https://github.com/djmdjm/openssh-wip/pull/29 rebased on 9.8p1 + fixes

0001-Improve-OpenSSH-performance-on-high-BDP-links.patch

From 17c4f66061dca281cc5c4e3b4c28545aec1c50af Mon Sep 17 00:00:00 2001
From: Damien Miller <djm@mindrot.org>
Date: Wed, 21 Feb 2024 20:26:29 +1100
Subject: [PATCH] Improve OpenSSH performance on high BDP links

The SSH protocol performs flow control using a channel receive
window that may only be set at the time a channel is created and
cannot be subsequently modified.

The current default window size is sufficient for most uses, but
on high bandwidth x delay links it is not enough. Increasing the
default window size is possible, but carries the cost of additional
worst-case memory usage if the output side is slow.

This implements a protocol extension to vary the maximum channel
window after the channel has been created, and uses this extension
to grow the window if the TCP receive window increases in size.
TCP has had a half-century of optimisation, so we should copy
its homework wherever possible.

This is based on work by Denzel Strauss and Sally Sun during their
STEP iternship at Google.
---
 channels.c               | 141 ++++++++++++++++++++++++++++++++++++++-
 channels.h               |   2 +
 clientloop.c             |   3 +
 kex.c                    |  20 +++++-
 kex.h                    |   1 +
 packet.c                 |  38 +++++++++++
 packet.h                 |   1 +
 sandbox-seccomp-filter.c |   6 ++
 serverloop.c             |   3 +
 9 files changed, 210 insertions(+), 5 deletions(-)

diff --git a/channels.c b/channels.c
index 3ee694717..b832bf5f8 100644
--- a/channels.c
+++ b/channels.c
@@ -86,6 +86,7 @@
 #include "authfd.h"
 #include "pathnames.h"
 #include "match.h"
+#include "kex.h"
 
 /* XXX remove once we're satisfied there's no lurking bugs */
 /* #define DEBUG_CHANNEL_POLL 1 */
@@ -933,6 +934,23 @@ channel_still_open(struct ssh *ssh)
 	return 0;
 }
 
+
+static u_int
+channel_count_open(struct ssh *ssh)
+{
+	u_int i, n;
+	Channel *c;
+
+	for (i = n = 0; i < ssh->chanctxt->channels_alloc; i++) {
+		if ((c = ssh->chanctxt->channels[i]) == NULL ||
+		    c->type != SSH_CHANNEL_OPEN ||
+		    c->flags & (CHAN_CLOSE_SENT|CHAN_CLOSE_RCVD))
+			continue;
+		n++;
+	}
+	return n;
+}
+
 /* Returns true if a channel with a TTY is open. */
 int
 channel_tty_open(struct ssh *ssh)
@@ -2371,7 +2389,7 @@ channel_check_window(struct ssh *ssh, Channel *c)
 		    (r = sshpkt_send(ssh)) != 0) {
 			fatal_fr(r, "channel %i", c->self);
 		}
-		debug2("channel %d: window %d sent adjust %d", c->self,
+		debug3_f("channel %d: window %d sent adjust %d", c->self,
 		    c->local_window, c->local_consumed);
 		c->local_window += c->local_consumed;
 		c->local_consumed = 0;
@@ -2379,12 +2397,81 @@ channel_check_window(struct ssh *ssh, Channel *c)
 	return 1;
 }
 
