view svr-main.c @ 1861:2b3a8026a6ce

Add re-exec for server This allows ASLR to re-randomize the address space for every connection, preventing some vulnerabilities from being exploitable by repeated probing. Overhead (memory and time) is yet to be confirmed. At present this is only enabled on Linux. Other BSD platforms with fexecve() would probably also work though have not been tested.
author Matt Johnston <matt@ucc.asn.au>
date Sun, 30 Jan 2022 10:14:56 +0800
parents 6ea18ca8fc03
children adfcdfb161a4
line wrap: on
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/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002-2006 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 "dbutil.h"
#include "session.h"
#include "buffer.h"
#include "signkey.h"
#include "runopts.h"
#include "dbrandom.h"
#include "crypto_desc.h"

static size_t listensockets(int *sock, size_t sockcount, int *maxfd);
static void sigchld_handler(int dummy);
static void sigsegv_handler(int);
static void sigintterm_handler(int fish);
static void main_inetd(void);
static void main_noinetd(int argc, char ** argv);
static void commonsetup(void);

#if defined(DBMULTI_dropbear) || !DROPBEAR_MULTI
#if defined(DBMULTI_dropbear) && DROPBEAR_MULTI
int dropbear_main(int argc, char ** argv)
#else
int main(int argc, char ** argv)
#endif
{
	_dropbear_exit = svr_dropbear_exit;
	_dropbear_log = svr_dropbear_log;

	disallow_core();

	if (argc < 1) {
		dropbear_exit("Bad argc");
	}

	/* get commandline options */
	svr_getopts(argc, argv);

#if INETD_MODE
	/* service program mode */
	if (svr_opts.inetdmode) {
		main_inetd();
		/* notreached */
	}
#endif

#if DROPBEAR_DO_REEXEC
	if (svr_opts.reexec_child) {
#ifdef PR_SET_NAME
		/* Fix the "Name:" in /proc/pid/status, otherwise it's
		a FD number from fexecve.
		Failure doesn't really matter, it's mostly aesthetic */
		prctl(PR_SET_NAME, basename(argv[0]), 0, 0);
#endif
		main_inetd();
		/* notreached */
	}
#endif

#if NON_INETD_MODE
	main_noinetd(argc, argv);
	/* notreached */
#endif

	dropbear_exit("Compiled without normal mode, can't run without -i\n");
	return -1;
}
#endif

#if INETD_MODE || DROPBEAR_DO_REEXEC
static void main_inetd() {
	char *host, *port = NULL;

	/* Set up handlers, syslog */
	commonsetup();

	seedrandom();

	if (!svr_opts.reexec_child) {
		/* In case our inetd was lax in logging source addresses */
		get_socket_address(0, NULL, NULL, &host, &port, 0);
			dropbear_log(LOG_INFO, "Child connection from %s:%s", host, port);
		m_free(host);
		m_free(port);

		/* Don't check the return value - it may just fail since inetd has
		 * already done setsid() after forking (xinetd on Darwin appears to do
		 * this */
		setsid();
	}

	/* Start service program 
	 * -1 is a dummy childpipe, just something we can close() without 
	 * mattering. */
	svr_session(0, -1);

	/* notreached */
}
#endif /* INETD_MODE */

#if NON_INETD_MODE
static void main_noinetd(int argc, char ** argv) {
	fd_set fds;
	unsigned int i, j;
	int val;
	int maxsock = -1;
	int listensocks[MAX_LISTEN_ADDR];
	size_t listensockcount = 0;
	FILE *pidfile = NULL;
	int execfd = -1;

	int childpipes[MAX_UNAUTH_CLIENTS];
	char * preauth_addrs[MAX_UNAUTH_CLIENTS];

	int childsock;
	int childpipe[2];

	(void)argc;
	(void)argv;

	/* Note: commonsetup() must happen before we daemon()ise. Otherwise
	   daemon() will chdir("/"), and we won't be able to find local-dir
	   hostkeys. */
	commonsetup();

	/* sockets to identify pre-authenticated clients */
	for (i = 0; i < MAX_UNAUTH_CLIENTS; i++) {
		childpipes[i] = -1;
	}
	memset(preauth_addrs, 0x0, sizeof(preauth_addrs));

	/* Set up the listening sockets */
	listensockcount = listensockets(listensocks, MAX_LISTEN_ADDR, &maxsock);
	if (listensockcount == 0)
	{
		dropbear_exit("No listening ports available.");
	}

	for (i = 0; i < listensockcount; i++) {
		FD_SET(listensocks[i], &fds);
	}

#if DROPBEAR_DO_REEXEC
	execfd = open(argv[0], O_CLOEXEC|O_RDONLY);
	if (execfd < 0) {
		/* Just fallback to straight fork */
		TRACE(("Couldn't open own binary %s, disabling re-exec: %s", argv[0], strerror(errno)))
	}
#endif

