# HG changeset patch
# User Dirkjan Bussink <d.bussink@gmail.com>
# Date 1607613193 -3600
# Node ID 1fc0012b9c38156d52c596ffb7b8da437b20d908
# Parent  ce3ce75a6e04875f99f8956b9ea9dbff66f077e7
Fix handling of replies to global requests (#112)

The current code assumes that all global requests want / need a reply.
This isn't always true and the request itself indicates if it wants a
reply or not.

It causes a specific problem with hostkeys-00@openssh.com messages.
These are sent by OpenSSH after authentication to inform the client of
potential other host keys for the host. This can be used to add a new
type of host key or to rotate host keys.

The initial information message from the server is sent as a global
request, but with want_reply set to false. This means that the server
doesn't expect an answer to this message. Instead the client needs to
send a prove request as a reply if it wants to receive proof of
ownership for the host keys.

The bug doesn't cause any current problems with due to how OpenSSH
treats receiving the failure message. It instead treats it as a
keepalive message and further ignores it.

Arguably this is a protocol violation though of Dropbear and it is only
accidental that it doesn't cause a problem with OpenSSH.

The bug was found when adding host keys support to libssh, which is more
strict protocol wise and treats the unexpected failure message an error,
also see https://gitlab.com/libssh/libssh-mirror/-/merge_requests/145
for more information.

The fix here is to honor the want_reply flag in the global request and
to only send a reply if the other side expects a reply.

diff -r ce3ce75a6e04 -r 1fc0012b9c38 cli-session.c
--- a/cli-session.c	Mon Dec 07 20:03:24 2020 +0800
+++ b/cli-session.c	Thu Dec 10 16:13:13 2020 +0100
@@ -408,9 +408,19 @@
 }
 
 static void recv_msg_global_request_cli(void) {
-	TRACE(("recv_msg_global_request_cli"))
-	/* Send a proper rejection */
-	send_msg_request_failure();
+	unsigned int len = 0;
+	unsigned int wantreply = 0;
+
+	len = buf_getint(ses.payload);
+	buf_incrpos(ses.payload, len);
+	wantreply = buf_getbool(ses.payload);
+
+	TRACE(("recv_msg_global_request_cli: want_reply: %u", wantreply));
+
+	if (wantreply) {
+		/* Send a proper rejection */
+		send_msg_request_failure();
+	}
 }
 
 void cli_dropbear_exit(int exitcode, const char* format, va_list param) {
diff -r ce3ce75a6e04 -r 1fc0012b9c38 svr-tcpfwd.c
--- a/svr-tcpfwd.c	Mon Dec 07 20:03:24 2020 +0800
+++ b/svr-tcpfwd.c	Thu Dec 10 16:13:13 2020 +0100
@@ -39,8 +39,17 @@
 
 /* This is better than SSH_MSG_UNIMPLEMENTED */
 void recv_msg_global_request_remotetcp() {
-		TRACE(("recv_msg_global_request_remotetcp: remote tcp forwarding not compiled in"))
+	unsigned int len = 0;
+	unsigned int wantreply = 0;
+
+	TRACE(("recv_msg_global_request_remotetcp: remote tcp forwarding not compiled in"))
+
+	len = buf_getint(ses.payload);
+	buf_incrpos(ses.payload, len);
+	wantreply = buf_getbool(ses.payload);
+	if (wantreply) {
 		send_msg_request_failure();
+	}
 }
 
 /* */