netdev - Re: [PATCH v2] GRO: fix merging a paged skb after non-paged skbs

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Message-Id: <20110124.142741.71100065.davem@davemloft.net>
Date:	Mon, 24 Jan 2011 14:27:41 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	mschmidt@...hat.com, netdev@...r.kernel.org,
	herbert@...dor.hengli.com.au, bhutchings@...arflare.com
Subject: Re: [PATCH v2] GRO: fix merging a paged skb after non-paged skbs

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 24 Jan 2011 23:22:25 +0100

> Le lundi 24 janvier 2011 à 23:08 +0100, Michal Schmidt a écrit :
>> Suppose that several linear skbs of the same flow were received by GRO. They
>> were thus merged into one skb with a frag_list. Then a new skb of the same flow
>> arrives, but it is a paged skb with data starting in its frags[].
>> 
>> Before adding the skb to the frag_list skb_gro_receive() will of course adjust
>> the skb to throw away the headers. It correctly modifies the page_offset and
>> size of the frag, but it leaves incorrect information in the skb:
>>  ->data_len is not decreased at all.
>>  ->len is decreased only by headlen, as if no change were done to the frag.
>> Later in a receiving process this causes skb_copy_datagram_iovec() to return
>> -EFAULT and this is seen in userspace as the result of the recv() syscall.
>> 
>> In practice the bug can be reproduced with the sfc driver. By default the
>> driver uses an adaptive scheme when it switches between using
>> napi_gro_receive() (with skbs) and napi_gro_frags() (with pages). The bug is
>> reproduced when under rx load with enough successful GRO merging the driver
>> decides to switch from the former to the latter.
>> 
>> Manual control is also possible, so reproducing this is easy with netcat:
>>  - on machine1 (with sfc): nc -l 12345 > /dev/null
>>  - on machine2: nc machine1 12345 < /dev/zero
>>  - on machine1:
>>    echo 1 > /sys/module/sfc/parameters/rx_alloc_method  # use skbs
>>    echo 2 > /sys/module/sfc/parameters/rx_alloc_method  # use pages
>>  - See that nc has quit suddenly.
>> 
>> [v2: Modified by Eric Dumazet to avoid advancing skb->data past the end
>>      and to use a temporary variable.]
>> 
>> Signed-off-by: Michal Schmidt <mschmidt@...hat.com>
 ...
> Acked-by: Eric Dumazet <eric.dumazet@...il.com>

Applied and queued up for -stable, thanks!
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