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Start the process of splitting sshd into separate binaries. This step
splits sshd into a listener and a session binary. More splits are
planned.

After this changes, the listener binary will validate the configuration,
load the hostkeys, listen on port 22 and manage MaxStartups only. All
session handling will be performed by a new sshd-session binary that the
listener fork+execs.

This reduces the listener process to the minimum necessary and sets us
up for future work on the sshd-session binary.

feedback/ok markus@ deraadt@

NB. if you're updating via source, please restart sshd after installing,
otherwise you run the risk of locking yourself out.

/* $OpenBSD: auth2-gss.c,v 1.35 2024/05/17 00:30:23 djm Exp $ */

/*
 * Copyright (c) 2001-2003 Simon Wilkinson. 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.
 */

#ifdef GSSAPI

#include <sys/types.h>

#include "xmalloc.h"
#include "sshkey.h"
#include "hostfile.h"
#include "auth.h"
#include "ssh2.h"
#include "log.h"
#include "dispatch.h"
#include "sshbuf.h"
#include "ssherr.h"
#include "servconf.h"
#include "packet.h"
#include "kex.h"
#include "ssh-gss.h"
#include "monitor_wrap.h"

#define SSH_GSSAPI_MAX_MECHS	2048

extern ServerOptions options;
extern struct authmethod_cfg methodcfg_gssapi;

static int input_gssapi_token(int type, u_int32_t plen, struct ssh *ssh);
static int input_gssapi_mic(int type, u_int32_t plen, struct ssh *ssh);
static int input_gssapi_exchange_complete(int type, u_int32_t plen, struct ssh *ssh);
static int input_gssapi_errtok(int, u_int32_t, struct ssh *);

/*
 * We only support those mechanisms that we know about (ie ones that we know
 * how to check local user kuserok and the like)
 */
static int
userauth_gssapi(struct ssh *ssh, const char *method)
{
	Authctxt *authctxt = ssh->authctxt;
	gss_OID_desc goid = {0, NULL};
	Gssctxt *ctxt = NULL;
	int r, present;
	u_int mechs;
	OM_uint32 ms;
	size_t len;
	u_char *doid = NULL;

	if ((r = sshpkt_get_u32(ssh, &mechs)) != 0)
		fatal_fr(r, "parse packet");

	if (mechs == 0) {
		logit_f("mechanism negotiation is not supported");
		return (0);
	} else if (mechs > SSH_GSSAPI_MAX_MECHS) {
		logit_f("too many mechanisms requested %u > %u", mechs,
		    SSH_GSSAPI_MAX_MECHS);
		return (0);
	}

	do {
		mechs--;

		free(doid);

		present = 0;
		if ((r = sshpkt_get_string(ssh, &doid, &len)) != 0)
			fatal_fr(r, "parse oid");

		if (len > 2 && doid[0] == SSH_GSS_OIDTYPE &&
		    doid[1] == len - 2) {
			goid.elements = doid + 2;
			goid.length   = len - 2;
			ssh_gssapi_test_oid_supported(&ms, &goid, &present);
		} else {
			logit_f("badly formed OID received");
		}
	} while (mechs > 0 && !present);

	if (!present) {
		free(doid);
		authctxt->server_caused_failure = 1;
		return (0);
	}

	if (!authctxt->valid || authctxt->user == NULL) {
		debug2_f("disabled because of invalid user");
		free(doid);
		return (0);
	}

	if (GSS_ERROR(mm_ssh_gssapi_server_ctx(&ctxt, &goid))) {
		if (ctxt != NULL)
			ssh_gssapi_delete_ctx(&ctxt);
		free(doid);
		authctxt->server_caused_failure = 1;
		return (0);
	}

	authctxt->methoddata = (void *)ctxt;

	/* Return the OID that we received */
	if ((r = sshpkt_start(ssh, SSH2_MSG_USERAUTH_GSSAPI_RESPONSE)) != 0 ||
	    (r = sshpkt_put_string(ssh, doid, len)) != 0 ||
	    (r = sshpkt_send(ssh)) != 0)
		fatal_fr(r, "send packet");

	free(doid);

	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, &input_gssapi_token);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, &input_gssapi_errtok);
	authctxt->postponed = 1;

	return (0);
}

static int
input_gssapi_token(int type, u_int32_t plen, struct ssh *ssh)
{
	Authctxt *authctxt = ssh->authctxt;
	Gssctxt *gssctxt;
	gss_buffer_desc send_tok = GSS_C_EMPTY_BUFFER;
	gss_buffer_desc recv_tok;
	OM_uint32 maj_status, min_status, flags;
	u_char *p;
	size_t len;
	int r;

	if (authctxt == NULL)
		fatal("No authentication or GSSAPI context");

	gssctxt = authctxt->methoddata;
	if ((r = sshpkt_get_string(ssh, &p, &len)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		fatal_fr(r, "parse packet");

	recv_tok.value = p;
	recv_tok.length = len;
	maj_status = mm_ssh_gssapi_accept_ctx(gssctxt, &recv_tok,
	    &send_tok, &flags);

	free(p);

