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djm pushed a commit to branch V_9_6
in repository openssh.

commit 0cb50eefdd29f0fec31d0e71cc4b004a5f704e67
Author: djm@openbsd.org <djm@openbsd.org>
Date: Mon Dec 18 14:47:20 2023 +0000

upstream: stricter handling of channel window limits

This makes ssh/sshd more strict in handling non-compliant peers that
send more data than the advertised channel window allows. Previously
the additional data would be silently discarded. This change will
cause ssh/sshd to terminate the connection if the channel window is
exceeded by more than a small grace allowance.

ok markus@

OpenBSD-Commit-ID: 811e21b41831eba3dd7f67b3d409a438f20d3037
---
channels.c | 19 ++++++++++++++-----
channels.h | 3 ++-
2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/channels.c b/channels.c
index 38135e5a..20f31dad 100644
--- a/channels.c
+++ b/channels.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.c,v 1.434 2023/11/15 22:51:49 djm Exp $ */
+/* $OpenBSD: channels.c,v 1.435 2023/12/18 14:47:20 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -3407,11 +3407,20 @@ channel_input_data(int type, u_int32_t seq, struct ssh *ssh)
return 0;
}
if (win_len > c->local_window) {
- logit("channel %d: rcvd too much data %zu, win %u",
- c->self, win_len, c->local_window);
- return 0;
+ c->local_window_exceeded += win_len - c->local_window;
+ logit("channel %d: rcvd too much data %zu, win %u/%u "
+ "(excess %u)", c->self, win_len, c->local_window,
+ c->local_window_max, c->local_window_exceeded);
+ c->local_window = 0;
+ /* Allow 10% grace before bringing the hammer down */
+ if (c->local_window_exceeded > (c->local_window_max / 10)) {
+ ssh_packet_disconnect(ssh, "channel %d: peer ignored "
+ "channel window", c->self);
+ }
+ } else {
+ c->local_window -= win_len;
+ c->local_window_exceeded = 0;
}
- c->local_window -= win_len;

if (c->datagram) {
if ((r = sshbuf_put_string(c->output, data, data_len)) != 0)
diff --git a/channels.h b/channels.h
index 3054b04d..bb2650f6 100644
--- a/channels.h
+++ b/channels.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: channels.h,v 1.153 2023/11/15 22:51:49 djm Exp $ */
+/* $OpenBSD: channels.h,v 1.154 2023/12/18 14:47:20 djm Exp $ */

/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -170,6 +170,7 @@ struct Channel {
u_int remote_window;
u_int remote_maxpacket;
u_int local_window;
+ u_int local_window_exceeded;
u_int local_window_max;
u_int local_consumed;
u_int local_maxpacket;

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