+/*
+ * Try to update local_window_max when the TCP receive window grows,
+ * if the peer supports the channel-window-max@openssh.com extension.
+ * NB. must be called before channel_check_window(), as that clobbers
+ * c->local_consumed.
+ */
+static void
+channel_check_max_window(struct ssh *ssh, Channel *c)
+{
+	int r, tcp_rwin;
+	u_int new_window_max;
+
+	if (ssh->kex == NULL ||
+	    (ssh->kex->flags & KEX_HAS_CHANNEL_MAX_WINDOW) == 0)
+		return; /* no protocol extension support */
+	if (c->local_consumed == 0)
+		return; /* only modify window for active channels */
+	if (c->type != SSH_CHANNEL_OPEN ||
+	    (c->flags & (CHAN_CLOSE_SENT|CHAN_CLOSE_RCVD)) != 0)
+		return; /* channel not in open state */
+	if ((tcp_rwin = ssh_packet_get_tcp_rwin(ssh)) == -1)
+		return; /* no TCP window recorded */
+	if (c->local_window_max >= sshbuf_max_size(c->output))
+		return; /* Already at max */
+
+	/*
+	 * XXX the following rules are all heuristics with factors that
+	 * I just made up - djm
+	 * XXX this increases the SSH window to the TCP window in
+	 * one shot. Is this what we want? Alternately we could
+	 * increase by a fraction of the difference, so the SSH window
+	 * successively approximates the TCP window.
+	 * XXX there is only a naive fair-share the TCP window across
+	 * channel windows. It treats all channel identically (e.g. an
+	 * agent connection vs a shell) and doesn't try very hard to only
+	 * increase the window for channels that need it.
+	 * XXX maybe maintain a notion of "claimed TCP window" and
+	 * give it out only to channels that are active?
+	 * XXX regardless, a bit of oversubscription of the TCP window
+	 * is probably desirable as it's likely that not all channels will
+	 * be fully utilising their windows simultaneously.
+	 * XXX this never shrinks the window. This might be desirable for
+	 * idle channels if other are busy, but would be a fair bit of work.
+	 */
+	if (sshbuf_len(c->output) > c->local_window_max / 4)
+		return; /* Don't increase window if output is slow/filling */
+
+	/* Allow some oversubscription of the TCP window across channels */
+	new_window_max = (3 * (u_int)tcp_rwin) / channel_count_open(ssh);
+	if (new_window_max > (u_int)tcp_rwin)
+		new_window_max = (u_int)tcp_rwin;
+	if (new_window_max > sshbuf_max_size(c->output))
+		new_window_max = sshbuf_max_size(c->output);
+
+	if (new_window_max <= c->local_window_max)
+		return; /* don't shrink */
+
+	channel_request_start(ssh, c->self,
+	    "max-window@openssh.com", 0);
+	if ((r = sshpkt_put_u32(ssh, new_window_max)) != 0 ||
+	    (r = sshpkt_send(ssh)) != 0)
+		fatal_fr(r, "channel %i", c->self);
+	debug2_f("channel %d: sent max-window@openssh.com "
+	    "%d -> %d (+%d), peer TCP rwin %d", c->self, c->local_window_max,
+	    new_window_max, new_window_max - c->local_window_max, tcp_rwin);
+	c->local_window_max = new_window_max;
+}
+
 static void
 channel_post_open(struct ssh *ssh, Channel *c)
 {
 	channel_handle_rfd(ssh, c);
 	channel_handle_wfd(ssh, c);
 	channel_handle_efd(ssh, c);
+	channel_check_max_window(ssh, c);
 	channel_check_window(ssh, c);
 }
 
@@ -3582,7 +3669,7 @@ channel_input_open_confirmation(int type, u_int32_t seq, struct ssh *ssh)
 	}
 
 	c->have_remote_id = 1;
-	c->remote_window = remote_window;
+	c->remote_window = c->remote_window_max = remote_window;
 	c->remote_maxpacket = remote_maxpacket;
 	c->type = SSH_CHANNEL_OPEN;
 	if (c->open_confirm) {
@@ -3649,6 +3736,53 @@ channel_input_open_failure(int type, u_int32_t seq, struct ssh *ssh)
 	return 0;
 }
 
