view svr-tcpfwd.c @ 1788:1fc0012b9c38

Fix handling of replies to global requests (#112) The current code assumes that all global requests want / need a reply. This isn't always true and the request itself indicates if it wants a reply or not. It causes a specific problem with [email protected] messages. These are sent by OpenSSH after authentication to inform the client of potential other host keys for the host. This can be used to add a new type of host key or to rotate host keys. The initial information message from the server is sent as a global request, but with want_reply set to false. This means that the server doesn't expect an answer to this message. Instead the client needs to send a prove request as a reply if it wants to receive proof of ownership for the host keys. The bug doesn't cause any current problems with due to how OpenSSH treats receiving the failure message. It instead treats it as a keepalive message and further ignores it. Arguably this is a protocol violation though of Dropbear and it is only accidental that it doesn't cause a problem with OpenSSH. The bug was found when adding host keys support to libssh, which is more strict protocol wise and treats the unexpected failure message an error, also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145 for more information. The fix here is to honor the want_reply flag in the global request and to only send a reply if the other side expects a reply.
author Dirkjan Bussink <d.bussink@gmail.com>
date Thu, 10 Dec 2020 16:13:13 +0100
parents 9026f976eee8
children 249681d9ecda
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/*
 * Dropbear SSH
 * 
 * Copyright (c) 2002,2003 Matt Johnston
 * Copyright (c) 2004 by Mihnea Stoenescu
 * All rights reserved.
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

#include "includes.h"
#include "ssh.h"
#include "tcpfwd.h"
#include "dbutil.h"
#include "session.h"
#include "buffer.h"
#include "packet.h"
#include "listener.h"
#include "runopts.h"
#include "auth.h"
#include "netio.h"

#if !DROPBEAR_SVR_REMOTETCPFWD

/* This is better than SSH_MSG_UNIMPLEMENTED */
void recv_msg_global_request_remotetcp() {
	unsigned int len = 0;
	unsigned int wantreply = 0;

	TRACE(("recv_msg_global_request_remotetcp: remote tcp forwarding not compiled in"))

	len = buf_getint(ses.payload);
	buf_incrpos(ses.payload, len);
	wantreply = buf_getbool(ses.payload);
	if (wantreply) {
		send_msg_request_failure();
	}
}

/* */
#endif /* !DROPBEAR_SVR_REMOTETCPFWD */

static int svr_cancelremotetcp(void);
static int svr_remotetcpreq(int *allocated_listen_port);
static int newtcpdirect(struct Channel * channel);

#if DROPBEAR_SVR_REMOTETCPFWD
static const struct ChanType svr_chan_tcpremote = {
	1, /* sepfds */
	"forwarded-tcpip",
	tcp_prio_inithandler,
	NULL,
	NULL,
	NULL,
	NULL
};

/* At the moment this is completely used for tcp code (with the name reflecting
 * that). If new request types are added, this should be replaced with code
 * similar to the request-switching in chansession.c */
void recv_msg_global_request_remotetcp() {

	char* reqname = NULL;
	unsigned int namelen;
	unsigned int wantreply = 0;
	int ret = DROPBEAR_FAILURE;

	TRACE(("enter recv_msg_global_request_remotetcp"))

	if (svr_opts.noremotetcp || !svr_pubkey_allows_tcpfwd()) {
		TRACE(("leave recv_msg_global_request_remotetcp: remote tcp forwarding disabled"))
		goto out;
	}

	reqname = buf_getstring(ses.payload, &namelen);
	wantreply = buf_getbool(ses.payload);

	if (namelen > MAX_NAME_LEN) {
		TRACE(("name len is wrong: %d", namelen))
		goto out;
	}

	if (strcmp("tcpip-forward", reqname) == 0) {
		int allocated_listen_port = 0;
		ret = svr_remotetcpreq(&allocated_listen_port);
		/* client expects-port-number-to-make-use-of-server-allocated-ports */
		if (DROPBEAR_SUCCESS == ret) {
			CHECKCLEARTOWRITE();
			buf_putbyte(ses.writepayload, SSH_MSG_REQUEST_SUCCESS);
			buf_putint(ses.writepayload, allocated_listen_port);
			encrypt_packet();
			wantreply = 0; /* avoid out: below sending another reply */
		}
	} else if (strcmp("cancel-tcpip-forward", reqname) == 0) {
		ret = svr_cancelremotetcp();
	} else {
		TRACE(("reqname isn't tcpip-forward: '%s'", reqname))
	}

out:
	if (wantreply) {
		if (ret == DROPBEAR_SUCCESS) {
			send_msg_request_success();
		} else {
			send_msg_request_failure();
		}
	}

	m_free(reqname);

	TRACE(("leave recv_msg_global_request"))
}

static int matchtcp(const void* typedata1, const void* typedata2) {

	const struct TCPListener *info1 = (struct TCPListener*)typedata1;
	const struct TCPListener *info2 = (struct TCPListener*)typedata2;

	return (info1->listenport == info2->listenport)
			&& (info1->chantype == info2->chantype)
			&& (strcmp(info1->listenaddr, info2->listenaddr) == 0);
}

static int svr_cancelremotetcp() {

	int ret = DROPBEAR_FAILURE;
	char * bindaddr = NULL;
	unsigned int addrlen;
	unsigned int port;
	struct Listener * listener = NULL;
	struct TCPListener tcpinfo;

