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Message-Id: <20240708115940.892-1-hdanton@sina.com>
Date: Mon, 8 Jul 2024 19:59:40 +0800
From: Hillf Danton <hdanton@...a.com>
To: James Chapman <jchapman@...alix.com>
Cc: netdev@...r.kernel.org,
linux-kernel@...r.kernel.org,
syzkaller-bugs@...glegroups.com,
tparkin@...alix.com,
syzbot+b471b7c936301a59745b@...kaller.appspotmail.com,
syzbot+c041b4ce3a6dfd1e63e2@...kaller.appspotmail.com
Subject: Re: [PATCH net-next v2] l2tp: fix possible UAF when cleaning up tunnels
On Mon, 8 Jul 2024 11:06:25 +0100 James Chapman <jchapman@...alix.com>
> On 05/07/2024 11:32, Hillf Danton wrote:
> > On Thu, 4 Jul 2024 16:25:08 +0100 James Chapman <jchapman@...alix.com>
> >> --- 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.
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