upstream: Add a sshd_config UnusedConnectionTimeout option to terminate

client connections that have no open channels for some length of time. This
complements the recently-added ChannelTimeout option that terminates inactive
channels after a timeout.

ok markus@

OpenBSD-Commit-ID: ca983be74c0350364c11f8ba3bd692f6f24f5da9

servconf.c

@@ -1,5 +1,5 @@
 
-/* $OpenBSD: servconf.c,v 1.389 2023/01/06 02:47:18 djm Exp $ */
+/* $OpenBSD: servconf.c,v 1.390 2023/01/17 09:44:48 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -198,6 +198,7 @@ initialize_server_options(ServerOptions *options)
 	options->required_rsa_size = -1;
 	options->channel_timeouts = NULL;
 	options->num_channel_timeouts = 0;
+	options->unused_connection_timeout = -1;
 }
 
 /* Returns 1 if a string option is unset or set to "none" or 0 otherwise. */
@@ -446,6 +447,8 @@ fill_default_server_options(ServerOptions *options)
 		options->sk_provider = xstrdup("internal");
 	if (options->required_rsa_size == -1)
 		options->required_rsa_size = SSH_RSA_MINIMUM_MODULUS_SIZE;
+	if (options->unused_connection_timeout == -1)
+		options->unused_connection_timeout = 0;
 
 	assemble_algorithms(options);
 
@@ -529,7 +532,7 @@ typedef enum {
 	sStreamLocalBindMask, sStreamLocalBindUnlink,
 	sAllowStreamLocalForwarding, sFingerprintHash, sDisableForwarding,
 	sExposeAuthInfo, sRDomain, sPubkeyAuthOptions, sSecurityKeyProvider,
-	sRequiredRSASize, sChannelTimeout,
+	sRequiredRSASize, sChannelTimeout, sUnusedConnectionTimeout,
 	sDeprecated, sIgnore, sUnsupported
 } ServerOpCodes;
 
@@ -691,6 +694,7 @@ static struct {
 	{ "securitykeyprovider", sSecurityKeyProvider, SSHCFG_GLOBAL },
 	{ "requiredrsasize", sRequiredRSASize, SSHCFG_ALL },
 	{ "channeltimeout", sChannelTimeout, SSHCFG_ALL },
+	{ "unusedconnectiontimeout", sUnusedConnectionTimeout, SSHCFG_ALL },
 	{ NULL, sBadOption, 0 }
 };
 
@@ -2537,6 +2541,17 @@ process_server_config_line_depth(ServerOptions *options, char *line,
 		}
 		break;
 
+	case sUnusedConnectionTimeout:
+		intptr = &options->unused_connection_timeout;
+		/* peek at first arg for "none" so we can reuse parse_time */
+		if (av[0] != NULL && strcasecmp(av[0], "none") == 0) {
+			(void)argv_next(&ac, &av); /* consume arg */
+			if (*activep)
+				*intptr = 0;
+			break;
+		}
+		goto parse_time;
+
 	case sDeprecated:
 	case sIgnore:
 	case sUnsupported:
@@ -2709,6 +2724,7 @@ copy_set_server_options(ServerOptions *dst, ServerOptions *src, int preauth)
 	M_CP_INTOPT(rekey_interval);
 	M_CP_INTOPT(log_level);
 	M_CP_INTOPT(required_rsa_size);
+	M_CP_INTOPT(unused_connection_timeout);
 
 	/*
 	 * The bind_mask is a mode_t that may be unsigned, so we can't use
@@ -2861,6 +2877,10 @@ fmt_intarg(ServerOpCodes code, int val)
 static void
 dump_cfg_int(ServerOpCodes code, int val)
 {
+	if (code == sUnusedConnectionTimeout && val == 0) {
+		printf("%s none\n", lookup_opcode_name(code));
+		return;
+	}
 	printf("%s %d\n", lookup_opcode_name(code), val);
 }
 
@@ -2977,6 +2997,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_int(sClientAliveCountMax, o->client_alive_count_max);
 	dump_cfg_int(sRequiredRSASize, o->required_rsa_size);
 	dump_cfg_oct(sStreamLocalBindMask, o->fwd_opts.streamlocal_bind_mask);
+	dump_cfg_int(sUnusedConnectionTimeout, o->unused_connection_timeout);
 
 	/* formatted integer arguments */
 	dump_cfg_fmtint(sPermitRootLogin, o->permit_root_login);

servconf.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.158 2023/01/06 02:47:19 djm Exp $ */
+/* $OpenBSD: servconf.h,v 1.159 2023/01/17 09:44:48 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -233,6 +233,8 @@ typedef struct {
 
 	char	**channel_timeouts;	/* inactivity timeout by channel type */
 	u_int	num_channel_timeouts;
+
+	int	unused_connection_timeout;
 }       ServerOptions;
 
 /* Information about the incoming connection as used by Match */

serverloop.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: serverloop.c,v 1.233 2023/01/06 02:38:23 djm Exp $ */
+/* $OpenBSD: serverloop.c,v 1.234 2023/01/17 09:44:48 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -172,17 +172,19 @@ wait_until_can_do_something(struct ssh *ssh,
     int *conn_in_readyp, int *conn_out_readyp)
 {
 	struct timespec timeout;
+	char remote_id[512];
 	int ret;
 	int client_alive_scheduled = 0;
 	u_int p;
-	/* time we last heard from the client OR sent a keepalive */
-	static time_t last_client_time;
+	time_t now;
+	static time_t last_client_time, unused_connection_expiry;
 
