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Add protection for private keys at rest in RAM against speculation
and memory sidechannel attacks like Spectre, Meltdown, Rowhammer and
Rambleed. This change encrypts private keys when they are not in use
with a symmetic key that is derived from a relatively large "prekey"
consisting of random data (currently 16KB).

Attackers must recover the entire prekey with high accuracy before
they can attempt to decrypt the shielded private key, but the current
generation of attacks have bit error rates that, when applied
cumulatively to the entire prekey, make this unlikely.

Implementation-wise, keys are encrypted "shielded" when loaded and then
automatically and transparently unshielded when used for signatures or
when being saved/serialised.

Hopefully we can remove this in a few years time when computer
architecture has become less unsafe.

been in snaps for a bit already; thanks deraadt@

ok dtucker@ deraadt@

/* $OpenBSD: authfd.c,v 1.114 2019/06/21 04:21:04 djm Exp $ */
/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Functions for connecting the local authentication agent.
 *
 * As far as I am concerned, the code I have written for this software
 * can be used freely for any purpose.  Any derived versions of this
 * software must be clearly marked as such, and if the derived work is
 * incompatible with the protocol description in the RFC file, it must be
 * called by a name other than "ssh" or "Secure Shell".
 *
 * SSH2 implementation,
 * Copyright (c) 2000 Markus Friedl.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */


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

#include <fcntl.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#include "xmalloc.h"
#include "ssh.h"
#include "sshbuf.h"
#include "sshkey.h"
#include "authfd.h"
#include "cipher.h"
#include "compat.h"
#include "log.h"
#include "atomicio.h"
#include "misc.h"
#include "ssherr.h"

#define MAX_AGENT_IDENTITIES	2048		/* Max keys in agent reply */
#define MAX_AGENT_REPLY_LEN	(256 * 1024) 	/* Max bytes in agent reply */

/* macro to check for "agent failure" message */
#define agent_failed(x) \
    ((x == SSH_AGENT_FAILURE) || \
    (x == SSH_COM_AGENT2_FAILURE) || \
    (x == SSH2_AGENT_FAILURE))

/* Convert success/failure response from agent to a err.h status */
static int
decode_reply(u_char type)
{
	if (agent_failed(type))
		return SSH_ERR_AGENT_FAILURE;
	else if (type == SSH_AGENT_SUCCESS)
		return 0;
	else
		return SSH_ERR_INVALID_FORMAT;
}

/* Returns the number of the authentication fd, or -1 if there is none. */
int
ssh_get_authentication_socket(int *fdp)
{
	const char *authsocket;
	int sock, oerrno;
	struct sockaddr_un sunaddr;

	if (fdp != NULL)
		*fdp = -1;

	authsocket = getenv(SSH_AUTHSOCKET_ENV_NAME);
	if (authsocket == NULL || *authsocket == '\0')
		return SSH_ERR_AGENT_NOT_PRESENT;

	memset(&sunaddr, 0, sizeof(sunaddr));
	sunaddr.sun_family = AF_UNIX;
	strlcpy(sunaddr.sun_path, authsocket, sizeof(sunaddr.sun_path));

	if ((sock = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)
		return SSH_ERR_SYSTEM_ERROR;

	/* close on exec */
	if (fcntl(sock, F_SETFD, FD_CLOEXEC) == -1 ||
	    connect(sock, (struct sockaddr *)&sunaddr, sizeof(sunaddr)) < 0) {
		oerrno = errno;
		close(sock);
		errno = oerrno;
		return SSH_ERR_SYSTEM_ERROR;
	}
	if (fdp != NULL)
		*fdp = sock;
	else
		close(sock);
	return 0;
}

/* Communicate with agent: send request and read reply */
static int
ssh_request_reply(int sock, struct sshbuf *request, struct sshbuf *reply)
{
	int r;
	size_t l, len;
	char buf[1024];

	/* Get the length of the message, and format it in the buffer. */
	len = sshbuf_len(request);
	POKE_U32(buf, len);

	/* Send the length and then the packet to the agent. */
	if (atomicio(vwrite, sock, buf, 4) != 4 ||
	    atomicio(vwrite, sock, sshbuf_mutable_ptr(request),
	    sshbuf_len(request)) != sshbuf_len(request))
		return SSH_ERR_AGENT_COMMUNICATION;
	/*
	 * Wait for response from the agent.  First read the length of the
	 * response packet.
	 */
	if (atomicio(read, sock, buf, 4) != 4)
	    return SSH_ERR_AGENT_COMMUNICATION;

