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

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Date:	Mon, 24 Jan 2011 23:22:25 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Schmidt <mschmidt@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH v2] GRO: fix merging a paged skb after non-paged skbs

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>
> ---
> Eric,
> I think skb->data is pretty much irrelevant at that point, because the
> skb's headlen is going to become zero, but admittedly it seems cleaner
> this way.
> Thanks.
> 
>  net/core/skbuff.c |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index d31bb36..7cd1bc8 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2744,8 +2744,12 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
>  
>  merge:
>  	if (offset > headlen) {
> -		skbinfo->frags[0].page_offset += offset - headlen;
> -		skbinfo->frags[0].size -= offset - headlen;
> +		unsigned int eat = offset - headlen;
> +
> +		skbinfo->frags[0].page_offset += eat;
> +		skbinfo->frags[0].size -= eat;
> +		skb->data_len -= eat;
> +		skb->len -= eat;
>  		offset = headlen;
>  	}
>  

Acked-by: Eric Dumazet <eric.dumazet@...il.com>

Thanks !


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