net/handshake: duplicate handshake cancellations leak socket

When a handshake request is cancelled it is removed from the
handshake_net->hn_requests list, but it is still present in the
handshake_rhashtbl until it is destroyed.

If a second cancellation request arrives for the same handshake request,
then remove_pending() will return false... and assuming
HANDSHAKE_F_REQ_COMPLETED isn't set in req->hr_flags, we'll continue
processing through the out_true label, where we put another reference on
the sock and a refcount underflow occurs.

This can happen for example if a handshake times out - particularly if
the SUNRPC client sends the AUTH_TLS probe to the server but doesn't
follow it up with the ClientHello due to a problem with tlshd.  When the
timeout is hit on the server, the server will send a FIN, which triggers
a cancellation request via xs_reset_transport().  When the timeout is
hit on the client, another cancellation request happens via
xs_tls_handshake_sync().

Add a test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED) in the pending cancel
path so duplicate cancels can be detected.

Fixes: 3b3009e ("net/handshake: Create a NETLINK service for handling handshake requests")
Suggested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20251209193015.3032058-1-smayhew@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: scottmayhew

net/handshake/request.c

@@ -326,7 +326,11 @@ bool handshake_req_cancel(struct sock *sk)
 
 	hn = handshake_pernet(net);
 	if (hn && remove_pending(hn, req)) {
-		/* Request hadn't been accepted */
+		/* Request hadn't been accepted - mark cancelled */
+		if (test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED, &req->hr_flags)) {
+			trace_handshake_cancel_busy(net, req, sk);
+			return false;
+		}
 		goto out_true;
 	}
 	if (test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED, &req->hr_flags)) {