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

1 files changed, 34 insertions, 1 deletions

diff --git a/monitor_wrap.c b/monitor_wrap.c
index 7807895c2..9720bdb43 100644
--- a/monitor_wrap.c
+++ b/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 <provos@citi.umich.edu>
  * Copyright 2002 Markus Friedl <markus@openbsd.org>
@@ -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));