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

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")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
---
 net/ipv4/gre_offload.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

> 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
>

On Fri, Jul 31, 2020 at 08:12:05PM +0200, Lorenzo Bianconi wrote:
> 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")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>

Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>

From: Lorenzo Bianconi <lorenzo@kernel.org>
Date: Fri, 31 Jul 2020 20:12:05 +0200

> 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")
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>

Applied with Fixes: tag corrected and queued up for -stable.

Thanks.