	TRACE(("enter cancelremotetcp"))

	bindaddr = buf_getstring(ses.payload, &addrlen);
	if (addrlen > MAX_IP_LEN) {
		TRACE(("addr len too long: %d", addrlen))
		goto out;
	}

	port = buf_getint(ses.payload);

	tcpinfo.sendaddr = NULL;
	tcpinfo.sendport = 0;
	tcpinfo.listenaddr = bindaddr;
	tcpinfo.listenport = port;
	listener = get_listener(CHANNEL_ID_TCPFORWARDED, &tcpinfo, matchtcp);
	if (listener) {
		remove_listener( listener );
		ret = DROPBEAR_SUCCESS;
	}

out:
	m_free(bindaddr);
	TRACE(("leave cancelremotetcp"))
	return ret;
}

static int svr_remotetcpreq(int *allocated_listen_port) {

	int ret = DROPBEAR_FAILURE;
	char * request_addr = NULL;
	unsigned int addrlen;
	struct TCPListener *tcpinfo = NULL;
	unsigned int port;
	struct Listener *listener = NULL;

	TRACE(("enter remotetcpreq"))

	request_addr = buf_getstring(ses.payload, &addrlen);
	if (addrlen > MAX_IP_LEN) {
		TRACE(("addr len too long: %d", addrlen))
		goto out;
	}

	port = buf_getint(ses.payload);

	if (port != 0) {
		if (port < 1 || port > 65535) {
			TRACE(("invalid port: %d", port))
			goto out;
		}

		if (!ses.allowprivport && port < IPPORT_RESERVED) {
			TRACE(("can't assign port < 1024 for non-root"))
			goto out;
		}
	}

	tcpinfo = (struct TCPListener*)m_malloc(sizeof(struct TCPListener));
	tcpinfo->sendaddr = NULL;
	tcpinfo->sendport = 0;
	tcpinfo->listenport = port;
	tcpinfo->chantype = &svr_chan_tcpremote;
	tcpinfo->tcp_type = forwarded;

	tcpinfo->request_listenaddr = request_addr;
	if (!opts.listen_fwd_all || (strcmp(request_addr, "localhost") == 0) ) {
		/* NULL means "localhost only" */
		tcpinfo->listenaddr = NULL;
	}
	else
	{
		tcpinfo->listenaddr = m_strdup(request_addr);
	}

	ret = listen_tcpfwd(tcpinfo, &listener);
	if (DROPBEAR_SUCCESS == ret) {
		tcpinfo->listenport = get_sock_port(listener->socks[0]);
		*allocated_listen_port = tcpinfo->listenport;
	}

out:
	if (ret == DROPBEAR_FAILURE) {
		/* we only free it if a listener wasn't created, since the listener
		 * has to remember it if it's to be cancelled */
		m_free(request_addr);
		m_free(tcpinfo);
	}

	TRACE(("leave remotetcpreq"))

	return ret;
}

#endif /* DROPBEAR_SVR_REMOTETCPFWD */

#if DROPBEAR_SVR_LOCALTCPFWD

const struct ChanType svr_chan_tcpdirect = {
	1, /* sepfds */
	"direct-tcpip",
	newtcpdirect, /* init */
	NULL, /* checkclose */
	NULL, /* reqhandler */
	NULL, /* closehandler */
	NULL /* cleanup */
};

/* Called upon creating a new direct tcp channel (ie we connect out to an
 * address */
static int newtcpdirect(struct Channel * channel) {

	char* desthost = NULL;
	unsigned int destport;
	char* orighost = NULL;
	unsigned int origport;
	char portstring[NI_MAXSERV];
	unsigned int len;
	int err = SSH_OPEN_ADMINISTRATIVELY_PROHIBITED;

	TRACE(("newtcpdirect channel %d", channel->index))

	if (svr_opts.nolocaltcp || !svr_pubkey_allows_tcpfwd()) {
		TRACE(("leave newtcpdirect: local tcp forwarding disabled"))
		goto out;
	}

	desthost = buf_getstring(ses.payload, &len);
	if (len > MAX_HOST_LEN) {
		TRACE(("leave newtcpdirect: desthost too long"))
		goto out;
	}

	destport = buf_getint(ses.payload);
	
	orighost = buf_getstring(ses.payload, &len);
	if (len > MAX_HOST_LEN) {
		TRACE(("leave newtcpdirect: orighost too long"))
		goto out;
	}

	origport = buf_getint(ses.payload);

	/* best be sure */
	if (origport > 65535 || destport > 65535) {
		TRACE(("leave newtcpdirect: port > 65535"))
		goto out;
	}

	channel->prio = DROPBEAR_CHANNEL_PRIO_UNKNOWABLE;

	snprintf(portstring, sizeof(portstring), "%u", destport);
	channel->conn_pending = connect_remote(desthost, portstring, channel_connect_done, channel, NULL, NULL);
	
	err = SSH_OPEN_IN_PROGRESS;

out:
	m_free(desthost);
	m_free(orighost);
	TRACE(("leave newtcpdirect: err %d", err))
	return err;
}

#endif /* DROPBEAR_SVR_LOCALTCPFWD */