+int
+channel_input_max_window(struct ssh *ssh, Channel *c)
+{
+	u_int old_window, new_max_window;
+	int r;
+
+	if (c->type != SSH_CHANNEL_OPEN) {
+		debug_f("channel %d: not open", c->self);
+		return SSH_ERR_INVALID_ARGUMENT;
+	}
+	if ((r = sshpkt_get_u32(ssh, &new_max_window)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0) {
+		error_fr(r, "channel %d: parse", c->self);
+		return r;
+	}
+	old_window = c->remote_window;
+	if (new_max_window > c->remote_window_max) {
+		/* Usual case: remote grows max window */
+		c->remote_window += new_max_window - c->remote_window_max;
+	} else if (new_max_window < c->remote_window_max) {
+		/*
+		 * Only allow shrinking the window if there is
+		 * sufficient capacity to do so.
+		 */
+		if (c->remote_window_max - new_max_window > c->remote_window) {
+			error_f("channel %d: remote requested invalid remote "
+			    "window size", c->self);
+			/* XXX what to do? close chan? set remote_window=0? */
+			/* XXX could gracefully ratchet window back */
+			/* XXX maybe better to just ban shrinking? */
+			return SSH_ERR_INVALID_ARGUMENT;
+		}
+		c->remote_window -= c->remote_window_max - new_max_window;
+	} else {
+		/* Same size shouldn't happen */
+		debug_f("channel %d: useless max-window update", c->self);
+	}
+	debug2_f("channel %d: new remote window max %d -> %d (%+d)",
+	    c->self, c->remote_window_max, new_max_window,
+	    new_max_window - c->remote_window_max);
+	debug3_f("channel %d: implicit window adjust %d -> %d (%+d)",
+	    c->self, old_window, c->remote_window,
+	    c->remote_window - old_window);
+	c->remote_window_max = new_max_window;
+	return 0;
+}
+
 int
 channel_input_window_adjust(int type, u_int32_t seq, struct ssh *ssh)
 {
@@ -3670,11 +3804,12 @@ channel_input_window_adjust(int type, u_int32_t seq, struct ssh *ssh)
 		error_fr(r, "parse adjust");
 		ssh_packet_disconnect(ssh, "Invalid window adjust message");
 	}
-	debug2("channel %d: rcvd adjust %u", c->self, adjust);
 	if ((new_rwin = c->remote_window + adjust) < c->remote_window) {
 		fatal("channel %d: adjust %u overflows remote window %u",
 		    c->self, adjust, c->remote_window);
 	}
+	debug3_f("channel %d: rcvd adjust %u window %u/%u", c->self, adjust,
+	    c->remote_window, c->remote_window_max);
 	c->remote_window = new_rwin;
 	return 0;
 }
diff --git a/channels.h b/channels.h
index fdea9f574..631f90fcb 100644
--- a/channels.h
+++ b/channels.h
@@ -167,6 +167,7 @@ struct Channel {
 	char   *remote_name;	/* remote hostname */
 
 	u_int	remote_window;
+	u_int	remote_window_max;
 	u_int	remote_maxpacket;
 	u_int	local_window;
 	u_int	local_window_exceeded;
@@ -326,6 +327,7 @@ int	 channel_input_open_confirmation(int, u_int32_t, struct ssh *);
 int	 channel_input_open_failure(int, u_int32_t, struct ssh *);
 int	 channel_input_window_adjust(int, u_int32_t, struct ssh *);
 int	 channel_input_status_confirm(int, u_int32_t, struct ssh *);
+int	 channel_input_max_window(struct ssh *ssh, Channel *c);
 
 /* file descriptor handling (read/write) */
 struct pollfd;
diff --git a/clientloop.c b/clientloop.c
index 8ed8b1c34..3b1aed840 100644
--- a/clientloop.c
+++ b/clientloop.c
@@ -1977,6 +1977,9 @@ client_input_channel_req(int type, u_int32_t seq, struct ssh *ssh)
 		if ((r = sshpkt_get_end(ssh)) != 0)
 			goto out;
 		chan_rcvd_eow(ssh, c);
+	} else if (strcmp(rtype, "max-window@openssh.com") == 0) {
+		if ((r = channel_input_max_window(ssh, c)) == 0)
+			success = 1;
 	} else if (strcmp(rtype, "exit-status") == 0) {
 		if ((r = sshpkt_get_u32(ssh, &exitval)) != 0)
 			goto out;
diff --git a/kex.c b/kex.c
index 63aae5d71..a2a8659c9 100644
--- a/kex.c
+++ b/kex.c
@@ -301,13 +301,16 @@ kex_compose_ext_info_server(struct ssh *ssh, struct sshbuf *m)
 	if (ssh->kex->server_sig_algs == NULL &&
 	    (ssh->kex->server_sig_algs = sshkey_alg_list(0, 1, 1, ',')) == NULL)
 		return SSH_ERR_ALLOC_FAIL;
-	if ((r = sshbuf_put_u32(m, 3)) != 0 ||
+	if ((r = sshbuf_put_u32(m, 4)) != 0 ||
 	    (r = sshbuf_put_cstring(m, "server-sig-algs")) != 0 ||
 	    (r = sshbuf_put_cstring(m, ssh->kex->server_sig_algs)) != 0 ||
 	    (r = sshbuf_put_cstring(m,
 	    "publickey-hostbound@openssh.com")) != 0 ||
 	    (r = sshbuf_put_cstring(m, "0")) != 0 ||
 	    (r = sshbuf_put_cstring(m, "ping@openssh.com")) != 0 ||
+	    (r = sshbuf_put_cstring(m, "0")) != 0 ||
+	    (r = sshbuf_put_cstring(m,
+	    "channel-max-window@openssh.com")) != 0 ||
 	    (r = sshbuf_put_cstring(m, "0")) != 0) {
 		error_fr(r, "compose");
 		return r;
@@ -320,8 +323,11 @@ kex_compose_ext_info_client(struct ssh *ssh, struct sshbuf *m)
 {
 	int r;
 
