ssh/sshd/altprivsep.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 *
 * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
 * Use is subject to license terms.
 */

#include <fcntl.h>
#include <sys/types.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>

#include <pwd.h>

#include "includes.h"
#include "atomicio.h"
#include "auth.h"
#include "bufaux.h"
#include "buffer.h"
#include "cipher.h"
#include "compat.h"
#include "dispatch.h"
#include "getput.h"
#include "kex.h"
#include "log.h"
#include "mac.h"
#include "packet.h"
#include "uidswap.h"
#include "ssh2.h"
#include "sshlogin.h"
#include "xmalloc.h"
#include "altprivsep.h"
#include "canohost.h"
#include "engine.h"
#include "servconf.h"

#ifdef HAVE_BSM
#include "bsmaudit.h"
adt_session_data_t *ah = NULL;
#endif /* HAVE_BSM */

#ifdef GSSAPI
#include "ssh-gss.h"
extern Gssctxt *xxx_gssctxt;
#endif /* GSSAPI */

extern Kex *xxx_kex;
extern u_char *session_id2;
extern int session_id2_len;

static Buffer to_monitor;
static Buffer from_monitor;

/*
 * Sun's Alternative Privilege Separation basics:
 *
 * Abstract
 * --------
 *
 * sshd(1M) fork()s and drops privs in the child while retaining privs
 * in the parent (a.k.a., the monitor).  The unprivileged sshd and the
 * monitor talk over a pipe using a simple protocol.
 *
 * The monitor protocol is all about having the monitor carry out the
 * only operations that require privileges OR access to privileged
 * resources.  These are: utmpx/wtmpx record keeping, auditing, and
 * SSHv2 re-keying.
 *
 * Re-Keying
 * ---------
 *
 * Re-keying is the only protocol version specific aspect of sshd in
 * which the monitor gets involved.
 *
 * The monitor processes all SSHv2 re-key protocol packets, but the
 * unprivileged sshd process does the transport layer crypto for those
 * packets.
 *
 * The monitor and its unprivileged sshd child process treat
 * SSH_MSG_NEWKEYS SSH2 messages specially: a) the monitor does not call
 * set_newkeys(), but b) the child asks the monitor for the set of
 * negotiated algorithms, key, IV and what not for the relevant
 * transport direction and then calls set_newkeys().
 *
 * Monitor Protocol
 * ----------------
 *
 * Monitor IPC message formats are similar to SSHv2 messages, minus
 * compression, encryption, padding and MACs:
 *
 *  - 4 octet message length
 *  - message data
 *     - 1 octet message type
 *     - message data
 *
 * In broad strokes:
 *
 *  - IPC: pipe, exit(2)/wait4(2)
 *
 *  - threads: the monitor and child are single-threaded
 *
 *  - monitor main loop: a variant of server_loop2(), for re-keying only
 *  - unpriv child main loop: server_loop2(), as usual
 *
 *  - protocol:
 *     - key exchange packets are always forwarded as is to the monitor
 *     - newkeys, record_login(), record_logout() are special packets
 *     using the packet type range reserved for local extensions
 *
 *  - the child drops privs and runs like a normal sshd, except that it
 *  sets dispatch handlers for key exchange packets that forward the
 *  packets to the monitor
 *
 * Event loops:
 *
 *  - all monitor protocols are synchronous: because the SSHv2 rekey
 *  protocols are synchronous and because the other monitor operations
 *  are synchronous (or have no replies),
 *
 *  - server_loop2() is modified to check the monitor pipe for rekey
 *  packets to forward to the client
 *
 *  - and dispatch handlers are set, upon receipt of KEXINIT (and reset
 *  when NEWKEYS is sent out) to forward incoming rekey packets to the
 *  monitor.
 *
 *  - the monitor runs an event loop not unlike server_loop2() and runs
 *  key exchanges almost exactly as a pre-altprivsep sshd would
 *
 *  - unpriv sshd exit -> monitor cleanup (including audit logout) and exit
 *
 *  - fatal() in monitor -> forcibly shutdown() socket and kill/wait for
 *  child (so that the audit event for the logout better reflects
 *  reality -- i.e., logged out means logged out, but for bg jobs)
 *
 * Message formats:
 *
 *  - key exchange packets/replies forwarded "as is"
 *
 *  - all other monitor requests are sent as SSH2_PRIV_MSG_ALTPRIVSEP and have a
 *  sub-type identifier (one octet)
 *  - private request sub-types include:
 *     - get new shared secret from last re-key
 *     - record login  (utmpx/wtmpx), request data contains three arguments:
 *     pid, ttyname, program name
 *     - record logout (utmpx/wtmpx), request data contains one argument: pid
 *
 * Reply sub-types include:
 *
 *  - NOP (for record_login/logout)
 *  - new shared secret from last re-key
 */

static int aps_started = 0;
static int is_monitor = 0;

static pid_t monitor_pid, child_pid;
static int pipe_fds[2];
static int pipe_fd = -1;
static Buffer input_pipe, output_pipe; /* for pipe I/O */

static Authctxt *xxx_authctxt;

