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

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Message-ID: <cfee59ae-4807-f384-b525-ce47fa4135e6@katalix.com>
Date: Mon, 8 Jul 2024 11:06:25 +0100
From: James Chapman <jchapman@...alix.com>
To: Hillf Danton <hdanton@...a.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 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?