Re: [PATCH net-next v2] l2tp: fix possible UAF when cleaning up tunnels

[James Chapman]

On 05/07/2024 11:32, Hillf Danton wrote:
> On Thu,  4 Jul 2024 16:25:08 +0100 James Chapman <jchapman@xxxxxxxxxxx>
> > --- a/net/l2tp/l2tp_core.c
> > +++ b/net/l2tp/l2tp_core.c
> > @@ -1290,17 +1290,20 @@ static void l2tp_session_unhash(struct l2tp_session *session)
> >   static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
> >   {
> >   	struct l2tp_session *session;
> > -	struct list_head *pos;
> > -	struct list_head *tmp;
> >   
> >   	spin_lock_bh(&tunnel->list_lock);
> >   	tunnel->acpt_newsess = false;
> > -	list_for_each_safe(pos, tmp, &tunnel->session_list) {
> > -		session = list_entry(pos, struct l2tp_session, list);
> > +	for (;;) {
> > +		session = list_first_entry_or_null(&tunnel->session_list,
> > +						   struct l2tp_session, list);
> > +		if (!session)
> > +			break;
> > +		l2tp_session_inc_refcount(session);
> >   		list_del_init(&session->list);
> >   		spin_unlock_bh(&tunnel->list_lock);
> >   		l2tp_session_delete(session);
> >   		spin_lock_bh(&tunnel->list_lock);
> > +		l2tp_session_dec_refcount(session);
>
> Bumping refcount up makes it safe for the current cpu to go thru race
> after releasing lock, and if it wins the race, dropping refcount makes
> the peer head on uaf.

Thanks for reviewing this. Can you elaborate on what you mean by "makes 
the peer head on uaf", please?