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

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 24 janvier 2011 à 18:47 +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.
> 
> Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
> ---
>  net/core/skbuff.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index d31bb36..c231f5b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2746,7 +2746,7 @@ merge:
>  	if (offset > headlen) {
>  		skbinfo->frags[0].page_offset += offset - headlen;
>  		skbinfo->frags[0].size -= offset - headlen;
> -		offset = headlen;
> +		skb->data_len -= offset - headlen;
>  	}
>  
>  	__skb_pull(skb, offset);

Hi Michal

Hmm, I dont really understand how __skb_pull(skb, offset) can be ok if
offset > headlen

skb->data might reach tail/end ?

Maybe I am too confused, this code is a bit complex :(

Thanks !

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;
 	}