view process-packet.c @ 1715:3974f087d9c0

Disallow leading lines before the ident for server (#102) Per RFC4253 4.2 clients must be able to process other lines of data before the version string, server behavior is not defined neither with MUST/SHOULD nor with MAY. If server process up to 50 lines too - it may cause too long hanging session with invalid/evil client that consume host resources and potentially may lead to DDoS on poor embedded boxes. Let's require first line from client to be version string and fail early if it's not - matches both RFC and real OpenSSH behavior.
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 15 Jun 2020 18:22:18 +0500
parents 41bf8f216644
children
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/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002-2004 Matt Johnston
 * 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 "packet.h"
#include "session.h"
#include "dbutil.h"
#include "ssh.h"
#include "algo.h"
#include "buffer.h"
#include "kex.h"
#include "dbrandom.h"
#include "service.h"
#include "auth.h"
#include "channel.h"

#define MAX_UNAUTH_PACKET_TYPE SSH_MSG_USERAUTH_PK_OK

static void recv_unimplemented(void);

/* process a decrypted packet, call the appropriate handler */
void process_packet() {

	unsigned char type;
	unsigned int i;
	time_t now;

	TRACE2(("enter process_packet"))

	type = buf_getbyte(ses.payload);
	TRACE(("process_packet: packet type = %d,  len %d", type, ses.payload->len))

	now = monotonic_now();
	ses.last_packet_time_keepalive_recv = now;

	/* These packets we can receive at any time */
	switch(type) {

		case SSH_MSG_IGNORE:
			goto out;
		case SSH_MSG_DEBUG:
			goto out;

		case SSH_MSG_UNIMPLEMENTED:
			/* debugging XXX */
			TRACE(("SSH_MSG_UNIMPLEMENTED"))
			goto out;
			
		case SSH_MSG_DISCONNECT:
			/* TODO cleanup? */
			dropbear_close("Disconnect received");
	}

	/* Ignore these packet types so that keepalives don't interfere with
	idle detection. This is slightly incorrect since a tcp forwarded
	global request with failure won't trigger the idle timeout,
	but that's probably acceptable */
	if (!(type == SSH_MSG_GLOBAL_REQUEST 
		|| type == SSH_MSG_REQUEST_FAILURE
		|| type == SSH_MSG_CHANNEL_FAILURE)) {
		ses.last_packet_time_idle = now;
	}

	/* This applies for KEX, where the spec says the next packet MUST be
	 * NEWKEYS */
	if (ses.requirenext != 0) {
		if (ses.requirenext == type)
		{
			/* Got what we expected */
			TRACE(("got expected packet %d during kexinit", type))
		}
		else
		{
			/* RFC4253 7.1 - various messages are allowed at this point.
			The only ones we know about have already been handled though,
			so just return "unimplemented" */
			if (type >= 1 && type <= 49
				&& type != SSH_MSG_SERVICE_REQUEST
				&& type != SSH_MSG_SERVICE_ACCEPT
				&& type != SSH_MSG_KEXINIT)
			{
				TRACE(("unknown allowed packet during kexinit"))
				recv_unimplemented();
				goto out;
			}
			else
			{
				TRACE(("disallowed packet during kexinit"))
				dropbear_exit("Unexpected packet type %d, expected %d", type,
						ses.requirenext);
			}
		}
	}

	/* Check if we should ignore this packet. Used currently only for
	 * KEX code, with first_kex_packet_follows */
	if (ses.ignorenext) {
		TRACE(("Ignoring packet, type = %d", type))
		ses.ignorenext = 0;
		goto out;
	}

	/* Only clear the flag after we have checked ignorenext */
	if (ses.requirenext != 0 && ses.requirenext == type)
	{
		ses.requirenext = 0;
	}


	/* Kindly the protocol authors gave all the preauth packets type values
	 * less-than-or-equal-to 60 ( == MAX_UNAUTH_PACKET_TYPE ).
	 * NOTE: if the protocol changes and new types are added, revisit this 
	 * assumption */
	if ( !ses.authstate.authdone && type > MAX_UNAUTH_PACKET_TYPE ) {
		dropbear_exit("Received message %d before userauth", type);
	}

	for (i = 0; ; i++) {
		if (ses.packettypes[i].type == 0) {
			/* end of list */
			break;
		}

		if (ses.packettypes[i].type == type) {
			ses.packettypes[i].handler();
			goto out;
		}
	}

	
	/* TODO do something more here? */
	TRACE(("preauth unknown packet"))
	recv_unimplemented();

out:
	ses.lastpacket = type;
	buf_free(ses.payload);
	ses.payload = NULL;

	TRACE2(("leave process_packet"))
}



/* This must be called directly after receiving the unimplemented packet.
 * Isn't the most clean implementation, it relies on packet processing
 * occurring directly after decryption (direct use of ses.recvseq).
 * This is reasonably valid, since there is only a single decryption buffer */
static void recv_unimplemented() {

	CHECKCLEARTOWRITE();

	buf_putbyte(ses.writepayload, SSH_MSG_UNIMPLEMENTED);
	/* the decryption routine increments the sequence number, we must
	 * decrement */
	buf_putint(ses.writepayload, ses.recvseq - 1);

	encrypt_packet();
}