upstream: Add a facility to sshd(8) to penalise particular

problematic client behaviours, controlled by two new sshd_config(5) options:
PerSourcePenalties and PerSourcePenaltyExemptList.

When PerSourcePenalties are enabled, sshd(8) will monitor the exit
status of its child pre-auth session processes. Through the exit
status, it can observe situations where the session did not
authenticate as expected. These conditions include when the client
repeatedly attempted authentication unsucessfully (possibly indicating
an attack against one or more accounts, e.g. password guessing), or
when client behaviour caused sshd to crash (possibly indicating
attempts to exploit sshd).

When such a condition is observed, sshd will record a penalty of some
duration (e.g. 30 seconds) against the client's address. If this time
is above a minimum threshold specified by the PerSourcePenalties, then
connections from the client address will be refused (along with any
others in the same PerSourceNetBlockSize CIDR range).

Repeated offenses by the same client address will accrue greater
penalties, up to a configurable maximum. A PerSourcePenaltyExemptList
option allows certain address ranges to be exempt from all penalties.

We hope these options will make it significantly more difficult for
attackers to find accounts with weak/guessable passwords or exploit
bugs in sshd(8) itself.

PerSourcePenalties is off by default, but we expect to enable it
automatically in the near future.

much feedback markus@ and others, ok markus@

OpenBSD-Commit-ID: 89ded70eccb2b4926ef0366a4d58a693de366cca

misc.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: misc.c,v 1.195 2024/05/17 06:11:17 deraadt Exp $ */
+/* $OpenBSD: misc.c,v 1.196 2024/06/06 17:15:25 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2005-2020 Damien Miller.  All rights reserved.
@@ -3100,3 +3100,19 @@ lib_contains_symbol(const char *path, const char *s)
 	return ret;
 #endif /* HAVE_NLIST_H */
 }
+
+int
+signal_is_crash(int sig)
+{
+	switch (sig) {
+	case SIGSEGV:
+	case SIGBUS:
+	case SIGTRAP:
+	case SIGSYS:
+	case SIGFPE:
+	case SIGILL:
+	case SIGABRT:
+		return 1;
+	}
+	return 0;
+}

misc.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: misc.h,v 1.108 2024/05/17 00:30:24 djm Exp $ */
+/* $OpenBSD: misc.h,v 1.109 2024/06/06 17:15:25 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <[email protected]>
@@ -252,6 +252,7 @@ void	notify_complete(struct notifier_ctx *, const char *, ...)
 
 typedef void (*sshsig_t)(int);
 sshsig_t ssh_signal(int, sshsig_t);
+int signal_is_crash(int);
 
 /* On OpenBSD time_t is int64_t which is long long. */
 /* #define SSH_TIME_T_MAX LLONG_MAX */

monitor.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: monitor.c,v 1.239 2024/05/17 06:42:04 jsg Exp $ */
+/* $OpenBSD: monitor.c,v 1.240 2024/06/06 17:15:25 djm Exp $ */
 /*
  * Copyright 2002 Niels Provos <[email protected]>
  * Copyright 2002 Markus Friedl <[email protected]>
@@ -161,6 +161,7 @@ static char *auth_submethod = NULL;
 static u_int session_id2_len = 0;
 static u_char *session_id2 = NULL;
 static pid_t monitor_child_pid;
+int auth_attempted = 0;
 
 struct mon_table {
 	enum monitor_reqtype type;
@@ -296,6 +297,10 @@ monitor_child_preauth(struct ssh *ssh, struct monitor *pmonitor)
 		authenticated = (monitor_read(ssh, pmonitor,
 		    mon_dispatch, &ent) == 1);
 
+		/* Record that auth was attempted to set exit status later */
+		if ((ent->flags & MON_AUTH) != 0)
+			auth_attempted = 1;
+
 		/* Special handling for multiple required authentications */
 		if (options.num_auth_methods != 0) {
 			if (authenticated &&
@@ -353,6 +358,7 @@ monitor_child_preauth(struct ssh *ssh, struct monitor *pmonitor)
 		fatal_f("authentication method name unknown");
 
 	debug_f("user %s authenticated by privileged process", authctxt->user);
+	auth_attempted = 0;
 	ssh->authctxt = NULL;
 	ssh_packet_set_log_preamble(ssh, "user %s", authctxt->user);
 

monitor_wrap.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: monitor_wrap.c,v 1.130 2024/05/17 00:30:24 djm Exp $ */
+/* $OpenBSD: monitor_wrap.c,v 1.131 2024/06/06 17:15:25 djm Exp $ */
 /*
  * Copyright 2002 Niels Provos <[email protected]>
  * Copyright 2002 Markus Friedl <[email protected]>
@@ -29,6 +29,7 @@
 
 #include <sys/types.h>
 #include <sys/uio.h>
+#include <sys/wait.h>
 
 #include <errno.h>
 #include <pwd.h>
@@ -73,6 +74,7 @@
 #include "session.h"
 #include "servconf.h"
 #include "monitor_wrap.h"
+#include "srclimit.h"
 
 #include "ssherr.h"
 
@@ -137,6 +139,36 @@ mm_request_send(int sock, enum monitor_reqtype type, struct sshbuf *m)
 		fatal_f("write: %s", strerror(errno));
 }
 
+static void
+mm_reap(void)
+{
+	int status = -1;
+
+	if (!mm_is_monitor())
+		return;
+	while (waitpid(pmonitor->m_pid, &status, 0) == -1) {
+		if (errno == EINTR)
+			continue;
+		pmonitor->m_pid = -1;
+		fatal_f("waitpid: %s", strerror(errno));
+	}
+	if (WIFEXITED(status)) {
+		if (WEXITSTATUS(status) != 0) {
+			debug_f("preauth child exited with status %d",
+			    WEXITSTATUS(status));
+			cleanup_exit(255);
+		}
+	} else if (WIFSIGNALED(status)) {
+		error_f("preauth child terminated by signal %d",
+		    WTERMSIG(status));
+		cleanup_exit(signal_is_crash(WTERMSIG(status)) ?
+		    EXIT_CHILD_CRASH : 255);
+	} else {
+		error_f("preauth child terminated abnormally");
+		cleanup_exit(EXIT_CHILD_CRASH);
+	}
+}
+
 void
 mm_request_receive(int sock, struct sshbuf *m)
 {
@@ -149,6 +181,7 @@ mm_request_receive(int sock, struct sshbuf *m)
 	if (atomicio(read, sock, buf, sizeof(buf)) != sizeof(buf)) {
 		if (errno == EPIPE) {
 			debug3_f("monitor fd closed");
+			mm_reap();
 			cleanup_exit(255);
 		}
 		fatal_f("read: %s", strerror(errno));

servconf.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.c,v 1.407 2024/05/17 01:17:40 djm Exp $ */
+/* $OpenBSD: servconf.c,v 1.408 2024/06/06 17:15:25 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
  *                    All rights reserved
@@ -163,6 +163,16 @@ initialize_server_options(ServerOptions *options)
 	options->per_source_max_startups = -1;
 	options->per_source_masklen_ipv4 = -1;
 	options->per_source_masklen_ipv6 = -1;
+	options->per_source_penalty_exempt = NULL;
+	options->per_source_penalty.enabled = -1;
+	options->per_source_penalty.max_sources = -1;
+	options->per_source_penalty.overflow_mode = -1;
+	options->per_source_penalty.penalty_crash = -1;
+	options->per_source_penalty.penalty_authfail = -1;
+	options->per_source_penalty.penalty_noauth = -1;
+	options->per_source_penalty.penalty_grace = -1;
+	options->per_source_penalty.penalty_max = -1;
+	options->per_source_penalty.penalty_min = -1;
 	options->max_authtries = -1;
 	options->max_sessions = -1;
 	options->banner = NULL;
@@ -402,6 +412,24 @@ fill_default_server_options(ServerOptions *options)
 		options->per_source_masklen_ipv4 = 32;
 	if (options->per_source_masklen_ipv6 == -1)
 		options->per_source_masklen_ipv6 = 128;
+	if (options->per_source_penalty.enabled == -1)
+		options->per_source_penalty.enabled = 0;
+	if (options->per_source_penalty.max_sources == -1)
+		options->per_source_penalty.max_sources = 65536;
+	if (options->per_source_penalty.overflow_mode == -1)
+		options->per_source_penalty.overflow_mode = PER_SOURCE_PENALTY_OVERFLOW_PERMISSIVE;
+	if (options->per_source_penalty.penalty_crash == -1)
+		options->per_source_penalty.penalty_crash = 90;
+	if (options->per_source_penalty.penalty_grace == -1)
+		options->per_source_penalty.penalty_grace = 20;
+	if (options->per_source_penalty.penalty_authfail == -1)
+		options->per_source_penalty.penalty_authfail = 5;
+	if (options->per_source_penalty.penalty_noauth == -1)
+		options->per_source_penalty.penalty_noauth = 1;
+	if (options->per_source_penalty.penalty_min == -1)
+		options->per_source_penalty.penalty_min = 15;
+	if (options->per_source_penalty.penalty_max == -1)
+		options->per_source_penalty.penalty_max = 600;
 	if (options->max_authtries == -1)
 		options->max_authtries = DEFAULT_AUTH_FAIL_MAX;
 	if (options->max_sessions == -1)
@@ -479,6 +507,7 @@ fill_default_server_options(ServerOptions *options)
 	CLEAR_ON_NONE(options->chroot_directory);
 	CLEAR_ON_NONE(options->routing_domain);
 	CLEAR_ON_NONE(options->host_key_agent);
+	CLEAR_ON_NONE(options->per_source_penalty_exempt);
 
 	for (i = 0; i < options->num_host_key_files; i++)
 		CLEAR_ON_NONE(options->host_key_files[i]);
@@ -513,6 +542,7 @@ typedef enum {
 	sBanner, sUseDNS, sHostbasedAuthentication,
 	sHostbasedUsesNameFromPacketOnly, sHostbasedAcceptedAlgorithms,
 	sHostKeyAlgorithms, sPerSourceMaxStartups, sPerSourceNetBlockSize,
+	sPerSourcePenalties, sPerSourcePenaltyExemptList,
 	sClientAliveInterval, sClientAliveCountMax, sAuthorizedKeysFile,
 	sGssAuthentication, sGssCleanupCreds, sGssStrictAcceptor,
 	sAcceptEnv, sSetEnv, sPermitTunnel,
@@ -645,6 +675,8 @@ static struct {
 	{ "maxstartups", sMaxStartups, SSHCFG_GLOBAL },
 	{ "persourcemaxstartups", sPerSourceMaxStartups, SSHCFG_GLOBAL },
 	{ "persourcenetblocksize", sPerSourceNetBlockSize, SSHCFG_GLOBAL },
+	{ "persourcepenalties", sPerSourcePenalties, SSHCFG_GLOBAL },
+	{ "persourcepenaltyexemptlist", sPerSourcePenaltyExemptList, SSHCFG_GLOBAL },
 	{ "maxauthtries", sMaxAuthTries, SSHCFG_ALL },
 	{ "maxsessions", sMaxSessions, SSHCFG_ALL },
 	{ "banner", sBanner, SSHCFG_ALL },
@@ -1945,6 +1977,89 @@ process_server_config_line_depth(ServerOptions *options, char *line,
 			options->per_source_max_startups = value;
 		break;
 