	/* Extract the length, and check it for sanity. */
	len = PEEK_U32(buf);
	if (len > MAX_AGENT_REPLY_LEN)
		return SSH_ERR_INVALID_FORMAT;

	/* Read the rest of the response in to the buffer. */
	sshbuf_reset(reply);
	while (len > 0) {
		l = len;
		if (l > sizeof(buf))
			l = sizeof(buf);
		if (atomicio(read, sock, buf, l) != l)
			return SSH_ERR_AGENT_COMMUNICATION;
		if ((r = sshbuf_put(reply, buf, l)) != 0)
			return r;
		len -= l;
	}
	return 0;
}

/*
 * Closes the agent socket if it should be closed (depends on how it was
 * obtained).  The argument must have been returned by
 * ssh_get_authentication_socket().
 */
void
ssh_close_authentication_socket(int sock)
{
	if (getenv(SSH_AUTHSOCKET_ENV_NAME))
		close(sock);
}

/* Lock/unlock agent */
int
ssh_lock_agent(int sock, int lock, const char *password)
{
	int r;
	u_char type = lock ? SSH_AGENTC_LOCK : SSH_AGENTC_UNLOCK;
	struct sshbuf *msg;

	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if ((r = sshbuf_put_u8(msg, type)) != 0 ||
	    (r = sshbuf_put_cstring(msg, password)) != 0)
		goto out;
	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	r = decode_reply(type);
 out:
	sshbuf_free(msg);
	return r;
}


static int
deserialise_identity2(struct sshbuf *ids, struct sshkey **keyp, char **commentp)
{
	int r;
	char *comment = NULL;
	const u_char *blob;
	size_t blen;

	if ((r = sshbuf_get_string_direct(ids, &blob, &blen)) != 0 ||
	    (r = sshbuf_get_cstring(ids, &comment, NULL)) != 0)
		goto out;
	if ((r = sshkey_from_blob(blob, blen, keyp)) != 0)
		goto out;
	if (commentp != NULL) {
		*commentp = comment;
		comment = NULL;
	}
	r = 0;
 out:
	free(comment);
	return r;
}

/*
 * Fetch list of identities held by the agent.
 */
int
ssh_fetch_identitylist(int sock, struct ssh_identitylist **idlp)
{
	u_char type;
	u_int32_t num, i;
	struct sshbuf *msg;
	struct ssh_identitylist *idl = NULL;
	int r;

	/*
	 * Send a message to the agent requesting for a list of the
	 * identities it can represent.
	 */
	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if ((r = sshbuf_put_u8(msg, SSH2_AGENTC_REQUEST_IDENTITIES)) != 0)
		goto out;

	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;

	/* Get message type, and verify that we got a proper answer. */
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	if (agent_failed(type)) {
		r = SSH_ERR_AGENT_FAILURE;
		goto out;
	} else if (type != SSH2_AGENT_IDENTITIES_ANSWER) {
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}

	/* Get the number of entries in the response and check it for sanity. */
	if ((r = sshbuf_get_u32(msg, &num)) != 0)
		goto out;
	if (num > MAX_AGENT_IDENTITIES) {
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if (num == 0) {
		r = SSH_ERR_AGENT_NO_IDENTITIES;
		goto out;
	}

	/* Deserialise the response into a list of keys/comments */
	if ((idl = calloc(1, sizeof(*idl))) == NULL ||
	    (idl->keys = calloc(num, sizeof(*idl->keys))) == NULL ||
	    (idl->comments = calloc(num, sizeof(*idl->comments))) == NULL) {
		r = SSH_ERR_ALLOC_FAIL;
		goto out;
	}
	for (i = 0; i < num;) {
		if ((r = deserialise_identity2(msg, &(idl->keys[i]),
		    &(idl->comments[i]))) != 0) {
			if (r == SSH_ERR_KEY_TYPE_UNKNOWN) {
				/* Gracefully skip unknown key types */
				num--;
				continue;
			} else
				goto out;
		}
		i++;
	}
	idl->nkeys = num;
	*idlp = idl;
	idl = NULL;
	r = 0;
 out:
	sshbuf_free(msg);
	if (idl != NULL)
		ssh_free_identitylist(idl);
	return r;
}

void
ssh_free_identitylist(struct ssh_identitylist *idl)
{
	size_t i;

	if (idl == NULL)
		return;
	for (i = 0; i < idl->nkeys; i++) {
		if (idl->keys != NULL)
			sshkey_free(idl->keys[i]);
		if (idl->comments != NULL)
			free(idl->comments[i]);
	}
	free(idl);
}

