netdev - Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net

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Message-ID: <CACGkMEsnPmbo8t6PbD8YsgKrZWHXG=Rz8ZwTDBJkSbmyzkNGSA@mail.gmail.com>
Date: Tue, 10 Sep 2024 14:12:17 +0800
From: Jason Wang <jasowang@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	edumazet@...gle.com, pabeni@...hat.com, stable@...r.kernel.org, 
	nsz@...t70.net, mst@...hat.com, yury.khrustalev@....com, broonie@...nel.org, 
	sudeep.holla@....com, Willem de Bruijn <willemb@...gle.com>, stable@...r.kernel.net
Subject: Re: [PATCH net] net: tighten bad gso csum offset check in virtio_net_hdr

On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> 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/20240909094527.GA3048202@port70.net/
> Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr")
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> Cc: <stable@...r.kernel.net>
>
> ---
>
> 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

Not for this patch, but I see this in UDP_L4:

                       if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
                               return -EINVAL;

This seems to forbid VIRTIO_NET_HDR_F_DATA_VALID. I wonder what's the
reason for doing this.

> 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
>

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks