[PATCH] net/handshake: a handshake can only be cancelled once

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().

Fixes: 3b3009ea8abb ("net/handshake: Create a NETLINK service for handling handshake requests")
Signed-off-by: Scott Mayhew <[email protected]>
---
 net/handshake/request.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/handshake/request.c b/net/handshake/request.c
index 274d2c89b6b2..c7b20d167a55 100644
--- a/net/handshake/request.c
+++ b/net/handshake/request.c
@@ -333,6 +333,10 @@ bool handshake_req_cancel(struct sock *sk)
 		return false;
 	}
 
+	/* Duplicate cancellation request */
+	trace_handshake_cancel_none(net, req, sk);
+	return false;
+
 out_true:
 	trace_handshake_cancel(net, req, sk);
 
-- 
2.51.0