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

[James Chapman]

On 08/07/2024 12:59, Hillf Danton wrote:
> On Mon, 8 Jul 2024 11:06:25 +0100 James Chapman <jchapman@xxxxxxxxxxx>
> > 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?
> Given race, there are winner and loser. If the current cpu wins the race,
> the loser hits uaf once winner drops refcount.

I think the session's dead flag would protect against threads racing in 
l2tp_session_delete to delete the same session.
Any thread with a pointer to a session should hold a reference on it to 
prevent the session going away while it is accessed. Am I missing a 
codepath where that's not the case?