/* Monitor functions */
extern void aps_monitor_loop(Authctxt *authctxt, pid_t child_pid);
static void aps_record_login(void);
static void aps_record_logout(void);
static void aps_start_rekex(void);
Authctxt *aps_read_auth_context(void);

/* main functions for handling the monitor */
static pid_t	altprivsep_start_monitor(Authctxt **authctxt);
static void	altprivsep_do_monitor(Authctxt *authctxt, pid_t child_pid);
static int	altprivsep_started(void);
static int	altprivsep_is_monitor(void);

/* calls _to_ monitor from unprivileged process */
static void	altprivsep_get_newkeys(enum kex_modes mode);

/* monitor-side fatal_cleanup callbacks */
static void	altprivsep_shutdown_sock(void *arg);

/* Altprivsep packet utilities for communication with the monitor */
static void	altprivsep_packet_start(u_char);
static int	altprivsep_packet_send(void);
static int	altprivsep_fwd_packet(u_char type);

static int	altprivsep_packet_read(void);
static void	altprivsep_packet_read_expect(int type);

static void	altprivsep_packet_put_char(int ch);
static void	altprivsep_packet_put_int(u_int value);
static void	altprivsep_packet_put_cstring(const char *str);
static void	altprivsep_packet_put_raw(const void *buf, u_int len);

static u_int	 altprivsep_packet_get_char(void);
static void	*altprivsep_packet_get_raw(u_int *length_ptr);
static void	*altprivsep_packet_get_string(u_int *length_ptr);

Kex		*prepare_for_ssh2_kex(void);

/*
 * Start monitor from privileged sshd process.
 *
 * Return values are like fork(2); the parent is the monitor.  The caller should
 * fatal() on error.
 *
 * Note that the monitor waits until the still privileged child finishes the
 * authentication. The child drops its privileges after the authentication.
 */
static pid_t
altprivsep_start_monitor(Authctxt **authctxt)
{
	pid_t pid;
	int junk;

	if (aps_started)
		fatal("Monitor startup failed: missing state");

	buffer_init(&output_pipe);
	buffer_init(&input_pipe);

	if (pipe(pipe_fds) != 0) {
		error("Monitor startup failure: could not create pipes: %s",
			strerror(errno));
		return (-1);
	}

	(void) fcntl(pipe_fds[0], F_SETFD, FD_CLOEXEC);
	(void) fcntl(pipe_fds[1], F_SETFD, FD_CLOEXEC);

	monitor_pid = getpid();

	if ((pid = fork()) > 0) {
		/*
		 * From now on, all debug messages from monitor will have prefix
		 * "monitor "
		 */
		set_log_txt_prefix("monitor ");
		(void) prepare_for_ssh2_kex();
		packet_set_server();
		/* parent */
		child_pid = pid;

		debug2("Monitor pid %ld, unprivileged child pid %ld",
			monitor_pid, child_pid);

		(void) close(pipe_fds[1]);
		pipe_fd = pipe_fds[0];

		/*
		 * Signal readiness of the monitor and then read the
		 * authentication context from the child.
		 */
		(void) write(pipe_fd, &pid, sizeof (pid));
		packet_set_monitor(pipe_fd);
		xxx_authctxt = *authctxt = aps_read_auth_context();

		if (fcntl(pipe_fd, F_SETFL, O_NONBLOCK) < 0)
			error("fcntl O_NONBLOCK: %.100s", strerror(errno));

		aps_started = 1;
		is_monitor = 1;

		debug2("Monitor started");

		return (pid);
	}

	if (pid < 0) {
		debug2("Monitor startup failure: could not fork unprivileged"
			" process:  %s", strerror(errno));
		return (pid);
	}

	/* this is the child that will later drop privileges */

	/* note that Solaris has bi-directional pipes so one pipe is enough */
	(void) close(pipe_fds[0]);
	pipe_fd = pipe_fds[1];