	if (GSS_ERROR(maj_status)) {
		if (send_tok.length != 0) {
			if ((r = sshpkt_start(ssh,
			    SSH2_MSG_USERAUTH_GSSAPI_ERRTOK)) != 0 ||
			    (r = sshpkt_put_string(ssh, send_tok.value,
			    send_tok.length)) != 0 ||
			    (r = sshpkt_send(ssh)) != 0)
				fatal_fr(r, "send ERRTOK packet");
		}
		authctxt->postponed = 0;
		ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
		userauth_finish(ssh, 0, "gssapi-with-mic", NULL);
	} else {
		if (send_tok.length != 0) {
			if ((r = sshpkt_start(ssh,
			    SSH2_MSG_USERAUTH_GSSAPI_TOKEN)) != 0 ||
			    (r = sshpkt_put_string(ssh, send_tok.value,
			    send_tok.length)) != 0 ||
			    (r = sshpkt_send(ssh)) != 0)
				fatal_fr(r, "send TOKEN packet");
		}
		if (maj_status == GSS_S_COMPLETE) {
			ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
			if (flags & GSS_C_INTEG_FLAG)
				ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_MIC,
				    &input_gssapi_mic);
			else
				ssh_dispatch_set(ssh,
				    SSH2_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE,
				    &input_gssapi_exchange_complete);
		}
	}

	gss_release_buffer(&min_status, &send_tok);
	return 0;
}

static int
input_gssapi_errtok(int type, u_int32_t plen, struct ssh *ssh)
{
	Authctxt *authctxt = ssh->authctxt;
	Gssctxt *gssctxt;
	gss_buffer_desc send_tok = GSS_C_EMPTY_BUFFER;
	gss_buffer_desc recv_tok;
	OM_uint32 maj_status;
	int r;
	u_char *p;
	size_t len;

	if (authctxt == NULL)
		fatal("No authentication or GSSAPI context");

	gssctxt = authctxt->methoddata;
	if ((r = sshpkt_get_string(ssh, &p, &len)) != 0 ||
	    (r = sshpkt_get_end(ssh)) != 0)
		fatal_fr(r, "parse packet");
	recv_tok.value = p;
	recv_tok.length = len;

	/* Push the error token into GSSAPI to see what it says */
	maj_status = mm_ssh_gssapi_accept_ctx(gssctxt, &recv_tok,
	    &send_tok, NULL);

	free(recv_tok.value);

	/* We can't return anything to the client, even if we wanted to */
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, NULL);

	/* The client will have already moved on to the next auth */

	gss_release_buffer(&maj_status, &send_tok);
	return 0;
}

/*
 * This is called when the client thinks we've completed authentication.
 * It should only be enabled in the dispatch handler by the function above,
 * which only enables it once the GSSAPI exchange is complete.
 */

static int
input_gssapi_exchange_complete(int type, u_int32_t plen, struct ssh *ssh)
{
	Authctxt *authctxt = ssh->authctxt;
	int r, authenticated;
	const char *displayname;

	if (authctxt == NULL)
		fatal("No authentication or GSSAPI context");

	/*
	 * We don't need to check the status, because we're only enabled in
	 * the dispatcher once the exchange is complete
	 */

	if ((r = sshpkt_get_end(ssh)) != 0)
		fatal_fr(r, "parse packet");

	authenticated = mm_ssh_gssapi_userok(authctxt->user);

	authctxt->postponed = 0;
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_MIC, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE, NULL);
	userauth_finish(ssh, authenticated, "gssapi-with-mic", NULL);
	return 0;
}

static int
input_gssapi_mic(int type, u_int32_t plen, struct ssh *ssh)
{
	Authctxt *authctxt = ssh->authctxt;
	Gssctxt *gssctxt;
	int r, authenticated = 0;
	struct sshbuf *b;
	gss_buffer_desc mic, gssbuf;
	const char *displayname;
	u_char *p;
	size_t len;

	if (authctxt == NULL)
		fatal("No authentication or GSSAPI context");

	gssctxt = authctxt->methoddata;

	if ((r = sshpkt_get_string(ssh, &p, &len)) != 0)
		fatal_fr(r, "parse packet");
	if ((b = sshbuf_new()) == NULL)
		fatal_f("sshbuf_new failed");
	mic.value = p;
	mic.length = len;
	ssh_gssapi_buildmic(b, authctxt->user, authctxt->service,
	    "gssapi-with-mic", ssh->kex->session_id);

	if ((gssbuf.value = sshbuf_mutable_ptr(b)) == NULL)
		fatal_f("sshbuf_mutable_ptr failed");
	gssbuf.length = sshbuf_len(b);

	if (!GSS_ERROR(mm_ssh_gssapi_checkmic(gssctxt, &gssbuf, &mic)))
		authenticated = mm_ssh_gssapi_userok(authctxt->user);
	else
		logit("GSSAPI MIC check failed");

	sshbuf_free(b);
	free(mic.value);

	authctxt->postponed = 0;
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_ERRTOK, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_MIC, NULL);
	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE, NULL);
	userauth_finish(ssh, authenticated, "gssapi-with-mic", NULL);
	return 0;
}

Authmethod method_gssapi = {
	&methodcfg_gssapi,
	userauth_gssapi,
};
#endif