/*
 * Sends a challenge (typically from a server via ssh(1)) to the agent,
 * and waits for a response from the agent.
 * Returns true (non-zero) if the agent gave the correct answer, zero
 * otherwise.
 */


/* encode signature algorithm in flag bits, so we can keep the msg format */
static u_int
agent_encode_alg(const struct sshkey *key, const char *alg)
{
	if (alg != NULL && sshkey_type_plain(key->type) == KEY_RSA) {
		if (strcmp(alg, "rsa-sha2-256") == 0 ||
		    strcmp(alg, "rsa-sha2-256-cert-v01@openssh.com") == 0)
			return SSH_AGENT_RSA_SHA2_256;
		if (strcmp(alg, "rsa-sha2-512") == 0 ||
		    strcmp(alg, "rsa-sha2-512-cert-v01@openssh.com") == 0)
			return SSH_AGENT_RSA_SHA2_512;
	}
	return 0;
}

/* ask agent to sign data, returns err.h code on error, 0 on success */
int
ssh_agent_sign(int sock, const struct sshkey *key,
    u_char **sigp, size_t *lenp,
    const u_char *data, size_t datalen, const char *alg, u_int compat)
{
	struct sshbuf *msg;
	u_char *sig = NULL, type = 0;
	size_t len = 0;
	u_int flags = 0;
	int r = SSH_ERR_INTERNAL_ERROR;

	*sigp = NULL;
	*lenp = 0;

	if (datalen > SSH_KEY_MAX_SIGN_DATA_SIZE)
		return SSH_ERR_INVALID_ARGUMENT;
	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	flags |= agent_encode_alg(key, alg);
	if ((r = sshbuf_put_u8(msg, SSH2_AGENTC_SIGN_REQUEST)) != 0 ||
	    (r = sshkey_puts(key, msg)) != 0 ||
	    (r = sshbuf_put_string(msg, data, datalen)) != 0 ||
	    (r = sshbuf_put_u32(msg, flags)) != 0)
		goto out;
	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	if (agent_failed(type)) {
		r = SSH_ERR_AGENT_FAILURE;
		goto out;
	} else if (type != SSH2_AGENT_SIGN_RESPONSE) {
		r = SSH_ERR_INVALID_FORMAT;
		goto out;
	}
	if ((r = sshbuf_get_string(msg, &sig, &len)) != 0)
		goto out;
	/* Check what we actually got back from the agent. */
	if ((r = sshkey_check_sigtype(sig, len, alg)) != 0)
		goto out;
	/* success */
	*sigp = sig;
	*lenp = len;
	sig = NULL;
	len = 0;
	r = 0;
 out:
	freezero(sig, len);
	sshbuf_free(msg);
	return r;
}

/* Encode key for a message to the agent. */


static int
encode_constraints(struct sshbuf *m, u_int life, u_int confirm, u_int maxsign)
{
	int r;

	if (life != 0) {
		if ((r = sshbuf_put_u8(m, SSH_AGENT_CONSTRAIN_LIFETIME)) != 0 ||
		    (r = sshbuf_put_u32(m, life)) != 0)
			goto out;
	}
	if (confirm != 0) {
		if ((r = sshbuf_put_u8(m, SSH_AGENT_CONSTRAIN_CONFIRM)) != 0)
			goto out;
	}
	if (maxsign != 0) {
		if ((r = sshbuf_put_u8(m, SSH_AGENT_CONSTRAIN_MAXSIGN)) != 0 ||
		    (r = sshbuf_put_u32(m, maxsign)) != 0)
			goto out;
	}
	r = 0;
 out:
	return r;
}

