netdev - Re: [RFC net] net: stream: don't purge sk_error

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Date:   Tue, 14 Sep 2021 09:32:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     willemb@...gle.com, netdev@...r.kernel.org
Subject: Re: [RFC net] net: stream: don't purge sk_error_queue without holding
 its lock



On 9/14/21 7:18 AM, Jakub Kicinski wrote:
> On Mon, 13 Sep 2021 22:14:00 -0700 Eric Dumazet wrote:
>> On 9/13/21 3:38 PM, Jakub Kicinski wrote:
>>> sk_stream_kill_queues() can be called when there are still
>>> outstanding skbs to transmit. Those skbs may try to queue
>>> notifications to the error queue (e.g. timestamps).
>>> If sk_stream_kill_queues() purges the queue without taking
>>> its lock the queue may get corrupted.
>>>
>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
>>> ---
>>> Sending as an RFC for review, compile-tested only.
>>>
>>> Seems far more likely that I'm missing something than that
>>> this has been broken forever and nobody noticed :S
>>> ---
>>>  net/core/stream.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/net/core/stream.c b/net/core/stream.c
>>> index 4f1d4aa5fb38..7c585088f394 100644
>>> --- a/net/core/stream.c
>>> +++ b/net/core/stream.c
>>> @@ -196,7 +196,7 @@ void sk_stream_kill_queues(struct sock *sk)
>>>  	__skb_queue_purge(&sk->sk_receive_queue);
>>>  
>>>  	/* Next, the error queue. */
>>> -	__skb_queue_purge(&sk->sk_error_queue);
>>> +	skb_queue_purge(&sk->sk_error_queue);
>>>  
>>>  	/* Next, the write queue. */
>>>  	WARN_ON(!skb_queue_empty(&sk->sk_write_queue));
>>
>> This should not be needed.
>>
>> By definition, sk_stream_kill_queues() is only called when there is no
>> more references on the sockets.
>>
>> So all outstanding packets must have been orphaned or freed.
> 
> I don't see the wait anywhere, would you mind spelling it out?
> My (likely flawed) understanding is that inet_sock_destruct() gets
> called when refs are gone (via sk->sk_destruct).


> 
> But tcp_disconnect() + tcp_close() seem to happily call
> inet_csk_destroy_sock() -> sk_stream_kill_queues() with outstanding
> sk_wmem_alloc refs.

tcp_disconnect() should probably leave the error queue as is.

For some reason I thought your report was about inet_sock_destruct()

tcp_disconnect() has always been full of bugs, it is surprising real applications
(not fuzzers) are still trying to use it.

> 
>> Anyway, Linux-2.6.12-rc2 had no timestamps yet.
> 
> I see, thanks, if some form of the patch stands perhaps:
> 
> Fixes: cb9eff097831 ("net: new user space API for time stamping of incoming and outgoing packets")
> 

Except that this patch wont prevent a packet being added to sk_error_queue
right after skb_queue_purge(&sk->sk_error_queue).

If you think there is a bug, it must be fixed in another way.

IMO, preventing err packets from a prior session being queued after a tcp_disconnect()
is rather hard. We should not even try (packets could be stuck for hours in a qdisc)