netdev - Re: [PATCH v2] net: Do not include padding in TCP GRO checksum

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Date:	Fri, 15 Nov 2013 13:44:55 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
	herbert@...dor.apana.org
Subject: Re: [PATCH v2] net: Do not include padding in TCP GRO checksum

On Fri, 2013-11-15 at 12:03 -0800, Alexander Duyck wrote:
> In some recent tests I found the TCP checksum was being treated as valid
> for certain frames with padding on them.  On closer inspection I found the
> issue was that GRO was using the skb->len instead of the length recorded in
> the IP/IPv6 header to determine the number of bytes to checksum.  As such
> padded frames that actually had invalid checksums generated by adding the
> padding to the checksum were being incorrectly tagged as valid.
> 
> This change corrects that by using the tot_len from IPv4 headers and the
> payload_len from IPv6 headers to compute the total number of bytes to be
> included in the checksum.
> 
> To address the fact that skb->csum is invalid when a padded frame is
> received I have updated the code to fall though to the CHECKSUM_NONE path
> for CHECKSUM_COMPLETE frames that contain padding.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---
> 
> v2: Update byte ordering of tot_len and payload_len so it is in host order.
>     Updated CHECKSUM_COMPLETE path so it falls back through CHECKSUM_NONE for
>     padded frames since this is how it is handled in ip_rcv.
> 
>     I have tested and verified the CHECKSUM_NONE path works, but I don't have
>     any adapters that generate CHECKSUM_COMPLETE to test with.
> 
>  net/ipv4/tcp_offload.c   |   26 +++++++++++++++++---------
>  net/ipv6/tcpv6_offload.c |   27 +++++++++++++++++----------
>  2 files changed, 34 insertions(+), 19 deletions(-)
> 
> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> index a2b68a1..17b5d14 100644
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -273,26 +273,34 @@ static int tcp_v4_gso_send_check(struct sk_buff *skb)
>  static struct sk_buff **tcp4_gro_receive(struct sk_buff **head, struct sk_buff *skb)
>  {
>  	const struct iphdr *iph = skb_gro_network_header(skb);
> +	int length = ntohs(iph->tot_len);


Sorry, but couldn't iph->tot_len be larger than skb_gro_len(skb) ?

It's not validated in inet_gro_complete() ...

skb_checksum(skb, ..., length, wsum)  could crash.




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