[PATCH] client/server: fix TCP forwarding hang when a service is unresponsive

[Corey Hickey]

From: Corey Hickey <chickey at tagged.com>

When a remote-forwarded TCP service is unresponsive, the SSH client can
hang after the SSH server sends an EOF.

The SSH client receives the EOF and drains the channel output, but
continues to wait for data on the channel input. The SSH server won't
close the channel until the client does, and the client won't close the
channel until it receives data (or an error) from the channel. The
client can end up waiting forever if the forwarded TCP service never
responds.

The analagous situation happens for local-forwarded TCP services, with
the roles of the SSH server and client reversed.

My attempted fix for each case is to configure the channel to force an
input drain upon receiving an EOF. I do not know if this is the right
approach, but the test suite continues to pass for me, at least.
---
 clientloop.c | 1 +
 serverloop.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/clientloop.c b/clientloop.c
index 0050f3eb..cd482e5d 100644
--- a/clientloop.c
+++ b/clientloop.c
@@ -1514,6 +1514,7 @@ client_request_forwarded_tcpip(struct ssh *ssh, const char *request_type,
 	sshbuf_free(b);
 	free(originator_address);
 	free(listen_address);
+	c->force_drain = 1;
 	return c;
 }
 
diff --git a/serverloop.c b/serverloop.c
index b4c0d82b..486e5277 100644
--- a/serverloop.c
+++ b/serverloop.c
@@ -470,6 +470,7 @@ server_request_direct_tcpip(struct ssh *ssh, int *reason, const char **errmsg)
  out:
 	free(originator);
 	free(target);
+	c->force_drain = 1;
 	return c;
 }
 
-- 
2.35.1