	/* fork */
	if (svr_opts.forkbg) {
		int closefds = 0;
#if !DEBUG_TRACE
		if (!opts.usingsyslog) {
			closefds = 1;
		}
#endif
		if (daemon(0, closefds) < 0) {
			dropbear_exit("Failed to daemonize: %s", strerror(errno));
		}
	}

	/* should be done after syslog is working */
	if (svr_opts.forkbg) {
		dropbear_log(LOG_INFO, "Running in background");
	} else {
		dropbear_log(LOG_INFO, "Not backgrounding");
	}

	/* create a PID file so that we can be killed easily */
	pidfile = fopen(svr_opts.pidfile, "w");
	if (pidfile) {
		fprintf(pidfile, "%d\n", getpid());
		fclose(pidfile);
	}

	/* incoming connection select loop */
	for(;;) {

		DROPBEAR_FD_ZERO(&fds);

		/* listening sockets */
		for (i = 0; i < listensockcount; i++) {
			FD_SET(listensocks[i], &fds);
		}

		/* pre-authentication clients */
		for (i = 0; i < MAX_UNAUTH_CLIENTS; i++) {
			if (childpipes[i] >= 0) {
				FD_SET(childpipes[i], &fds);
				maxsock = MAX(maxsock, childpipes[i]);
			}
		}

		val = select(maxsock+1, &fds, NULL, NULL, NULL);

		if (ses.exitflag) {
			unlink(svr_opts.pidfile);
			dropbear_exit("Terminated by signal");
		}

		if (val == 0) {
			/* timeout reached - shouldn't happen. eh */
			continue;
		}

		if (val < 0) {
			if (errno == EINTR) {
				continue;
			}
			dropbear_exit("Listening socket error");
		}

		/* close fds which have been authed or closed - svr-auth.c handles
		 * closing the auth sockets on success */
		for (i = 0; i < MAX_UNAUTH_CLIENTS; i++) {
			if (childpipes[i] >= 0 && FD_ISSET(childpipes[i], &fds)) {
				m_close(childpipes[i]);
				childpipes[i] = -1;
				m_free(preauth_addrs[i]);
			}
		}

		/* handle each socket which has something to say */
		for (i = 0; i < listensockcount; i++) {
			size_t num_unauthed_for_addr = 0;
			size_t num_unauthed_total = 0;
			char *remote_host = NULL, *remote_port = NULL;
			pid_t fork_ret = 0;
			size_t conn_idx = 0;
			struct sockaddr_storage remoteaddr;
			socklen_t remoteaddrlen;

			if (!FD_ISSET(listensocks[i], &fds)) 
				continue;

			remoteaddrlen = sizeof(remoteaddr);
			childsock = accept(listensocks[i], 
					(struct sockaddr*)&remoteaddr, &remoteaddrlen);

			if (childsock < 0) {
				/* accept failed */
				continue;
			}

			/* Limit the number of unauthenticated connections per IP */
			getaddrstring(&remoteaddr, &remote_host, NULL, 0);

			num_unauthed_for_addr = 0;
			num_unauthed_total = 0;
			for (j = 0; j < MAX_UNAUTH_CLIENTS; j++) {
				if (childpipes[j] >= 0) {
					num_unauthed_total++;
					if (strcmp(remote_host, preauth_addrs[j]) == 0) {
						num_unauthed_for_addr++;
					}
				} else {
					/* a free slot */
					conn_idx = j;
				}
			}

			if (num_unauthed_total >= MAX_UNAUTH_CLIENTS
					|| num_unauthed_for_addr >= MAX_UNAUTH_PER_IP) {
				goto out;
			}

			seedrandom();

			if (pipe(childpipe) < 0) {
				TRACE(("error creating child pipe"))
				goto out;
			}

#if DEBUG_NOFORK
			fork_ret = 0;
#else
			fork_ret = fork();
#endif
			if (fork_ret < 0) {
				dropbear_log(LOG_WARNING, "Error forking: %s", strerror(errno));
				goto out;
			}

			addrandom((void*)&fork_ret, sizeof(fork_ret));

			if (fork_ret > 0) {

				/* parent */
				childpipes[conn_idx] = childpipe[0];
				m_close(childpipe[1]);
				preauth_addrs[conn_idx] = remote_host;
				remote_host = NULL;

			} else {

				/* child */
				getaddrstring(&remoteaddr, NULL, &remote_port, 0);
				dropbear_log(LOG_INFO, "Child connection from %s:%s", remote_host, remote_port);
				m_free(remote_host);
				m_free(remote_port);

#ifndef DEBUG_NOFORK
				if (setsid() < 0) {
					dropbear_exit("setsid: %s", strerror(errno));
				}
#endif

