netdev - Re: skb_can_coalesce() merges tcp frags with XFS-related slab objects

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Message-ID: <ed413bee-796e-1c3f-4829-4871c032e1ff@virtuozzo.com>
Date:   Wed, 20 Feb 2019 19:19:06 +0300
From:   Vasily Averin <vvs@...tuozzo.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Ilya Dryomov <idryomov@...il.com>
Subject: Re: skb_can_coalesce() merges tcp frags with XFS-related slab objects

On 2/20/19 6:53 PM, Eric Dumazet wrote:
> On 02/20/2019 05:34 AM, Vasily Averin wrote:
>> Dear David,
>>
>> currently do_tcp_sendpages() calls skb_can_coalesce() to merge proper tcp fragments.
>> If these fragments are slab objects and the data is not transferred out of the local host
>> then tcp_recvmsg() can crash host on BUG_ON (see [2] below).
>>
>> There is known usecase when slab objects are provided to tcp_sendpage:
>> XFS over locally landed network blockdevice.
>>
>> I found few such cases:
>> - _drbd_send_page() had PageSlab() check log time ago.
>> - recently Ilya Dryomov fixed it in ceph 
>>  by commit 7e241f647dc7 "libceph: fall back to sendmsg for slab pages"
>>
>> Recently OpenVZ team noticed this problem during experiments with
>> XFS over locally-landed iscsi target.
>>
>> I would note: triggered BUG is not a real problem but false alert,
>> that though crashes host.
>>
>> I can fix last problem by adding PageSlab() into iscsi_tcp_segment_map(),
>> however it does not fix the problem completely,
>> there are chances that the problem will be reproduced again with some other filesystems 
>> or with some other kind of network blockdevice.
>>
>> David, what do you think, is it probably better to add PageSlab() check
>> directly into skb_can_coalesce()? (see [1] below)
>>
> 
> No, this would be wrong.
> 
> There is no way a page fragment can be backed by slab object,
> since a page fragment can be shared (the page refcount needs to be manipulated, without slab/slub
> being aware of this)

Thank you for explanation, 
though this happen in real life and triggers BUG_ON only if receiving side is located on the same host.
Is it probably makes sense to add WARN_ON into skb_can_coalesce to detect such cases?

> Please fix the callers.

Ok, will do it.