	/* wait for monitor to be ready */
	debug2("Waiting for monitor");
	(void) read(pipe_fd, &junk, sizeof (junk));
	debug2("Monitor signalled readiness");

	buffer_init(&to_monitor);
	buffer_init(&from_monitor);

	/* AltPrivSep interfaces are set up */
	aps_started = 1;
	return (pid);
}

int
altprivsep_get_pipe_fd(void)
{
	return (pipe_fd);
}

/*
 * This function is used in the unprivileged child for all packets in the range
 * between SSH2_MSG_KEXINIT and SSH2_MSG_TRANSPORT_MAX.
 */
void
altprivsep_rekey(int type, u_int32_t seq, void *ctxt)
{
	Kex *kex = (Kex *)ctxt;

	if (kex == NULL)
		fatal("Missing key exchange context in unprivileged process");

	if (type != SSH2_MSG_NEWKEYS) {
		debug2("Forwarding re-key packet (%d) to monitor", type);
		if (!altprivsep_fwd_packet(type))
			fatal("Monitor not responding");
	}

	/* tell server_loop2() that we're re-keying */
	kex->done = 0;

	/* NEWKEYS is special: get the new keys for client->server direction */
	if (type == SSH2_MSG_NEWKEYS) {
		debug2("received SSH2_MSG_NEWKEYS packet - "
		    "getting new inbound keys from the monitor");
		altprivsep_get_newkeys(MODE_IN);
		kex->done = 1;
	}
}

void
altprivsep_process_input(fd_set *rset)
{
	void	*data;
	int	 type;
	u_int	 dlen;

	if (pipe_fd == -1)
		return;

	if (!FD_ISSET(pipe_fd, rset))
		return;

	debug2("reading from pipe to monitor (%d)", pipe_fd);
	if ((type = altprivsep_packet_read()) == -1)
		fatal("Monitor not responding");

	if (!compat20)
		return; /* shouldn't happen! but be safe */

	if (type == 0)
		return;	/* EOF -- nothing to do here */

	if (type >= SSH2_MSG_MAX)
		fatal("Received garbage from monitor");

	debug2("Read packet type %d from pipe to monitor", (u_int)type);

	if (type == SSH2_PRIV_MSG_ALTPRIVSEP)
		return; /* shouldn't happen! */

	/* NEWKEYS is special: get the new keys for server->client direction */
	if (type == SSH2_MSG_NEWKEYS) {
		debug2("forwarding SSH2_MSG_NEWKEYS packet we got from monitor to "
		    "the client");
		packet_start(SSH2_MSG_NEWKEYS);
		packet_send();
		debug2("getting new outbound keys from the monitor");
		altprivsep_get_newkeys(MODE_OUT);
		return;
	}

	data = altprivsep_packet_get_raw(&dlen);

	packet_start((u_char)type);

	if (data != NULL && dlen > 0)
		packet_put_raw(data, dlen);

	packet_send();
}

static void
altprivsep_do_monitor(Authctxt *authctxt, pid_t child_pid)
{
	aps_monitor_loop(authctxt, child_pid);
}

static int
altprivsep_started(void)
{
	return (aps_started);
}

static int
altprivsep_is_monitor(void)
{
	return (is_monitor);
}

/*
 * A fatal cleanup function to forcibly shutdown the connection socket
 */
static void
altprivsep_shutdown_sock(void *arg)
{
	int sock;

	if (arg == NULL)
		return;

	sock = *(int *)arg;

	(void) shutdown(sock, SHUT_RDWR);
}

/* Calls _to_ monitor from unprivileged process */
static int
altprivsep_fwd_packet(u_char type)
{
	u_int len;
	void  *data;

	altprivsep_packet_start(type);
	data = packet_get_raw(&len);
	altprivsep_packet_put_raw(data, len);

	/* packet_send()s any replies from the monitor to the client */
	return (altprivsep_packet_send());
}

extern Newkeys *current_keys[MODE_MAX];

