view svr-session.c @ 1790:42745af83b7d

Introduce extra delay before closing unauthenticated sessions To make it harder for attackers, introduce a delay to keep an unauthenticated session open a bit longer, thus blocking a connection slot until after the delay. Without this, while there is a limit on the amount of attempts an attacker can make at the same time (MAX_UNAUTH_PER_IP), the time taken by dropbear to handle one attempt is still short and thus for each of the allowed parallel attempts many attempts can be chained one after the other. The attempt rate is then: "MAX_UNAUTH_PER_IP / <process time of one attempt>". With the delay, this rate becomes: "MAX_UNAUTH_PER_IP / UNAUTH_CLOSE_DELAY".
author Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
date Wed, 15 Feb 2017 13:53:04 +0100
parents a6da10ac64b5
children
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/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002,2003 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 "session.h"
#include "dbutil.h"
#include "packet.h"
#include "algo.h"
#include "buffer.h"
#include "dss.h"
#include "ssh.h"
#include "dbrandom.h"
#include "kex.h"
#include "channel.h"
#include "chansession.h"
#include "atomicio.h"
#include "tcpfwd.h"
#include "service.h"
#include "auth.h"
#include "runopts.h"
#include "crypto_desc.h"
#include "fuzz.h"

static void svr_remoteclosed(void);
static void svr_algos_initialise(void);

struct serversession svr_ses; /* GLOBAL */

static const packettype svr_packettypes[] = {
	{SSH_MSG_CHANNEL_DATA, recv_msg_channel_data},
	{SSH_MSG_CHANNEL_WINDOW_ADJUST, recv_msg_channel_window_adjust},
	{SSH_MSG_USERAUTH_REQUEST, recv_msg_userauth_request}, /* server */
	{SSH_MSG_SERVICE_REQUEST, recv_msg_service_request}, /* server */
	{SSH_MSG_KEXINIT, recv_msg_kexinit},
	{SSH_MSG_KEXDH_INIT, recv_msg_kexdh_init}, /* server */
	{SSH_MSG_NEWKEYS, recv_msg_newkeys},
	{SSH_MSG_GLOBAL_REQUEST, recv_msg_global_request_remotetcp},
	{SSH_MSG_CHANNEL_REQUEST, recv_msg_channel_request},
	{SSH_MSG_CHANNEL_OPEN, recv_msg_channel_open},
	{SSH_MSG_CHANNEL_EOF, recv_msg_channel_eof},
	{SSH_MSG_CHANNEL_CLOSE, recv_msg_channel_close},
	{SSH_MSG_CHANNEL_SUCCESS, ignore_recv_response},
	{SSH_MSG_CHANNEL_FAILURE, ignore_recv_response},
	{SSH_MSG_REQUEST_FAILURE, ignore_recv_response}, /* for keepalive */
	{SSH_MSG_REQUEST_SUCCESS, ignore_recv_response}, /* client */
#if DROPBEAR_LISTENERS
	{SSH_MSG_CHANNEL_OPEN_CONFIRMATION, recv_msg_channel_open_confirmation},
	{SSH_MSG_CHANNEL_OPEN_FAILURE, recv_msg_channel_open_failure},
#endif
	{0, NULL} /* End */
};

static const struct ChanType *svr_chantypes[] = {
	&svrchansess,
#if DROPBEAR_SVR_LOCALTCPFWD
	&svr_chan_tcpdirect,
#endif
	NULL /* Null termination is mandatory. */
};

static void
svr_session_cleanup(void) {
	/* free potential public key options */
	svr_pubkey_options_cleanup();

	m_free(svr_ses.addrstring);
	m_free(svr_ses.remotehost);
	m_free(svr_ses.childpids);
	svr_ses.childpidsize = 0;

#if DROPBEAR_PLUGIN
        if (svr_ses.plugin_handle != NULL) {
            if (svr_ses.plugin_instance) {
                svr_ses.plugin_instance->delete_plugin(svr_ses.plugin_instance);
                svr_ses.plugin_instance = NULL;
            }

            dlclose(svr_ses.plugin_handle);
            svr_ses.plugin_handle = NULL;
        }
#endif
}

void svr_session(int sock, int childpipe) {
	char *host, *port;
	size_t len;

	common_session_init(sock, sock);

	/* Initialise server specific parts of the session */
	svr_ses.childpipe = childpipe;
#if DROPBEAR_VFORK
	svr_ses.server_pid = getpid();
#endif

	/* for logging the remote address */
	get_socket_address(ses.sock_in, NULL, NULL, &host, &port, 0);
	len = strlen(host) + strlen(port) + 2;
	svr_ses.addrstring = m_malloc(len);
	snprintf(svr_ses.addrstring, len, "%s:%s", host, port);
	m_free(host);
	m_free(port);