 	*conn_in_readyp = *conn_out_readyp = 0;
 
 	/* Prepare channel poll. First two pollfd entries are reserved */
 	ptimeout_init(&timeout);
 	channel_prepare_poll(ssh, pfdp, npfd_allocp, npfd_activep, 2, &timeout);
+	now = monotime();
 	if (*npfd_activep < 2)
 		fatal_f("bad npfd %u", *npfd_activep); /* shouldn't happen */
 	if (options.rekey_interval > 0 && !ssh_packet_is_rekeying(ssh)) {
@@ -190,6 +192,18 @@ wait_until_can_do_something(struct ssh *ssh,
 		    ssh_packet_get_rekey_timeout(ssh));
 	}
 
+	/*
+	 * If no channels are open and UnusedConnectionTimeout is set, then
+	 * start the clock to terminate the connection.
+	 */
+	if (options.unused_connection_timeout != 0) {
+		if (channel_still_open(ssh) || unused_connection_expiry == 0) {
+			unused_connection_expiry = now +
+			    options.unused_connection_timeout;
+		}
+		ptimeout_deadline_monotime(&timeout, unused_connection_expiry);
+	}
+
 	/*
 	 * if using client_alive, set the max timeout accordingly,
 	 * and indicate that this particular timeout was for client
@@ -199,8 +213,9 @@ wait_until_can_do_something(struct ssh *ssh,
 	 * analysis more difficult, but we're not doing it yet.
 	 */
 	if (options.client_alive_interval) {
+		/* Time we last heard from the client OR sent a keepalive */
 		if (last_client_time == 0)
-			last_client_time = monotime();
+			last_client_time = now;
 		ptimeout_deadline_sec(&timeout, options.client_alive_interval);
 		/* XXX ? deadline_monotime(last_client_time + alive_interval) */
 		client_alive_scheduled = 1;
@@ -237,9 +252,9 @@ wait_until_can_do_something(struct ssh *ssh,
 	*conn_in_readyp = (*pfdp)[0].revents != 0;
 	*conn_out_readyp = (*pfdp)[1].revents != 0;
 
+	now = monotime(); /* need to reset after ppoll() */
 	/* ClientAliveInterval probing */
 	if (client_alive_scheduled) {
-		time_t now = monotime();
 		if (ret == 0 &&
 		    now > last_client_time + options.client_alive_interval) {
 			/* ppoll timed out and we're due to probe */
@@ -250,6 +265,14 @@ wait_until_can_do_something(struct ssh *ssh,
 			last_client_time = now;
 		}
 	}
+
+	/* UnusedConnectionTimeout handling */
+	if (unused_connection_expiry != 0 &&
+	    now > unused_connection_expiry && !channel_still_open(ssh)) {
+		sshpkt_fmt_connection_id(ssh, remote_id, sizeof(remote_id));
+		logit("terminating inactive connection from %s", remote_id);
+		cleanup_exit(255);
+	}
 }
 
 /*

sshd_config.5

@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: sshd_config.5,v 1.345 2023/01/06 08:44:11 jmc Exp $
-.Dd $Mdocdate: January 6 2023 $
+.\" $OpenBSD: sshd_config.5,v 1.346 2023/01/17 09:44:48 djm Exp $
+.Dd $Mdocdate: January 17 2023 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -459,6 +459,9 @@ close the SSH connection, nor does it prevent a client from
 requesting another channel of the same type.
 In particular, expiring an inactive forwarding session does not prevent
 another identical forwarding from being subsequently created.
+See also
+.Cm UnusedConnectionTimeout ,
+which may be used in conjunction with this option.
 .Pp
 The default is not to expire channels of any type for inactivity.
 .It Cm ChrootDirectory
@@ -1256,6 +1259,7 @@ Available keywords are
 .Cm AuthorizedPrincipalsFile ,
 .Cm Banner ,
 .Cm CASignatureAlgorithms ,
+.Cm ChannelTimeout ,
 .Cm ChrootDirectory ,
 .Cm ClientAliveCountMax ,
 .Cm ClientAliveInterval ,
@@ -1295,6 +1299,7 @@ Available keywords are
 .Cm StreamLocalBindMask ,
 .Cm StreamLocalBindUnlink ,
 .Cm TrustedUserCAKeys ,
+.Cm UnusedConnectionTimeout ,
 .Cm X11DisplayOffset ,
 .Cm X11Forwarding
 and
@@ -1811,6 +1816,33 @@ for authentication using
 .Cm TrustedUserCAKeys .
 For more details on certificates, see the CERTIFICATES section in
 .Xr ssh-keygen 1 .
+.It Cm UnusedConnectionTimeout
+Specifies whether and how quickly
+.Xr sshd 8
+should close client connections with no open channels.
+Open channels include active shell, command execution or subsystem
+sessions, connected network, socket, agent of X11 forwardings.
+Forwarding listeners, such as those from the
+.Xr ssh 1
+.Fl R
+flag are not considered as open channels and do not prevent the timeout.
+The timeout value
+is specified in seconds or may use any of the units documented in the
+.Sx TIME FORMATS
+section.
+.Pp
+Note that this timeout starts when the client connection completes
+user authentication but before the client has an opportunity to open any
+channels.
+Caution should be used when using short timeout values, as they may not
+provide sufficient time for the client to request and open its channels
+before terminating the connection.
+.Pp
+The default
+.Cm none
+is to never expire connections for having no open channels.
+This option may be useful in conjunction with
+.Cm ChannelTimeout .
 .It Cm UseDNS
 Specifies whether
 .Xr sshd 8