/* To be called from packet.c:set_newkeys() before referencing current_keys */
static void
altprivsep_get_newkeys(enum kex_modes mode)
{
	Newkeys	*newkeys;
	Comp	*comp;
	Enc	*enc;
	Mac	*mac;
	u_int	 len;

	if (!altprivsep_started())
		return;

	if (altprivsep_is_monitor())
		return; /* shouldn't happen */

	/* request new keys */
	altprivsep_packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	altprivsep_packet_put_char(APS_MSG_NEWKEYS_REQ);
	altprivsep_packet_put_int((u_int)mode);
	altprivsep_packet_send();
	altprivsep_packet_read_expect(SSH2_PRIV_MSG_ALTPRIVSEP);
	if (altprivsep_packet_get_char() != APS_MSG_NEWKEYS_REP)
		fatal("Received garbage from monitor during re-keying");

	newkeys = xmalloc(sizeof (*newkeys));
	memset(newkeys, 0, sizeof (*newkeys));

	enc = &newkeys->enc;
	mac = &newkeys->mac;
	comp = &newkeys->comp;

	/* Cipher name, key, IV */
	enc->name = altprivsep_packet_get_string(NULL);
	if ((enc->cipher = cipher_by_name(enc->name)) == NULL)
		fatal("Monitor negotiated an unknown cipher during re-key");

	enc->key = altprivsep_packet_get_string(&enc->key_len);
	enc->iv = altprivsep_packet_get_string(&enc->block_size);

	/* MAC name */
	mac->name = altprivsep_packet_get_string(NULL);
	if (mac_setup(mac, mac->name) < 0)
		fatal("Monitor negotiated an unknown MAC algorithm "
			"during re-key");

	mac->key = altprivsep_packet_get_string(&len);
	if (len > mac->key_len)
		fatal("%s: bad mac key length: %d > %d", __func__, len,
			mac->key_len);

	/* Compression algorithm name */
	comp->name = altprivsep_packet_get_string(NULL);
	if (strcmp(comp->name, "zlib") != 0 && strcmp(comp->name, "none") != 0)
		fatal("Monitor negotiated an unknown compression "
			"algorithm during re-key");

	comp->type = 0;
	comp->enabled = 0; /* forces compression re-init, as per-spec */
	if (strcmp(comp->name, "zlib") == 0)
		comp->type = 1;

	/*
	 * Now install new keys
	 *
	 * For now abuse kex.c/packet.c non-interfaces.  Someday, when
	 * the many internal interfaces are parametrized, made reentrant
	 * and thread-safe, made more consistent, and when necessary-but-
	 * currently-missing interfaces are added then this bit of
	 * ugliness can be revisited.
	 *
	 * The ugliness is in the set_newkeys(), its name and the lack
	 * of a (Newkeys *) parameter, which forces us to pass the
	 * newkeys through current_keys[mode].  But this saves us some
	 * lines of code for now, though not comments.
	 *
	 * Also, we've abused, in the code above, knowledge of what
	 * set_newkeys() expects the current_keys[mode] to contain.
	 */
	current_keys[mode] = newkeys;
	set_newkeys(mode);

}

void
altprivsep_record_login(pid_t pid, const char *ttyname)
{
	altprivsep_packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	altprivsep_packet_put_char(APS_MSG_RECORD_LOGIN);
	altprivsep_packet_put_int(pid);
	altprivsep_packet_put_cstring(ttyname);
	altprivsep_packet_send();
	altprivsep_packet_read_expect(SSH2_PRIV_MSG_ALTPRIVSEP);
}

void
altprivsep_record_logout(pid_t pid)
{
	altprivsep_packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	altprivsep_packet_put_char(APS_MSG_RECORD_LOGOUT);
	altprivsep_packet_put_int(pid);
	altprivsep_packet_send();
	altprivsep_packet_read_expect(SSH2_PRIV_MSG_ALTPRIVSEP);
}

void
altprivsep_start_rekex(void)
{
	altprivsep_packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	altprivsep_packet_put_char(APS_MSG_START_REKEX);
	altprivsep_packet_send();
	altprivsep_packet_read_expect(SSH2_PRIV_MSG_ALTPRIVSEP);
}

/*
 * The monitor needs some information that its child learns during the
 * authentication process. Since the child was forked before the key exchange
 * and authentication started it must send some context to the monitor after the
 * authentication is finished. Less obvious part - monitor needs the session ID
 * since it is used in the key generation process after the key (re-)exchange is
 * finished.
 */
void
altprivsep_send_auth_context(Authctxt *authctxt)
{
	debug("sending auth context to the monitor");
	altprivsep_packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	altprivsep_packet_put_char(APS_MSG_AUTH_CONTEXT);
	altprivsep_packet_put_int(authctxt->pw->pw_uid);
	altprivsep_packet_put_int(authctxt->pw->pw_gid);
	altprivsep_packet_put_cstring(authctxt->pw->pw_name);
	altprivsep_packet_put_raw(session_id2, session_id2_len);
	debug("will send %d bytes of auth context to the monitor",
	    buffer_len(&to_monitor));
	altprivsep_packet_send();
	altprivsep_packet_read_expect(SSH2_PRIV_MSG_ALTPRIVSEP);
}

static void aps_send_newkeys(void);