+	case sPerSourcePenaltyExemptList:
+		charptr = &options->per_source_penalty_exempt;
+		arg = argv_next(&ac, &av);
+		if (!arg || *arg == '\0')
+			fatal("%s line %d: missing file name.",
+			    filename, linenum);
+		if (addr_match_list(NULL, arg) != 0) {
+			fatal("%s line %d: keyword %s "
+			    "invalid address argument.",
+			    filename, linenum, keyword);
+		}
+		if (*activep && *charptr == NULL)
+			*charptr = xstrdup(arg);
+		break;
+
+	case sPerSourcePenalties:
+		while ((arg = argv_next(&ac, &av)) != NULL) {
+			found = 1;
+			value = -1;
+			value2 = 0;
+			p = NULL;
+			/* Allow no/yes only in first position */
+			if (strcasecmp(arg, "no") == 0 ||
+			    (value2 = (strcasecmp(arg, "yes") == 0))) {
+				if (ac > 0) {
+					fatal("%s line %d: keyword %s \"%s\" "
+					    "argument must appear alone.",
+					    filename, linenum, keyword, arg);
+				}
+				if (*activep &&
+				    options->per_source_penalty.enabled == -1)
+					options->per_source_penalty.enabled = value2;
+				continue;
+			} else if (strncmp(arg, "crash:", 6) == 0) {
+				p = arg + 6;
+				intptr = &options->per_source_penalty.penalty_crash;
+			} else if (strncmp(arg, "authfail:", 9) == 0) {
+				p = arg + 9;
+				intptr = &options->per_source_penalty.penalty_authfail;
+			} else if (strncmp(arg, "noauth:", 7) == 0) {
+				p = arg + 7;
+				intptr = &options->per_source_penalty.penalty_noauth;
+			} else if (strncmp(arg, "grace-exceeded:", 15) == 0) {
+				p = arg + 15;
+				intptr = &options->per_source_penalty.penalty_grace;
+			} else if (strncmp(arg, "max:", 4) == 0) {
+				p = arg + 4;
+				intptr = &options->per_source_penalty.penalty_max;
+			} else if (strncmp(arg, "min:", 4) == 0) {
+				p = arg + 4;
+				intptr = &options->per_source_penalty.penalty_min;
+			} else if (strncmp(arg, "max-sources:", 12) == 0) {
+				intptr = &options->per_source_penalty.max_sources;
+				if ((errstr = atoi_err(arg+12, &value)) != NULL)
+					fatal("%s line %d: %s value %s.",
+					    filename, linenum, keyword, errstr);
+			} else if (strcmp(arg, "overflow:deny-all") == 0) {
+				intptr = &options->per_source_penalty.overflow_mode;
+				value = PER_SOURCE_PENALTY_OVERFLOW_DENY_ALL;
+			} else if (strcmp(arg, "overflow:permissive") == 0) {
+				intptr = &options->per_source_penalty.overflow_mode;
+				value = PER_SOURCE_PENALTY_OVERFLOW_PERMISSIVE;
+			} else {
+				fatal("%s line %d: unsupported %s keyword %s",
+				    filename, linenum, keyword, arg);
+			}
+			/* If no value was parsed above, assume it's a time */
+			if (value == -1 && (value = convtime(p)) == -1) {
+				fatal("%s line %d: invalid %s time value.",
+				    filename, linenum, keyword);
+			}
+			if (*activep && *intptr == -1) {
+				*intptr = value;
+				/* any option implicitly enables penalties */
+				options->per_source_penalty.enabled = 1;
+			}
+		}
+		if (!found) {
+			fatal("%s line %d: no %s specified",
+			    filename, linenum, keyword);
+		}
+		break;
+
 	case sMaxAuthTries:
 		intptr = &options->max_authtries;
 		goto parse_int;
@@ -3082,6 +3197,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_string(sRDomain, o->routing_domain);
 #endif
 	dump_cfg_string(sSshdSessionPath, o->sshd_session_path);
+	dump_cfg_string(sPerSourcePenaltyExemptList, o->per_source_penalty_exempt);
 
 	/* string arguments requiring a lookup */
 	dump_cfg_string(sLogLevel, log_level_name(o->log_level));
@@ -3169,4 +3285,20 @@ dump_config(ServerOptions *o)
 	if (o->pubkey_auth_options & PUBKEYAUTH_VERIFY_REQUIRED)
 		printf(" verify-required");
 	printf("\n");
+
+	if (o->per_source_penalty.enabled) {
+		printf("persourcepenalties crash:%d authfail:%d noauth:%d "
+		    "grace-exceeded:%d max:%d min:%d max-sources:%d "
+		    "overflow:%s\n", o->per_source_penalty.penalty_crash,
+		    o->per_source_penalty.penalty_authfail,
+		    o->per_source_penalty.penalty_noauth,
+		    o->per_source_penalty.penalty_grace,
+		    o->per_source_penalty.penalty_max,
+		    o->per_source_penalty.penalty_min,
+		    o->per_source_penalty.max_sources,
+		    o->per_source_penalty.overflow_mode ==
+		    PER_SOURCE_PENALTY_OVERFLOW_DENY_ALL ?
+		    "deny-all" : "permissive");
+	} else
+		printf("persourcepenalties no\n");
 }

servconf.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.163 2024/05/23 23:47:16 jsg Exp $ */
+/* $OpenBSD: servconf.h,v 1.164 2024/06/06 17:15:25 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <[email protected]>
@@ -65,6 +65,20 @@ struct listenaddr {
 	struct addrinfo *addrs;
 };
 
+#define PER_SOURCE_PENALTY_OVERFLOW_DENY_ALL	1
+#define PER_SOURCE_PENALTY_OVERFLOW_PERMISSIVE	2
+struct per_source_penalty {
+	int	enabled;
+	int	max_sources;
+	int	overflow_mode;
+	int	penalty_crash;
+	int	penalty_grace;
+	int	penalty_authfail;
+	int	penalty_noauth;
+	int	penalty_max;
+	int	penalty_min;
+};
+
 typedef struct {
 	u_int	num_ports;
 	u_int	ports_from_cmdline;
@@ -172,6 +186,8 @@ typedef struct {
 	int	per_source_max_startups;
 	int	per_source_masklen_ipv4;
 	int	per_source_masklen_ipv6;
+	char	*per_source_penalty_exempt;
+	struct per_source_penalty per_source_penalty;
 	int	max_authtries;
 	int	max_sessions;
 	char   *banner;			/* SSH-2 banner message */