/*
 * Adds an identity to the authentication server.
 * This call is intended only for use by ssh-add(1) and like applications.
 */
int
ssh_add_identity_constrained(int sock, struct sshkey *key,
    const char *comment, u_int life, u_int confirm, u_int maxsign)
{
	struct sshbuf *msg;
	int r, constrained = (life || confirm || maxsign);
	u_char type;

	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;

	switch (key->type) {
#ifdef WITH_OPENSSL
	case KEY_RSA:
	case KEY_RSA_CERT:
	case KEY_DSA:
	case KEY_DSA_CERT:
	case KEY_ECDSA:
	case KEY_ECDSA_CERT:
#endif
	case KEY_ED25519:
	case KEY_ED25519_CERT:
	case KEY_XMSS:
	case KEY_XMSS_CERT:
		type = constrained ?
		    SSH2_AGENTC_ADD_ID_CONSTRAINED :
		    SSH2_AGENTC_ADD_IDENTITY;
		if ((r = sshbuf_put_u8(msg, type)) != 0 ||
		    (r = sshkey_private_serialize_maxsign(key, msg, maxsign,
		    NULL)) != 0 ||
		    (r = sshbuf_put_cstring(msg, comment)) != 0)
			goto out;
		break;
	default:
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if (constrained &&
	    (r = encode_constraints(msg, life, confirm, maxsign)) != 0)
		goto out;
	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	r = decode_reply(type);
 out:
	sshbuf_free(msg);
	return r;
}

/*
 * Removes an identity from the authentication server.
 * This call is intended only for use by ssh-add(1) and like applications.
 */
int
ssh_remove_identity(int sock, struct sshkey *key)
{
	struct sshbuf *msg;
	int r;
	u_char type, *blob = NULL;
	size_t blen;

	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;

	if (key->type != KEY_UNSPEC) {
		if ((r = sshkey_to_blob(key, &blob, &blen)) != 0)
			goto out;
		if ((r = sshbuf_put_u8(msg,
		    SSH2_AGENTC_REMOVE_IDENTITY)) != 0 ||
		    (r = sshbuf_put_string(msg, blob, blen)) != 0)
			goto out;
	} else {
		r = SSH_ERR_INVALID_ARGUMENT;
		goto out;
	}
	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	r = decode_reply(type);
 out:
	if (blob != NULL) {
		explicit_bzero(blob, blen);
		free(blob);
	}
	sshbuf_free(msg);
	return r;
}

/*
 * Add/remove an token-based identity from the authentication server.
 * This call is intended only for use by ssh-add(1) and like applications.
 */
int
ssh_update_card(int sock, int add, const char *reader_id, const char *pin,
    u_int life, u_int confirm)
{
	struct sshbuf *msg;
	int r, constrained = (life || confirm);
	u_char type;

	if (add) {
		type = constrained ?
		    SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED :
		    SSH_AGENTC_ADD_SMARTCARD_KEY;
	} else
		type = SSH_AGENTC_REMOVE_SMARTCARD_KEY;

	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if ((r = sshbuf_put_u8(msg, type)) != 0 ||
	    (r = sshbuf_put_cstring(msg, reader_id)) != 0 ||
	    (r = sshbuf_put_cstring(msg, pin)) != 0)
		goto out;
	if (constrained &&
	    (r = encode_constraints(msg, life, confirm, 0)) != 0)
		goto out;
	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	r = decode_reply(type);
 out:
	sshbuf_free(msg);
	return r;
}

/*
 * Removes all identities from the agent.
 * This call is intended only for use by ssh-add(1) and like applications.
 *
 * This supports the SSH protocol 1 message to because, when clearing all
 * keys from an agent, we generally want to clear both protocol v1 and v2
 * keys.
 */
int
ssh_remove_all_identities(int sock, int version)
{
	struct sshbuf *msg;
	u_char type = (version == 1) ?
	    SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES :
	    SSH2_AGENTC_REMOVE_ALL_IDENTITIES;
	int r;

	if ((msg = sshbuf_new()) == NULL)
		return SSH_ERR_ALLOC_FAIL;
	if ((r = sshbuf_put_u8(msg, type)) != 0)
		goto out;
	if ((r = ssh_request_reply(sock, msg, msg)) != 0)
		goto out;
	if ((r = sshbuf_get_u8(msg, &type)) != 0)
		goto out;
	r = decode_reply(type);
 out:
	sshbuf_free(msg);
	return r;
}