netdev - Re: [PATCH] net: skb content must be visible for lockless skb

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Message-ID: <6aa849e4-3411-d71a-44a7-8ef927c1e270@ya.ru>
Date:   Sun, 7 Aug 2022 23:17:54 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: skb content must be visible for lockless skb_peek()
 and its variations

On 01.08.2022 21:45, Kirill Tkhai wrote:
> On 01.08.2022 10:39, Eric Dumazet wrote:
>> On Mon, Aug 1, 2022 at 9:00 AM Kirill Tkhai <tkhai@...ru> wrote:
>>>
>>> On 01.08.2022 09:52, Paolo Abeni wrote:
>>>> On Sun, 2022-07-31 at 23:39 +0300, Kirill Tkhai wrote:
>>>>> From: Kirill Tkhai <tkhai@...ru>
>>>>>
>>>>> Currently, there are no barriers, and skb->xxx update may become invisible on cpu2.
>>>>> In the below example var2 may point to intial_val0 instead of expected var1:
>>>>>
>>>>> [cpu1]                                       [cpu2]
>>>>> skb->xxx = initial_val0;
>>>>> ...
>>>>> skb->xxx = var1;                     skb = READ_ONCE(prev_skb->next);
>>>>> <no barrier>                         <no barrier>
>>>>> WRITE_ONCE(prev_skb->next, skb);     var2 = skb->xxx;
>>>>>
>>>>> This patch adds barriers and fixes the problem. Note, that __skb_peek() is not patched,
>>>>> since it's a lowlevel function, and a caller has to understand the things it does (and
>>>>> also __skb_peek() is used under queue lock in some places).
>>>>>
>>>>> Signed-off-by: Kirill Tkhai <tkhai@...ru>
>>>>> ---
>>>>> Hi, David, Eric and other developers,
>>>>>
>>>>> picking unix sockets code I found this problem,
>>>>
>>>> Could you please report exactly how/where the problem maifests (e.g.
>>>> the involved call paths/time sequence)?
>>>
>>> I didn't get why call paths in the patch description are not enough for you. Please, explain
>>> what you want.
>>>
>>>>> and for me it looks like it exists. If there
>>>>> are arguments that everything is OK and it's expected, please, explain.
>>>>
>>>> I don't see why such barriers are needed for the locked peek/tail
>>>> variants, as the spin_lock pair implies a full memory barrier.
>>>
>>> This is for lockless skb_peek() calls and the patch is called in that way :). For locked skb_peek()
>>> this is not needed. I'm not sure we need separate skb_peek() and skb_peek_lockless(). Do we?
>>
>> We prefer explicit _lockless variants to document the precise points
>> they are needed.
>>
>> A new helper (and its initial usage) will clearly point to the problem
>> you saw in af_unix.
> 
> The problem is:
> 
> unix_stream_sendmsg()	unix_stream_read_generic()
>   skb->len = size;	  skb = skb_peek();
>   skb_queue_tail(skb);	  unix_skb_len(skb); <- here we read wrong len

Oh, there are unix_state_lock(). Please, ignore this patch...