#if DROPBEAR_PLUGIN
        /* Initializes the PLUGIN Plugin */
        svr_ses.plugin_handle = NULL;
        svr_ses.plugin_instance = NULL;
        if (svr_opts.pubkey_plugin) {
#if DEBUG_TRACE
            const int verbose = debug_trace;
#else
            const int verbose = 0;
#endif
            PubkeyExtPlugin_newFn  pluginConstructor;

            /* RTLD_NOW: fails if not all the symbols are resolved now. Better fail now than at run-time */
            svr_ses.plugin_handle = dlopen(svr_opts.pubkey_plugin, RTLD_NOW);
            if (svr_ses.plugin_handle == NULL) {
                dropbear_exit("failed to load external pubkey plugin '%s': %s", svr_opts.pubkey_plugin, dlerror());
            }
            pluginConstructor = (PubkeyExtPlugin_newFn)dlsym(svr_ses.plugin_handle, DROPBEAR_PUBKEY_PLUGIN_FNNAME_NEW);
            if (!pluginConstructor) {
                dropbear_exit("plugin constructor method not found in external pubkey plugin");
            }

            /* Create an instance of the plugin */
            svr_ses.plugin_instance = pluginConstructor(verbose, svr_opts.pubkey_plugin_options, svr_ses.addrstring);
            if (svr_ses.plugin_instance == NULL) {
                dropbear_exit("external plugin initialization failed");
            }
            /* Check if the plugin is compatible */
            if ( (svr_ses.plugin_instance->api_version[0] != DROPBEAR_PLUGIN_VERSION_MAJOR) ||
                 (svr_ses.plugin_instance->api_version[1] < DROPBEAR_PLUGIN_VERSION_MINOR) ) {
                dropbear_exit("plugin version check failed: "
                              "Dropbear=%d.%d, plugin=%d.%d",
                        DROPBEAR_PLUGIN_VERSION_MAJOR, DROPBEAR_PLUGIN_VERSION_MINOR,
                        svr_ses.plugin_instance->api_version[0], svr_ses.plugin_instance->api_version[1]);
            }
            if (svr_ses.plugin_instance->api_version[1] > DROPBEAR_PLUGIN_VERSION_MINOR) {
                dropbear_log(LOG_WARNING, "plugin API newer than dropbear API: "
                              "Dropbear=%d.%d, plugin=%d.%d",
                        DROPBEAR_PLUGIN_VERSION_MAJOR, DROPBEAR_PLUGIN_VERSION_MINOR,
                        svr_ses.plugin_instance->api_version[0], svr_ses.plugin_instance->api_version[1]);
            }
            dropbear_log(LOG_INFO, "successfully loaded and initialized pubkey plugin '%s'", svr_opts.pubkey_plugin);
        }
#endif

	svr_authinitialise();
	chaninitialise(svr_chantypes);
	svr_chansessinitialise();
	svr_algos_initialise();

	get_socket_address(ses.sock_in, NULL, NULL, 
			&svr_ses.remotehost, NULL, 1);

	/* set up messages etc */
	ses.remoteclosed = svr_remoteclosed;
	ses.extra_session_cleanup = svr_session_cleanup;

	/* packet handlers */
	ses.packettypes = svr_packettypes;

	ses.isserver = 1;

	/* We're ready to go now */
	ses.init_done = 1;

	/* exchange identification, version etc */
	send_session_identification();
	
	kexfirstinitialise(); /* initialise the kex state */

	/* start off with key exchange */
	send_msg_kexinit();

#if DROPBEAR_FUZZ
    if (fuzz.fuzzing) {
        fuzz_svr_hook_preloop();
    }
#endif

	/* Run the main for loop. NULL is for the dispatcher - only the client
	 * code makes use of it */
	session_loop(svr_chansess_checksignal);

	/* Not reached */

}

/* failure exit - format must be <= 100 chars */
void svr_dropbear_exit(int exitcode, const char* format, va_list param) {
	char exitmsg[150];
	char fullmsg[300];
	char fromaddr[60];
	int i;
	int add_delay = 0;

#if DROPBEAR_PLUGIN
        if ((ses.plugin_session != NULL)) {
            svr_ses.plugin_instance->delete_session(ses.plugin_session);
        }
        ses.plugin_session = NULL;
#endif

	/* Render the formatted exit message */
	vsnprintf(exitmsg, sizeof(exitmsg), format, param);

	/* svr_ses.addrstring may not be set for some early exits, or for
	the listener process */
	fromaddr[0] = '\0';
	if (svr_ses.addrstring) {
	    snprintf(fromaddr, sizeof(fromaddr), " from <%s>", svr_ses.addrstring);
    }