/* Monitor side dispatch handler for SSH2_PRIV_MSG_ALTPRIVSEP */
/* ARGSUSED */
void
aps_input_altpriv_msg(int type, u_int32_t seq, void *ctxt)
{
	u_char req_type;

	req_type = packet_get_char();

	switch (req_type) {
	case APS_MSG_NEWKEYS_REQ:
		aps_send_newkeys();
		break;
	case APS_MSG_RECORD_LOGIN:
		aps_record_login();
		break;
	case APS_MSG_RECORD_LOGOUT:
		aps_record_logout();
		break;
	case APS_MSG_START_REKEX:
		aps_start_rekex();
		break;
	default:
		break;
	}
}

/* Monitor-side handlers for APS_MSG_* */
static
void
aps_send_newkeys(void)
{
	Newkeys *newkeys;
	Enc *enc;
	Mac *mac;
	Comp *comp;
	enum kex_modes mode;

	/* get direction for which newkeys are wanted */
	mode = (enum kex_modes) packet_get_int();
	packet_check_eom();

	/* get those newkeys */
	newkeys = kex_get_newkeys(mode);
	enc = &newkeys->enc;
	mac = &newkeys->mac;
	comp = &newkeys->comp;

	/*
	 * Negotiated algorithms, client->server and server->client, for
	 * cipher, mac and compression.
	 */
	packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	packet_put_char(APS_MSG_NEWKEYS_REP);
	packet_put_cstring(enc->name);
	packet_put_string(enc->key, enc->key_len);
	packet_put_string(enc->iv, enc->block_size);
	packet_put_cstring(mac->name);
	packet_put_string(mac->key, mac->key_len);
	packet_put_cstring(comp->name);

	packet_send();
	free_keys(newkeys);
}

struct _aps_login_rec {
	pid_t			lr_pid;
	char			*lr_tty;
	struct _aps_login_rec	*next;
};

typedef struct _aps_login_rec aps_login_rec;

static aps_login_rec *aps_login_list = NULL;

static
void
aps_record_login(void)
{
	aps_login_rec	*new_rec;
	struct stat	 sbuf;
	size_t		 proc_path_len;
	char		*proc_path;

	new_rec = xmalloc(sizeof (aps_login_rec));
	memset(new_rec, 0, sizeof (aps_login_rec));

	new_rec->lr_pid = packet_get_int();
	new_rec->lr_tty = packet_get_string(NULL);

	proc_path_len = snprintf(NULL, 0, "/proc/%d", new_rec->lr_pid);
	proc_path = xmalloc(proc_path_len + 1);
	(void) snprintf(proc_path, proc_path_len + 1, "/proc/%d",
			new_rec->lr_pid);

	if (stat(proc_path, &sbuf) ||
	    sbuf.st_uid != xxx_authctxt->pw->pw_uid ||
	    stat(new_rec->lr_tty, &sbuf) < 0 ||
	    sbuf.st_uid != xxx_authctxt->pw->pw_uid) {
		debug2("Spurious record_login request from unprivileged sshd");
		xfree(proc_path);
		xfree(new_rec->lr_tty);
		xfree(new_rec);
		return;
	}

	/* Insert new record on list */
	new_rec->next = aps_login_list;
	aps_login_list = new_rec;

	record_login(new_rec->lr_pid, new_rec->lr_tty, NULL,
		xxx_authctxt->user);

	packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	packet_send();

	xfree(proc_path);
}

static
void
aps_record_logout(void)
{
	aps_login_rec	**p, *q;
	pid_t		 pid;

	pid = packet_get_int();
	packet_check_eom();

	for (p = &aps_login_list; *p != NULL; p = &q->next) {
		q = *p;
		if (q->lr_pid == pid) {
			record_logout(q->lr_pid, q->lr_tty, NULL,
				xxx_authctxt->user);

			/* dequeue */
			*p = q->next;
			xfree(q->lr_tty);
			xfree(q);
			break;
		}
	}

	packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	packet_send();
}

static
void
aps_start_rekex(void)
{
	/*
	 * Send confirmation. We could implement it without that but it doesn't
	 * bring any harm to do that and we are consistent with other subtypes
	 * of our private SSH2_PRIV_MSG_ALTPRIVSEP message type.
	 */
	packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	packet_send();

