netdev - Re: [PATCH net] inetpeer: fix uninit-value in inet

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Date:   Mon, 9 Apr 2018 08:08:29 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>, edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net] inetpeer: fix uninit-value in inet_getpeer



On 04/09/2018 07:58 AM, David Miller wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Mon,  9 Apr 2018 06:43:27 -0700
> 
>> syzbot/KMSAN reported that p->dtime was read while it was
>> not yet initialized in :
>>
>> 	delta = (__u32)jiffies - p->dtime;
>> 	if (delta < ttl || !refcount_dec_if_one(&p->refcnt))
>> 		gc_stack[i] = NULL;
>>
>> This is a false positive, because the inetpeer wont be erased
>> from rb-tree if the refcount_dec_if_one(&p->refcnt) does not
>> succeed. And this wont happen before first inet_putpeer() call
>> for this inetpeer has been done, and ->dtime field is written
>> exactly before the refcount_dec_and_test(&p->refcnt).
>>
>> The KMSAN report was :
>  ...
>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> Reported-by: syzbot <syzkaller@...glegroups.com>
> 
> Applied, but it looks like we are just adding assignments simply
> to placate these reports when the tools and facilities cannot
> see through the logic properly.
> 

To be fair, this is because the check on ->dtime should be done a second time after
the refcount_dec_if_one(&p->refcnt)

It is a tiny race, and we do not really care given nature of inetpeer cache, best effort,
and DDOS candidate anyway.

If we purge one entry too soon, this is not a big deal.

I believe tool is fine.