	/* Add the prefix depending on session/auth state */
	if (!ses.init_done) {
		/* before session init */
		snprintf(fullmsg, sizeof(fullmsg), "Early exit%s: %s", fromaddr, exitmsg);
	} else if (ses.authstate.authdone) {
		/* user has authenticated */
		snprintf(fullmsg, sizeof(fullmsg),
				"Exit (%s)%s: %s", 
				ses.authstate.pw_name, fromaddr, exitmsg);
	} else if (ses.authstate.pw_name) {
		/* we have a potential user */
		snprintf(fullmsg, sizeof(fullmsg), 
				"Exit before auth%s: (user '%s', %u fails): %s",
				fromaddr, ses.authstate.pw_name, ses.authstate.failcount, exitmsg);
		add_delay = 1;
	} else {
		/* before userauth */
		snprintf(fullmsg, sizeof(fullmsg), "Exit before auth%s: %s", fromaddr, exitmsg);
		add_delay = 1;
	}

	dropbear_log(LOG_INFO, "%s", fullmsg);

	/* To make it harder for attackers, introduce a delay to keep an
	 * unauthenticated session open a bit longer, thus blocking a connection
	 * slot until after the delay. Without this, while there is a limit on
	 * the amount of attempts an attacker can make at the same time
	 * (MAX_UNAUTH_PER_IP), the time taken by dropbear to handle one attempt
	 * is still short and thus for each of the allowed parallel attempts
	 * many attempts can be chained one after the other. The attempt rate is
	 * then:
	 *     "MAX_UNAUTH_PER_IP / <process time of one attempt>".
	 * With the delay, this rate becomes:
	 *     "MAX_UNAUTH_PER_IP / UNAUTH_CLOSE_DELAY".
	 */
	if ((add_delay != 0) && (UNAUTH_CLOSE_DELAY > 0)) {
		TRACE(("svr_dropbear_exit: start delay of %d seconds", UNAUTH_CLOSE_DELAY));
		sleep(UNAUTH_CLOSE_DELAY);
		TRACE(("svr_dropbear_exit: end delay of %d seconds", UNAUTH_CLOSE_DELAY));
	}

#if DROPBEAR_VFORK
	/* For uclinux only the main server process should cleanup - we don't want
	 * forked children doing that */
	if (svr_ses.server_pid == getpid())
#endif
	{
		/* must be after we've done with username etc */
		session_cleanup();
	}

#if DROPBEAR_FUZZ
	/* longjmp before cleaning up svr_opts */
    if (fuzz.do_jmp) {
        longjmp(fuzz.jmp, 1);
    }
#endif

	if (svr_opts.hostkey) {
		sign_key_free(svr_opts.hostkey);
		svr_opts.hostkey = NULL;
	}
	for (i = 0; i < DROPBEAR_MAX_PORTS; i++) {
		m_free(svr_opts.addresses[i]);
		m_free(svr_opts.ports[i]);
	}

    
	exit(exitcode);

}

/* priority is priority as with syslog() */
void svr_dropbear_log(int priority, const char* format, va_list param) {

	char printbuf[1024];
	char datestr[20];
	time_t timesec;
	int havetrace = 0;

	vsnprintf(printbuf, sizeof(printbuf), format, param);

#ifndef DISABLE_SYSLOG
	if (opts.usingsyslog) {
		syslog(priority, "%s", printbuf);
	}
#endif

	/* if we are using DEBUG_TRACE, we want to print to stderr even if
	 * syslog is used, so it is included in error reports */
#if DEBUG_TRACE
	havetrace = debug_trace;
#endif

	if (!opts.usingsyslog || havetrace) {
		struct tm * local_tm = NULL;
		timesec = time(NULL);
		local_tm = localtime(&timesec);
		if (local_tm == NULL
			|| strftime(datestr, sizeof(datestr), "%b %d %H:%M:%S", 
						local_tm) == 0)
		{
			/* upon failure, just print the epoch-seconds time. */
			snprintf(datestr, sizeof(datestr), "%d", (int)timesec);
		}
		fprintf(stderr, "[%d] %s %s\n", getpid(), datestr, printbuf);
	}
}

/* called when the remote side closes the connection */
static void svr_remoteclosed() {

	m_close(ses.sock_in);
	if (ses.sock_in != ses.sock_out) {
		m_close(ses.sock_out);
	}
	ses.sock_in = -1;
	ses.sock_out = -1;
	dropbear_close("Exited normally");

}

static void svr_algos_initialise(void) {
	algo_type *algo;
	for (algo = sshkex; algo->name; algo++) {
#if DROPBEAR_DH_GROUP1 && DROPBEAR_DH_GROUP1_CLIENTONLY
		if (strcmp(algo->name, "diffie-hellman-group1-sha1") == 0) {
			algo->usable = 0;
		}
#endif
#if DROPBEAR_EXT_INFO
		if (strcmp(algo->name, SSH_EXT_INFO_C) == 0) {
			algo->usable = 0;
		}
#endif
	}
}