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

[Scott Mayhew <smayhew@redhat.com>]

On Sat, 06 Dec 2025, Chuck Lever wrote:

> 
> 
> On Sat, Dec 6, 2025, at 9:30 AM, Scott Mayhew wrote:
> > 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 <smayhew@redhat.com>
> > ---
> >  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
> 
> To help support engineers find this patch, I recommend using
> "net/handshake: duplicate handshake cancellations leak socket" as
> the short description.
> 
> The proposed solution might introduce a socket reference leak:
> 
> 1. Request submitted: sock_hold() called (line 271)
> 2. Request accepted by daemon via handshake_req_next()
>    (removes from pending list)
> 3. Cancel called:
>   - remove_pending() returns FALSE (not in pending list)
>   - test_and_set_bit() returns FALSE (sets the bit now)
>   - With patch: returns FALSE, sock_put() NOT called
> 4. handshake_complete() called: bit already set, skips sock_put()
> 
> What if we use test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED) in the
> pending cancel path so duplicate cancels can be detected?
> 
> Instead of:
> 
>         if (hn && remove_pending(hn, req)) {
>                 /* Request hadn't been accepted */
>                 goto out_true;
>         }
> 
> go with this bit of untested code:
> 
>         if (hn && remove_pending(hn, req)) {
>                 /* 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;
>         }

Thanks, Chuck.  That works.
> 
> -- 
> Chuck Lever
>