-	if ((r = sshbuf_put_u32(m, 1)) != 0 ||
+	if ((r = sshbuf_put_u32(m, 2)) != 0 ||
 	    (r = sshbuf_put_cstring(m, "ext-info-in-auth@openssh.com")) != 0 ||
+	    (r = sshbuf_put_cstring(m, "0")) != 0 ||
+	    (r = sshbuf_put_cstring(m,
+	    "channel-max-window@openssh.com")) != 0 ||
 	    (r = sshbuf_put_cstring(m, "0")) != 0) {
 		error_fr(r, "compose");
 		goto out;
@@ -451,6 +457,11 @@ kex_ext_info_client_parse(struct ssh *ssh, const char *name,
 		    "0", KEX_HAS_PING)) != 0) {
 			return r;
 		}
+	} else if (strcmp(name, "channel-max-window@openssh.com") == 0) {
+		if ((r = kex_ext_info_check_ver(ssh->kex, name, value, vlen,
+		    "0", KEX_HAS_CHANNEL_MAX_WINDOW)) != 0) {
+			return r;
+		}
 	} else
 		debug_f("%s (unrecognised)", name);
 
@@ -468,6 +479,11 @@ kex_ext_info_server_parse(struct ssh *ssh, const char *name,
 		    "0", KEX_HAS_EXT_INFO_IN_AUTH)) != 0) {
 			return r;
 		}
+	} else if (strcmp(name, "channel-max-window@openssh.com") == 0) {
+		if ((r = kex_ext_info_check_ver(ssh->kex, name, value, vlen,
+		    "0", KEX_HAS_CHANNEL_MAX_WINDOW)) != 0) {
+			return r;
+		}
 	} else
 		debug_f("%s (unrecognised)", name);
 	return 0;
diff --git a/kex.h b/kex.h
index 34665eb20..7a9432754 100644
--- a/kex.h
+++ b/kex.h
@@ -113,6 +113,7 @@ enum kex_exchange {
 #define KEX_RSA_SHA2_512_SUPPORTED	0x0010 /* only set in server for now */
 #define KEX_HAS_PING			0x0020
 #define KEX_HAS_EXT_INFO_IN_AUTH	0x0040
+#define KEX_HAS_CHANNEL_MAX_WINDOW	0x0080
 
 struct sshenc {
 	char	*name;
diff --git a/packet.c b/packet.c
index 4fca8d660..4dd3f46fd 100644
--- a/packet.c
+++ b/packet.c
@@ -219,6 +219,9 @@ struct session_state {
 	/* One-off warning about weak ciphers */
 	int cipher_warning_done;
 
+	/* Last observed TCP receive window size */
+	int tcp_rwin;
+
 	/* Hook for fuzzing inbound packets */
 	ssh_packet_hook_fn *hook_in;
 	void *hook_in_ctx;
@@ -248,6 +251,7 @@ ssh_alloc_session_state(void)
 	state->max_packet_size = 32768;
 	state->packet_timeout_ms = -1;
 	state->p_send.packets = state->p_read.packets = 0;
+	state->tcp_rwin = -1;
 	state->initialized = 1;
 	/*
 	 * ssh_packet_send2() needs to queue packets until
@@ -668,6 +672,37 @@ ssh_packet_rdomain_in(struct ssh *ssh)
 	return ssh->rdomain_in;
 }
 