	/*
	 * KEX_INIT message could be the one that reached the limit. In that
	 * case, it was already forwarded to us from the unnprivileged child,
	 * and maybe even acted upon. Obviously we must not send another
	 * KEX_INIT message.
	 */
	if (!(xxx_kex->flags & KEX_INIT_SENT))
		kex_send_kexinit(xxx_kex);
	else
		debug2("rekeying already in progress");
}

/*
 * This is the monitor side of altprivsep_send_auth_context().
 */
Authctxt *
aps_read_auth_context(void)
{
	unsigned char *tmp;
	Authctxt *authctxt;

	/*
	 * After the successful authentication we get the context. Getting
	 * end-of-file means that authentication failed and we can exit as well.
	 */
	debug("reading the context from the child");
	packet_read_expect(SSH2_PRIV_MSG_ALTPRIVSEP);
	debug3("got SSH2_PRIV_MSG_ALTPRIVSEP");
	if (packet_get_char() != APS_MSG_AUTH_CONTEXT) {
		fatal("APS_MSG_AUTH_CONTEXT message subtype expected.");
	}

	authctxt = xcalloc(1, sizeof(Authctxt));
	authctxt->pw = xcalloc(1, sizeof(struct passwd));

	/* uid_t and gid_t are integers (UNIX spec) */
	authctxt->pw->pw_uid = packet_get_int();
	authctxt->pw->pw_gid = packet_get_int();
	authctxt->pw->pw_name = packet_get_string(NULL);
	authctxt->user = xstrdup(authctxt->pw->pw_name);
	debug3("uid/gid/username %d/%d/%s", authctxt->pw->pw_uid,
	    authctxt->pw->pw_gid, authctxt->user);
	session_id2 = (unsigned char *)packet_get_raw((unsigned int*)&session_id2_len);

	/* we don't have this for SSH1. In that case, session_id2_len is 0. */
	if (session_id2_len > 0) {
		tmp = (unsigned char *)xmalloc(session_id2_len);
		memcpy(tmp, session_id2, session_id2_len);
		session_id2 = tmp;
		debug3("read session ID (%d B)", session_id2_len);
		xxx_kex->session_id = tmp;
		xxx_kex->session_id_len = session_id2_len;
	}
	debug("finished reading the context");

	/* send confirmation */
	packet_start(SSH2_PRIV_MSG_ALTPRIVSEP);
	packet_send();

	return (authctxt);
}


/* Utilities for communication with the monitor */
static void
altprivsep_packet_start(u_char type)
{
	buffer_clear(&to_monitor);
	buffer_put_char(&to_monitor, type);
}

static void
altprivsep_packet_put_char(int ch)
{
	buffer_put_char(&to_monitor, ch);
}

static void
altprivsep_packet_put_int(u_int value)
{
	buffer_put_int(&to_monitor, value);
}

static void
altprivsep_packet_put_cstring(const char *str)
{
	buffer_put_cstring(&to_monitor, str);
}

static void
altprivsep_packet_put_raw(const void *buf, u_int len)
{
	buffer_append(&to_monitor, buf, len);
}

/*
 * Send a monitor packet to the monitor.  This function is blocking.
 *
 * Returns -1 if the monitor pipe has been closed earlier, fatal()s if
 * there's any other problems.
 */
static int
altprivsep_packet_send(void)
{
	ssize_t len;
	u_int32_t plen;	/* packet length */
	u_char	plen_buf[sizeof (plen)];
	u_char padlen;	/* padding length */
	fd_set *setp;

	if (pipe_fd == -1)
		return (-1);

	if ((plen = buffer_len(&to_monitor)) == 0)
		return (0);

	/*
	 * We talk the SSHv2 binary packet protocol to the monitor,
	 * using the none cipher, mac and compression algorithms.
	 *
	 * But, interestingly, the none cipher has a block size of 8
	 * bytes, thus we must pad the packet.
	 *
	 * Also, encryption includes the packet length, so the padding
	 * must account for that field.  I.e., (sizeof (packet length) +
	 * sizeof (padding length) + packet length + padding length) %
	 * block_size must == 0.
	 *
	 * Also, there must be at least four (4) bytes of padding.
	 */
	padlen = (8 - ((plen + sizeof (plen) + sizeof (padlen)) % 8)) % 8;
	if (padlen < 4)
		padlen += 8;

