Subject: Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 10 Sep 2024 03:45:18 -0400

On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <[email-masked]>
> 
> The referenced commit drops bad input, but has false positives.
> Tighten the check to avoid these.
> 
> The check detects illegal checksum offload requests, which produce
> csum_start/csum_off beyond end of packet after segmentation.
> 
> But it is based on two incorrect assumptions:
> 
> 1. virtio_net_hdr_to_skb with VIRTIO_NET_HDR_GSO_TCP[46] implies GSO.
> True in callers that inject into the tx path, such as tap.
> But false in callers that inject into rx, like virtio-net.
> Here, the flags indicate GRO, and CHECKSUM_UNNECESSARY or
> CHECKSUM_NONE without VIRTIO_NET_HDR_F_NEEDS_CSUM is normal.
> 
> 2. TSO requires checksum offload, i.e., ip_summed == CHECKSUM_PARTIAL.
> False, as tcp[46]_gso_segment will fix up csum_start and offset for
> all other ip_summed by calling __tcp_v4_send_check.
> 
> Because of 2, we can limit the scope of the fix to virtio_net_hdr
> that do try to set these fields, with a bogus value.
> 
> Link: https://lore.kernel.org/netdev/[email-masked]/
> Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
> Signed-off-by: Willem de Bruijn <[email-masked]>
> Cc: <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

But I think netdev maintainers ask contributors not to CC
stable directly.

> ---
> 
> Verified that the syzbot repro is still caught.
> 
> An equivalent alternative would be to move the check for csum_offset
> to where the csum_start check is in segmentation:
> 
> -    if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb)))
> +    if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb) ||
> +                 skb->csum_offset != offsetof(struct tcphdr, check)))
> 
> Cleaner, but messier stable backport.
> 
> We'll need an equivalent patch to this for VIRTIO_NET_HDR_GSO_UDP_L4.
> But that csum_offset test was in a different commit, so different
> Fixes tag.
> ---
>  include/linux/virtio_net.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 6c395a2600e8d..276ca543ef44d 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  			break;
>  		case SKB_GSO_TCPV4:
>  		case SKB_GSO_TCPV6:
> -			if (skb->csum_offset != offsetof(struct tcphdr, check))
> +			if (skb->ip_summed == CHECKSUM_PARTIAL &&
> +			    skb->csum_offset != offsetof(struct tcphdr, check))
>  				return -EINVAL;
>  			break;
>  		}
> -- 
> 2.46.0.598.g6f2099f65c-goog