+static void
+ssh_packet_update_tcp_rwin(struct ssh *ssh)
+{
+	int rwin;
+	socklen_t len = sizeof(rwin);
+
+	if (ssh->state == NULL ||
+	    !ssh_packet_connection_is_on_socket(ssh))
+		return;
+	if (getsockopt(ssh->state->connection_in, SOL_SOCKET, SO_RCVBUF,
+	    &rwin, &len) != 0) {
+		debug_f("getsockopt(%d SO_RCVBUF): %s",
+		    ssh->state->connection_in, strerror(errno));
+		return;
+	}
+	if (ssh->state->tcp_rwin != -1 && rwin != ssh->state->tcp_rwin) {
+		debug3_f("TCP receive window %d -> %d (%+d)",
+		    ssh->state->tcp_rwin, rwin, rwin - ssh->state->tcp_rwin);
+	}
+	ssh->state->tcp_rwin = rwin;
+}
+
+int
+ssh_packet_get_tcp_rwin(struct ssh *ssh)
+{
+	if (ssh == NULL || ssh->state == NULL ||
+	    !ssh_packet_connection_is_on_socket(ssh))
+		return -1;
+	return ssh->state->tcp_rwin;
+}
+
 /* Closes the connection and clears and frees internal data structures. */
 
 static void
@@ -1501,6 +1536,8 @@ ssh_packet_read_seqnr(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)
 		/* Append it to the buffer. */
 		if ((r = ssh_packet_process_incoming(ssh, buf, len)) != 0)
 			goto out;
+
+		ssh_packet_update_tcp_rwin(ssh);
 	}
  out:
 	return r;
@@ -1917,6 +1954,7 @@ ssh_packet_process_read(struct ssh *ssh, int fd)
 
 	if ((r = sshbuf_read(fd, state->input, PACKET_MAX_SIZE, &rlen)) != 0)
 		return r;
+	ssh_packet_update_tcp_rwin(ssh);
 
 	if (state->packet_discard) {
 		if ((r = sshbuf_consume_end(state->input, rlen)) != 0)
diff --git a/packet.h b/packet.h
index 5ab1f409a..5568ff8b1 100644
--- a/packet.h
+++ b/packet.h
@@ -163,6 +163,7 @@ int	 ssh_remote_port(struct ssh *);
 const char *ssh_local_ipaddr(struct ssh *);
 int	 ssh_local_port(struct ssh *);
 const char *ssh_packet_rdomain_in(struct ssh *);
+int	 ssh_packet_get_tcp_rwin(struct ssh *);
 char	*ssh_remote_hostname(struct ssh *);
 
 void	 ssh_packet_set_rekey_limits(struct ssh *, u_int64_t, u_int32_t);
diff --git a/sandbox-seccomp-filter.c b/sandbox-seccomp-filter.c
index 23b40b643..a6b16b4f1 100644
--- a/sandbox-seccomp-filter.c
+++ b/sandbox-seccomp-filter.c
@@ -289,6 +289,9 @@ static const struct sock_filter preauth_insns[] = {
 #ifdef __NR_geteuid32
 	SC_ALLOW(__NR_geteuid32),
 #endif
+#ifdef __NR_getpeername
+	SC_ALLOW(__NR_getpeername),
+#endif
 #ifdef __NR_getpgid
 	SC_ALLOW(__NR_getpgid),
 #endif
@@ -298,6 +301,9 @@ static const struct sock_filter preauth_insns[] = {
 #ifdef __NR_getrandom
 	SC_ALLOW(__NR_getrandom),
 #endif
+#ifdef __NR_getsockopt
+	SC_ALLOW(__NR_getsockopt),
+#endif
 #ifdef __NR_gettid
 	SC_ALLOW(__NR_gettid),
 #endif
diff --git a/serverloop.c b/serverloop.c
index 757cc6f02..5e2f87be6 100644
--- a/serverloop.c
+++ b/serverloop.c
@@ -855,6 +855,9 @@ server_input_channel_req(int type, u_int32_t seq, struct ssh *ssh)
 		if ((r = sshpkt_get_end(ssh)) != 0)
 			sshpkt_fatal(ssh, r, "%s: parse packet", __func__);
 		chan_rcvd_eow(ssh, c);
+	} else if (strcmp(rtype, "max-window@openssh.com") == 0) {
+		if ((r = channel_input_max_window(ssh, c)) == 0)
+			success = 1;
 	} else if ((c->type == SSH_CHANNEL_LARVAL ||
 	    c->type == SSH_CHANNEL_OPEN) && strcmp(c->ctype, "session") == 0)
 		success = session_input_channel_req(ssh, c, rtype);
-- 
2.45.1