	/* packet length counts padding and padding length field */
	plen += padlen + sizeof (padlen);

	PUT_32BIT(plen_buf, plen);

	setp = xmalloc(howmany(pipe_fd + 1, NFDBITS) * sizeof (fd_mask));
	memset(setp, 0, howmany(pipe_fd + 1, NFDBITS) * sizeof (fd_mask));
	FD_SET(pipe_fd, setp);

	while (select(pipe_fd + 1, NULL, setp, NULL, NULL) == -1) {
		if (errno == EAGAIN || errno == EINTR)
			continue;
		else
			goto pipe_gone;
	}

	xfree(setp);

	/* packet length field */
	len = atomicio(write, pipe_fd, plen_buf, sizeof (plen));

	if (len != sizeof (plen))
		goto pipe_gone;

	/* padding length field */
	len = atomicio(write, pipe_fd, &padlen, sizeof (padlen));

	if (len != sizeof (padlen))
		goto pipe_gone;

	len = atomicio(write, pipe_fd, buffer_ptr(&to_monitor), plen - 1);

	if (len != (plen - 1))
		goto pipe_gone;

	buffer_clear(&to_monitor);

	return (1);

pipe_gone:

	(void) close(pipe_fd);

	pipe_fd = -1;

	fatal("Monitor not responding");

	/* NOTREACHED */
	return (0);
}

/*
 * Read a monitor packet from the monitor.  This function is blocking.
 */
static int
altprivsep_packet_read(void)
{
	ssize_t len = -1;
	u_int32_t plen;
	u_char plen_buf[sizeof (plen)];
	u_char padlen;
	fd_set *setp;

	if (pipe_fd == -1)
		return (-1);

	setp = xmalloc(howmany(pipe_fd + 1, NFDBITS) * sizeof (fd_mask));
	memset(setp, 0, howmany(pipe_fd + 1, NFDBITS) * sizeof (fd_mask));
	FD_SET(pipe_fd, setp);

	while (select(pipe_fd + 1, setp, NULL, NULL, NULL) == -1) {
		if (errno == EAGAIN || errno == EINTR)
			continue;
		else
			goto pipe_gone;
	}

	xfree(setp);

	/* packet length field */
	len = atomicio(read, pipe_fd, plen_buf, sizeof (plen));

	plen = GET_32BIT(plen_buf);

	if (len != sizeof (plen))
		goto pipe_gone;

	/* padding length field */
	len = atomicio(read, pipe_fd, &padlen, sizeof (padlen));

	if (len != sizeof (padlen))
		goto pipe_gone;

	plen -= sizeof (padlen);

	buffer_clear(&from_monitor);
	buffer_append_space(&from_monitor, plen);

	/* packet data + padding */
	len = atomicio(read, pipe_fd, buffer_ptr(&from_monitor), plen);

	if (len != plen)
		goto pipe_gone;

	/* remove padding */
	if (padlen > 0)
		buffer_consume_end(&from_monitor, padlen);

	/* packet type */
	return (buffer_get_char(&from_monitor));

pipe_gone:

	(void) close(pipe_fd);

	pipe_fd = -1;

	if (len < 0)
		fatal("Monitor not responding");

	debug2("Monitor pipe closed by monitor");
	return (0);
}

static void
altprivsep_packet_read_expect(int expected)
{
	int type;

	type = altprivsep_packet_read();

	if (type <= 0)
		fatal("Monitor not responding");

	if (type != expected)
		fatal("Protocol error in privilege separation; expected "
			"packet type %d, got %d", expected, type);
}

static u_int
altprivsep_packet_get_char(void)
{
	return (buffer_get_char(&from_monitor));
}
void
*altprivsep_packet_get_raw(u_int *length_ptr)
{
	if (length_ptr != NULL)
		*length_ptr = buffer_len(&from_monitor);

	return (buffer_ptr(&from_monitor));
}
void
*altprivsep_packet_get_string(u_int *length_ptr)
{
	return (buffer_get_string(&from_monitor, length_ptr));
}

void
altprivsep_start_and_do_monitor(int use_engine, int inetd, int newsock,
	int statup_pipe)
{
	pid_t aps_child;
	Authctxt *authctxt;

