Subject: Re: [PATCH] net: fix for a race condition in the inet frag code

From: Nikolay Aleksandrov <[email-masked]>
Date: Mon, 03 Mar 2014 15:49:56 +0100

On 03/03/2014 03:40 PM, Florian Westphal wrote:
> Nikolay Aleksandrov <[email-masked]> wrote:
>> I stumbled upon this very serious bug while hunting for another one,
>> it's a very subtle race condition between inet_frag_evictor,
>> inet_frag_intern and the IPv4/6 frag_queue and expire functions (basically
>> the users of inet_frag_kill/inet_frag_put).
>> What happens is that after a fragment has been added to the hash chain but
>> before it's been added to the lru_list (inet_frag_lru_add), it may get
>> deleted (either by an expired timer if the system load is high or the
>> timer sufficiently low, or by the fraq_queue function for different
>> reasons) before it's added to the lru_list
> 
> Sorry.  Not following here, see below.
> 
>> diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
>> index bb075fc9a14f..322dcebfc588 100644
>> --- a/net/ipv4/inet_fragment.c
>> +++ b/net/ipv4/inet_fragment.c
>> @@ -278,9 +278,10 @@ static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
>>  
>>  	atomic_inc(&qp->refcnt);
>>  	hlist_add_head(&qp->list, &hb->chain);
>> +	inet_frag_lru_add(nf, qp);
>>  	spin_unlock(&hb->chain_lock);
>>  	read_unlock(&f->lock);
> 
> If I understand correctly your're saying that qp can be free'd on
> another/cpu timer right after dropping the locks.  But how is it
> possible?
> 
> ->refcnt is bumped above when arming the timer (before dropping chain
> lock), so even if the frag_expire timer fires instantly it should not
> free qp.
> 
> What am I missing?
> 
> Thanks,
> Florian
> 
An important point is that inet_frag_kill removes both the timer's refcnt and
has an unconditional atomic_dec to remove the original/guarding refcnt, so it
basically removes everything that's in the way.