Re: [PATCH net] net: gre: recompute gre csum for sctp over gre tunnels

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> The GRE tunnel can be used to transport traffic that does not rely on a
> Internet checksum (e.g. SCTP). The issue can be triggered creating a GRE
> or GRETAP tunnel and transmitting SCTP traffic ontop of it where CRC
> offload has been disabled. In order to fix the issue we need to
> recompute the GRE csum in gre_gso_segment() not relying on the inner
> checksum.
> The issue is still present when we have the CRC offload enabled.
> In this case we need to disable the CRC offload if we require GRE
> checksum since otherwise skb_checksum() will report a wrong value.
> 
> Fixes: 4749c09c37030 ("gre: Call gso_make_checksum")

I put the wrong Fixes tag, reviewing it I guess the right one is:

Fixes: 90017accff61 ("sctp: Add GSO support")

sorry for the noise.

Regards,
Lorenzo

> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  net/ipv4/gre_offload.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/gre_offload.c b/net/ipv4/gre_offload.c
> index 2e6d1b7a7bc9..e0a246575887 100644
> --- a/net/ipv4/gre_offload.c
> +++ b/net/ipv4/gre_offload.c
> @@ -15,12 +15,12 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>  				       netdev_features_t features)
>  {
>  	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
> +	bool need_csum, need_recompute_csum, gso_partial;
>  	struct sk_buff *segs = ERR_PTR(-EINVAL);
>  	u16 mac_offset = skb->mac_header;
>  	__be16 protocol = skb->protocol;
>  	u16 mac_len = skb->mac_len;
>  	int gre_offset, outer_hlen;
> -	bool need_csum, gso_partial;
>  
>  	if (!skb->encapsulation)
>  		goto out;
> @@ -41,6 +41,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>  	skb->protocol = skb->inner_protocol;
>  
>  	need_csum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_GRE_CSUM);
> +	need_recompute_csum = skb->csum_not_inet;
>  	skb->encap_hdr_csum = need_csum;
>  
>  	features &= skb->dev->hw_enc_features;
> @@ -98,7 +99,15 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
>  		}
>  
>  		*(pcsum + 1) = 0;
> -		*pcsum = gso_make_checksum(skb, 0);
> +		if (need_recompute_csum && !skb_is_gso(skb)) {
> +			__wsum csum;
> +
> +			csum = skb_checksum(skb, gre_offset,
> +					    skb->len - gre_offset, 0);
> +			*pcsum = csum_fold(csum);
> +		} else {
> +			*pcsum = gso_make_checksum(skb, 0);
> +		}
>  	} while ((skb = skb->next));
>  out:
>  	return segs;
> -- 
> 2.26.2
> 

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