				/* make sure we close sockets */
				for (j = 0; j < listensockcount; j++) {
					m_close(listensocks[j]);
				}

				m_close(childpipe[0]);

				if (execfd >= 0) {
#if DROPBEAR_DO_REEXEC
					/* Add "-2" to the args and re-execute ourself */
					char **new_argv = m_malloc(sizeof(char*) * (argc+1));
					memcpy(new_argv, argv, sizeof(char*) * argc);
					new_argv[argc] = "-2";

					if ((dup2(childsock, STDIN_FILENO) < 0)) {
						dropbear_exit("dup2 failed: %s", strerror(errno));
					}
					m_close(childsock);
					/* Re-execute ourself */
					fexecve(execfd, new_argv, environ);
					/* Not reached on success */

					/* Fall back on plain fork otherwise */
					TRACE(("fexecve failed, disabling re-exec: %s", strerror(errno)))
					m_free(new_argv);
#endif /* DROPBEAR_DO_REEXEC */
				}

				/* start the session */
				svr_session(childsock, childpipe[1]);
				/* don't return */
				dropbear_assert(0);
			}

out:
			/* This section is important for the parent too */
			m_close(childsock);
			if (remote_host) {
				m_free(remote_host);
			}
		}
	} /* for(;;) loop */

	/* don't reach here */
}
#endif /* NON_INETD_MODE */


/* catch + reap zombie children */
static void sigchld_handler(int UNUSED(unused)) {
	struct sigaction sa_chld;

	const int saved_errno = errno;

	while(waitpid(-1, NULL, WNOHANG) > 0) {}

	sa_chld.sa_handler = sigchld_handler;
	sa_chld.sa_flags = SA_NOCLDSTOP;
	sigemptyset(&sa_chld.sa_mask);
	if (sigaction(SIGCHLD, &sa_chld, NULL) < 0) {
		dropbear_exit("signal() error");
	}
	errno = saved_errno;
}

/* catch any segvs */
static void sigsegv_handler(int UNUSED(unused)) {
	fprintf(stderr, "Aiee, segfault! You should probably report "
			"this as a bug to the developer\n");
	_exit(EXIT_FAILURE);
}

/* catch ctrl-c or sigterm */
static void sigintterm_handler(int UNUSED(unused)) {

	ses.exitflag = 1;
}

/* Things used by inetd and non-inetd modes */
static void commonsetup() {

	struct sigaction sa_chld;
#ifndef DISABLE_SYSLOG
	if (opts.usingsyslog) {
		startsyslog(PROGNAME);
	}
#endif

	/* set up cleanup handler */
	if (signal(SIGINT, sigintterm_handler) == SIG_ERR || 
#ifndef DEBUG_VALGRIND
		signal(SIGTERM, sigintterm_handler) == SIG_ERR ||
#endif
		signal(SIGPIPE, SIG_IGN) == SIG_ERR) {
		dropbear_exit("signal() error");
	}

	/* catch and reap zombie children */
	sa_chld.sa_handler = sigchld_handler;
	sa_chld.sa_flags = SA_NOCLDSTOP;
	sigemptyset(&sa_chld.sa_mask);
	if (sigaction(SIGCHLD, &sa_chld, NULL) < 0) {
		dropbear_exit("signal() error");
	}
	if (signal(SIGSEGV, sigsegv_handler) == SIG_ERR) {
		dropbear_exit("signal() error");
	}

	crypto_init();

	/* Now we can setup the hostkeys - needs to be after logging is on,
	 * otherwise we might end up blatting error messages to the socket */
	load_all_hostkeys();
}

/* Set up listening sockets for all the requested ports */
static size_t listensockets(int *socks, size_t sockcount, int *maxfd) {

	unsigned int i, n;
	char* errstring = NULL;
	size_t sockpos = 0;
	int nsock;

	TRACE(("listensockets: %d to try", svr_opts.portcount))

	for (i = 0; i < svr_opts.portcount; i++) {

		TRACE(("listening on '%s:%s'", svr_opts.addresses[i], svr_opts.ports[i]))

		nsock = dropbear_listen(svr_opts.addresses[i], svr_opts.ports[i], &socks[sockpos], 
				sockcount - sockpos,
				&errstring, maxfd);

		if (nsock < 0) {
			dropbear_log(LOG_WARNING, "Failed listening on '%s': %s", 
							svr_opts.ports[i], errstring);
			m_free(errstring);
			continue;
		}

		for (n = 0; n < (unsigned int)nsock; n++) {
			int sock = socks[sockpos + n];
			set_sock_priority(sock, DROPBEAR_PRIO_LOWDELAY);
#if DROPBEAR_SERVER_TCP_FAST_OPEN
			set_listen_fast_open(sock);
#endif
		}

		sockpos += nsock;

	}
	return sockpos;
}