIGNUM *, EC_KEY *) = NULL;
	RSA_METHOD *rsa_meth;
	EC_KEY_METHOD *ec_meth;

	if ((ec_meth = EC_KEY_METHOD_new(EC_KEY_OpenSSL())) == NULL)
		return -1;
	EC_KEY_METHOD_get_sign(ec_meth, &ec_sign, NULL, NULL);
	EC_KEY_METHOD_set_sign(ec_meth, ec_sign, NULL, ecdsa_do_sign);
	EC_KEY_METHOD_get_init(ec_meth, &ec_init, &helper->ec_finish,
	    &ec_copy, &ec_set_group, &ec_set_private, &ec_set_public);
	EC_KEY_METHOD_set_init(ec_meth, ec_init, ecdsa_do_finish,
	    ec_copy, ec_set_group, ec_set_private, ec_set_public);

	if ((rsa_meth = RSA_meth_dup(RSA_get_default_method())) == NULL)
		fatal_f("RSA_meth_dup failed");
	helper->rsa_finish = RSA_meth_get_finish(rsa_meth);
	if (!RSA_meth_set1_name(rsa_meth, "ssh-pkcs11-helper") ||
	    !RSA_meth_set_priv_enc(rsa_meth, rsa_encrypt) ||
	    !RSA_meth_set_finish(rsa_meth, rsa_finish))
		fatal_f("failed to prepare method");

	helper->ec_meth = ec_meth;
	helper->rsa_meth = rsa_meth;
	return 0;
}

static struct helper *
pkcs11_start_helper(const char *path)
{
	int pair[2];
	char *prog, *verbosity = NULL;
	struct helper *helper;
	pid_t pid;

	if (nhelpers >= INT_MAX)
		fatal_f("too many helpers");
	debug3_f("start helper for %s", path);
	if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair) == -1) {
		error_f("socketpair: %s", strerror(errno));
		return NULL;
	}
	helper = xcalloc(1, sizeof(*helper));
	if (pkcs11_start_helper_methods(helper) == -1) {
		error_f("pkcs11_start_helper_methods failed");
		goto fail;
	}
	if ((pid = fork()) == -1) {
		error_f("fork: %s", strerror(errno));
 fail:
		close(pair[0]);
		close(pair[1]);
		RSA_meth_free(helper->rsa_meth);
		EC_KEY_METHOD_free(helper->ec_meth);
		free(helper);
		return NULL;
	} else if (pid == 0) {
		if ((dup2(pair[1], STDIN_FILENO) == -1) ||
		    (dup2(pair[1], STDOUT_FILENO) == -1)) {
			fprintf(stderr, "dup2: %s\n", strerror(errno));
			_exit(1);
		}
		close(pair[0]);
		close(pair[1]);
		prog = getenv("SSH_PKCS11_HELPER");
		if (prog == NULL || strlen(prog) == 0)
			prog = _PATH_SSH_PKCS11_HELPER;
		if (log_level_get() >= SYSLOG_LEVEL_DEBUG1)
			verbosity = "-vvv";
		debug_f("starting %s %s", prog,
		    verbosity == NULL ? "" : verbosity);
		execlp(prog, prog, verbosity, (char *)NULL);
		fprintf(stderr, "exec: %s: %s\n", prog, strerror(errno));
		_exit(1);
	}
	close(pair[1]);
	helper->fd = pair[0];
	helper->path = xstrdup(path);
	helper->pid = pid;
	debug3_f("helper %zu for \"%s\" on fd %d pid %ld", nhelpers,
	    helper->path, helper->fd, (long)helper->pid);
	helpers = xrecallocarray(helpers, nhelpers,
	    nhelpers + 1, sizeof(*helpers));
	helpers[nhelpers++] = helper;
	return helper;
}

int
pkcs11_add_provider(char *name, char *pin, struct sshkey ***keysp,
    char ***labelsp)
{
	struct sshkey *k;
	int r, type;
	u_char *blob;
	char *label;
	size_t blen;
	u_int nkeys, i;
	struct sshbuf *msg;
	struct helper *helper;

	if ((helper = helper_by_provider(name)) == NULL &&
	    (helper = pkcs11_start_helper(name)) == NULL)
		return -1;

	if ((msg = sshbuf_new()) == NULL)
		fatal_f("sshbuf_new failed");
	if ((r = sshbuf_put_u8(msg, SSH_AGENTC_ADD_SMARTCARD_KEY)) != 0 ||
	    (r = sshbuf_put_cstring(msg, name)) != 0 ||
	    (r = sshbuf_put_cstring(msg, pin)) != 0)
		fatal_fr(r, "compose");
	send_msg(helper->fd, msg);
	sshbuf_reset(msg);

	type = recv_msg(helper->fd, msg);
	if (type == SSH2_AGENT_IDENTITIES_ANSWER) {
		if ((r = sshbuf_get_u32(msg, &nkeys)) != 0)
			fatal_fr(r, "parse nkeys");
		*keysp = xcalloc(nkeys, sizeof(struct sshkey *));
		if (labelsp)
			*labelsp = xcalloc(nkeys, sizeof(char *));
		for (i = 0; i < nkeys; i++) {
			/* XXX clean up properly instead of fatal() */
			if ((r = sshbuf_get_string(msg, &blob, &blen)) != 0 ||
			    (r = sshbuf_get_cstring(msg, &label, NULL)) != 0)
				fatal_fr(r, "parse key");
			if ((r = sshkey_from_blob(blob, blen, &k)) != 0)
				fatal_fr(r, "decode key");
			wrap_key(helper, k);
			(*keysp)[i] = k;
			if (labelsp)
				(*labelsp)[i] = label;
			else
				free(label);
			free(blob);
		}
	} else if (type == SSH2_AGENT_FAILURE) {
		if ((r = sshbuf_get_u32(msg, &nkeys)) != 0)
			nkeys = -1;
	} else {
		nkeys = -1;
	}
	sshbuf_free(msg);
	return (nkeys);
}

int
pkcs11_del_provider(char *name)
{
	struct helper *helper;

	/*
	 * ssh-agent deletes keys before calling this, so the helper entry
	 * should be gone before we get here.
	 */
	debug3_f("delete %s", name);
	if ((helper = helper_by_provider(name)) != NULL)
		helper_terminate(helper);
	return 0;
}