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

[Hillf Danton]

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.