Subject: Re: Fw: [Bug 86851] New: Reproducible panic on heavy UDP traffic

From: Nikolay Aleksandrov <[email-masked]>
Date: Mon, 27 Oct 2014 09:48:15 +0100

On 10/27/2014 01:47 AM, Eric Dumazet wrote:
> On Mon, 2014-10-27 at 00:28 +0100, Nikolay Aleksandrov wrote:
> 
>>
>> Thanks for CCing me.
>> I'll dig in the code tomorrow but my first thought when I saw this was
>> could it be possible that we have a race condition between
>> ip_frag_queue() and inet_frag_evict(), more precisely between the
>> ipq_kill() calls from ip_frag_queue and inet_frag_evict since the frag
>> could be found before we have entered the evictor which then can add it to
>> its expire list but the ipq_kill() from ip_frag_queue() can do a list_del
>> after we release the chain lock in the evictor so we may end up like this ?
> 
> Yes, either we use hlist_del_init() but loose poison aid, or test if
> frag was evicted :
> 
> Not sure about refcount.
> 
> diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
> index 9eb89f3f0ee4..894ec30c5896 100644
> --- a/net/ipv4/inet_fragment.c
> +++ b/net/ipv4/inet_fragment.c
> @@ -285,7 +285,8 @@ static inline void fq_unlink(struct inet_frag_queue *fq, struct inet_frags *f)
>  	struct inet_frag_bucket *hb;
>  
>  	hb = get_frag_bucket_locked(fq, f);
> -	hlist_del(&fq->list);
> +	if (!(fq->flags & INET_FRAG_EVICTED))
> +		hlist_del(&fq->list);
>  	spin_unlock(&hb->chain_lock);
>  }
>  
> 
> 

Exactly, I was thinking about a similar fix since the evict flag is only
set with the chain lock. IMO the refcount should be fine.
CCing the reporter.
Patrick could you please try Eric's patch ?