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expose PKCS#11 key labels/X.509 subjects as comments

Extract the key label or X.509 subject string when PKCS#11 keys
are retrieved from the token and plumb this through to places where
it may be used as a comment.

based on https://github.com/openssh/openssh-portable/pull/138
by Danielle Church

feedback and ok markus@

/* $OpenBSD: ssh-pkcs11-client.c,v 1.16 2020/01/25 00:03:36 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 <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 */

static int fd = -1;
static pid_t pid = -1;

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

	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("%s: buffer error: %s", __func__, ssh_err(r));
}

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

	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 */
	sshbuf_reset(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("%s: buffer error: %s", __func__, ssh_err(r));
		len -= l;
	}
	if ((r = sshbuf_get_u8(m, &c)) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	return c;
}

int
pkcs11_init(int interactive)
{
	return (0);
}

void
pkcs11_terminate(void)
{
	if (fd >= 0)
		close(fd);
}

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;

	if (padding != RSA_PKCS1_PADDING)
		goto fail;
	key = sshkey_new(KEY_UNSPEC);
	if (key == NULL) {
		error("%s: sshkey_new failed", __func__);
		goto fail;
	}
	key->type = KEY_RSA;
	RSA_up_ref(rsa);
	key->rsa = rsa;
	if ((r = sshkey_to_blob(key, &blob, &blen)) != 0) {
		error("%s: sshkey_to_blob: %s", __func__, ssh_err(r));
		goto fail;
	}
	if ((msg = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __func__);
	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("%s: buffer error: %s", __func__, ssh_err(r));
	send_msg(msg);
	sshbuf_reset(msg);

	if (recv_msg(msg) == SSH2_AGENT_SIGN_RESPONSE) {
		if ((r = sshbuf_get_string(msg, &signature, &slen)) != 0)
			fatal("%s: buffer error: %s", __func__, ssh_err(r));
		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 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;

	nid = sshkey_ecdsa_key_to_nid(ec);
	if (nid < 0) {
		error("%s: couldn't get curve nid", __func__);
		goto fail;
	}

	key = sshkey_new(KEY_UNSPEC);
	if (key == NULL) {
		error("%s: sshkey_new failed", __func__);
		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("%s: sshkey_to_blob: %s", __func__, ssh_err(r));
		goto fail;
	}
	if ((msg = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __func__);
	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("%s: buffer error: %s", __func__, ssh_err(r));
	send_msg(msg);
	sshbuf_reset(msg);

	if (recv_msg(msg) == SSH2_AGENT_SIGN_RESPONSE) {
		if ((r = sshbuf_get_string(msg, &signature, &slen)) != 0)
			fatal("%s: buffer error: %s", __func__, ssh_err(r));
		cp = signature;
		ret = d2i_ECDSA_SIG(NULL, &cp, slen);
		free(signature);
	}

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

static RSA_METHOD	*helper_rsa;
static EC_KEY_METHOD	*helper_ecdsa;

/* redirect private key crypto operations to the ssh-pkcs11-helper */
static void
wrap_key(struct sshkey *k)
{
	if (k->type == KEY_RSA)
		RSA_set_method(k->rsa, helper_rsa);
	else if (k->type == KEY_ECDSA)
		EC_KEY_set_method(k->ecdsa, helper_ecdsa);
	else
		fatal("%s: unknown key type", __func__);
}

static int
pkcs11_start_helper_methods(void)
{
	if (helper_ecdsa != NULL)
		return (0);

	int (*orig_sign)(int, const unsigned char *, int, unsigned char *,
	    unsigned int *, const BIGNUM *, const BIGNUM *, EC_KEY *) = NULL;
	if (helper_ecdsa != NULL)
		return (0);
	helper_ecdsa = EC_KEY_METHOD_new(EC_KEY_OpenSSL());
	if (helper_ecdsa == NULL)
		return (-1);
	EC_KEY_METHOD_get_sign(helper_ecdsa, &orig_sign, NULL, NULL);
	EC_KEY_METHOD_set_sign(helper_ecdsa, orig_sign, NULL, ecdsa_do_sign);

	if ((helper_rsa = RSA_meth_dup(RSA_get_default_method())) == NULL)
		fatal("%s: RSA_meth_dup failed", __func__);
	if (!RSA_meth_set1_name(helper_rsa, "ssh-pkcs11-helper") ||
	    !RSA_meth_set_priv_enc(helper_rsa, rsa_encrypt))
		fatal("%s: failed to prepare method", __func__);

	return (0);
}

static int
pkcs11_start_helper(void)
{
	int pair[2];
	char *helper, *verbosity = NULL;

	if (log_level_get() >= SYSLOG_LEVEL_DEBUG1)
		verbosity = "-vvv";

	if (pkcs11_start_helper_methods() == -1) {
		error("pkcs11_start_helper_methods failed");
		return (-1);
	}

	if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair) == -1) {
		error("socketpair: %s", strerror(errno));
		return (-1);
	}
	if ((pid = fork()) == -1) {
		error("fork: %s", strerror(errno));
		return (-1);
	} 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]);
		helper = getenv("SSH_PKCS11_HELPER");
		if (helper == NULL || strlen(helper) == 0)
			helper = _PATH_SSH_PKCS11_HELPER;
		debug("%s: starting %s %s", __func__, helper,
		    verbosity == NULL ? "" : verbosity);
		execlp(helper, helper, verbosity, (char *)NULL);
		fprintf(stderr, "exec: %s: %s\n", helper, strerror(errno));
		_exit(1);
	}
	close(pair[1]);
	fd = pair[0];
	return (0);
}

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;

	if (fd < 0 && pkcs11_start_helper() < 0)
		return (-1);

	if ((msg = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __func__);
	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("%s: buffer error: %s", __func__, ssh_err(r));
	send_msg(msg);
	sshbuf_reset(msg);

	type = recv_msg(msg);
	if (type == SSH2_AGENT_IDENTITIES_ANSWER) {
		if ((r = sshbuf_get_u32(msg, &nkeys)) != 0)
			fatal("%s: buffer error: %s", __func__, ssh_err(r));
		*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("%s: buffer error: %s",
				    __func__, ssh_err(r));
			if ((r = sshkey_from_blob(blob, blen, &k)) != 0)
				fatal("%s: bad key: %s", __func__, ssh_err(r));
			wrap_key(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)
{
	int r, ret = -1;
	struct sshbuf *msg;

	if ((msg = sshbuf_new()) == NULL)
		fatal("%s: sshbuf_new failed", __func__);
	if ((r = sshbuf_put_u8(msg, SSH_AGENTC_REMOVE_SMARTCARD_KEY)) != 0 ||
	    (r = sshbuf_put_cstring(msg, name)) != 0 ||
	    (r = sshbuf_put_cstring(msg, "")) != 0)
		fatal("%s: buffer error: %s", __func__, ssh_err(r));
	send_msg(msg);
	sshbuf_reset(msg);

	if (recv_msg(msg) == SSH_AGENT_SUCCESS)
		ret = 0;
	sshbuf_free(msg);
	return (ret);
}