	/*
	 * The monitor will packet_close() in packet_set_monitor() called from
	 * altprivsep_start_monitor() below to clean up the socket stuff before
	 * it switches to pipes for communication to the child. The socket fd is
	 * closed there so we must dup it here - monitor needs that socket to
	 * shutdown the connection in case of any problem; see comments below.
	 * Note that current newsock was assigned to connection_(in|out) which
	 * are the variables used in packet_close() to close the communication
	 * socket.
	 */
	newsock = dup(newsock);

	if ((aps_child = altprivsep_start_monitor(&authctxt)) == -1)
		fatal("Monitor could not be started.");

	if (aps_child > 0) {
		/* ALTPRIVSEP Monitor */

		/*
		 * The ALTPRIVSEP monitor here does:
		 *
		 *  - record keeping and auditing
		 *  - PAM cleanup
		 */

		/*
		 * Alarm signal handler is for our child only since that's the
		 * one that does the authentication.
		 */
		(void) alarm(0);
		(void) signal(SIGALRM, SIG_DFL);
		/* this is for MaxStartups and the child takes care of that */
		(void) close(statup_pipe);
		(void) pkcs11_engine_load(use_engine);

		/*
		 * If the monitor fatal()s it will audit/record a logout, so
		 * we'd better do something to really mean it: shutdown the
		 * socket but leave the child alone -- it's been disconnected
		 * and we hope it exits, but killing any pid from a privileged
		 * monitor could be dangerous.
		 *
		 * NOTE: Order matters -- these fatal cleanups must come before
		 * the audit logout fatal cleanup as these functions are called
		 * in LIFO.
		 */
		fatal_add_cleanup((void (*)(void *))altprivsep_shutdown_sock,
			(void *)&newsock);

		if (compat20) {
			debug3("Recording SSHv2 session login in wtmpx");
			/*
			 * record_login() relies on connection_in to be the
			 * socket to get the peer address. The problem is that
			 * connection_in had to be set to the pipe descriptor in
			 * altprivsep_start_monitor(). It's not nice but the
			 * easiest way to get the peer's address is to
			 * temporarily set connection_in to the socket's file
			 * descriptor.
			 */
			packet_set_fds(inetd == 1 ? -1 : newsock, 0);
			record_login(getpid(), NULL, "sshd", authctxt->user);
			packet_set_fds(0, 1);
		}

#ifdef HAVE_BSM
		/* Initialize the group list, audit sometimes needs it. */
		if (initgroups(authctxt->pw->pw_name,
		    authctxt->pw->pw_gid) < 0) {
			perror("initgroups");
			exit (1);
		}

		/*
		 * The monitor process fork()ed before the authentication
		 * process started so at this point we have an unaudited
		 * context.  Thus we need to obtain the audit session data
		 * from the authentication process (aps_child) which will
		 * have the correct audit context for the user logging in.
		 * To do so we pass along the process-ID of the aps_child
		 * process so that it is referenced for this audit session
		 * rather than referencing the monitor's unaudited context.
		 */
		audit_sshd_login(&ah, aps_child);

		fatal_add_cleanup((void (*)(void *))audit_sshd_logout,
		    (void *)&ah);
#endif /* HAVE_BSM */

#ifdef GSSAPI
		fatal_add_cleanup((void (*)(void *))ssh_gssapi_cleanup_creds,
			(void *)&xxx_gssctxt);
#endif /* GSSAPI */

		altprivsep_do_monitor(authctxt, aps_child);

		/* If we got here the connection is dead. */
		fatal_remove_cleanup((void (*)(void *))altprivsep_shutdown_sock,
			(void *)&newsock);

		if (compat20) {
			debug3("Recording SSHv2 session logout in wtmpx");
			record_logout(getpid(), NULL, "sshd", authctxt->user);
		}

		/*
		 * Make sure the socket is closed. The monitor can't call
		 * packet_close here as it's done a packet_set_connection()
		 * with the pipe to the child instead of the socket.
		 */
		(void) shutdown(newsock, SHUT_RDWR);

#ifdef GSSAPI
		fatal_remove_cleanup((void (*)(void *))ssh_gssapi_cleanup_creds,
			&xxx_gssctxt);
		ssh_gssapi_cleanup_creds(xxx_gssctxt);
		ssh_gssapi_server_mechs(NULL); /* release cached mechs list */
#endif /* GSSAPI */

#ifdef HAVE_BSM
		fatal_remove_cleanup((void (*)(void *))audit_sshd_logout, (void *)&ah);
		audit_sshd_logout(&ah);
#endif /* HAVE_BSM */

		exit(0);
	} else {
		/*
		 * This is the child, close the dup()ed file descriptor for a
		 * socket. It's not needed in the child.
		 */
		close(newsock);
	}
}