Re: [PATCH net] gso: fix GSO_DODGY bit handling for related protocols

[Willem de Bruijn]

On Wed, Jul 12, 2023 at 9:55 PM Yan Zhai <yan@xxxxxxxxxxxxxx> wrote:
>
> SKB_GSO_DODGY bit indicates a GSO packet comes from an untrusted source.
> The canonical way is to recompute the gso_segs to avoid device driver
> issues. Afterwards, the DODGY bit can be removed to avoid re-check at the
> egress of later devices, e.g. packets can egress to a vlan device backed
> by a real NIC.
>
> Commit 1fd54773c267 ("udp: allow header check for dodgy GSO_UDP_L4
> packets.") checks DODGY bit for UDP, but for packets that can be fed
> directly to the device after gso_segs reset, it actually falls through
> to fragmentation [1].
>
> Commit 90017accff61 ("sctp: Add GSO support") and commit 3820c3f3e417
> ("[TCP]: Reset gso_segs if packet is dodgy") both didn't remove the DODGY
> bit after recomputing gso_segs.
>
> This change fixes the GSO_UDP_L4 handling case, and remove the DODGY bit
> at other places.

These two things should not be conflated.

Only the USO fix is strictly needed to fix the reported issue.

> Fixes: 90017accff61 ("sctp: Add GSO support")
> Fixes: 3820c3f3e417 ("[TCP]: Reset gso_segs if packet is dodgy")
> Fixes: 1fd54773c267 ("udp: allow header check for dodgy GSO_UDP_L4 packets.")

Link: https://lore.kernel.org/all/CAJPywTKDdjtwkLVUW6LRA2FU912qcDmQOQGt2WaDo28KzYDg+A@xxxxxxxxxxxxxx/

> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
>
> ---
> [1]:
> https://lore.kernel.org/all/CAJPywTKDdjtwkLVUW6LRA2FU912qcDmQOQGt2WaDo28KzYDg+A@xxxxxxxxxxxxxx/
>
> ---
>  net/ipv4/tcp_offload.c |  1 +
>  net/ipv4/udp_offload.c | 19 +++++++++++++++----
>  net/ipv6/udp_offload.c | 19 +++++++++++++++----
>  net/sctp/offload.c     |  2 ++
>  4 files changed, 33 insertions(+), 8 deletions(-)
>
> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> index 8311c38267b5..f9b93708c22e 100644
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -87,6 +87,7 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
>                 /* Packet is from an untrusted source, reset gso_segs. */
>
>                 skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
> +               skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
>
>                 segs = NULL;
>                 goto out;
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 75aa4de5b731..bd29cf19bb6b 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -388,11 +388,22 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
>         if (!pskb_may_pull(skb, sizeof(struct udphdr)))
>                 goto out;
>
> -       if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> -           !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
> -               return __udp_gso_segment(skb, features, false);
> -
>         mss = skb_shinfo(skb)->gso_size;

Why move the block below this line?

> +
> +       if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
> +               if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
> +                       /* Packet is from an untrusted source, reset actual gso_segs */
> +                       skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
> +                                                                mss);
> +                       skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
> +
> +                       segs = NULL;
> +                       goto out;
> +               } else {
> +                       return __udp_gso_segment(skb, features, false);
> +               }
> +       }
> +

The validation should take place inside __udp_gso_segment.

Revert the previous patch to always enter that function for USO packets:

       if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4)
                return __udp_gso_segment(skb, features, false);

And in that function decide to return NULL after validation.


>         if (unlikely(skb->len <= mss))
>                 goto out;
>
> diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
> index ad3b8726873e..6857d9f7bd06 100644
> --- a/net/ipv6/udp_offload.c
> +++ b/net/ipv6/udp_offload.c
> @@ -43,11 +43,22 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,
>                 if (!pskb_may_pull(skb, sizeof(struct udphdr)))
>                         goto out;
>
> -               if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> -                   !skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST))
> -                       return __udp_gso_segment(skb, features, true);
> -
>                 mss = skb_shinfo(skb)->gso_size;
> +
> +               if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
> +                       if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
> +                               /* Packet is from an untrusted source, reset actual gso_segs */
> +                               skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
> +                                                                        mss);
> +                               skb_shinfo(skb)->gso_type &= ~SKB_GSO_DODGY;
> +
> +                               segs = NULL;
> +                               goto out;
> +                       } else {
> +                               return __udp_gso_segment(skb, features, true);
> +                       }
> +               }
> +
>                 if (unlikely(skb->len <= mss))
>                         goto out;
>
> diff --git a/net/sctp/offload.c b/net/sctp/offload.c
> index 502095173d88..3d2b44db0d42 100644
> --- a/net/sctp/offload.c
> +++ b/net/sctp/offload.c
> @@ -65,6 +65,8 @@ static struct sk_buff *sctp_gso_segment(struct sk_buff *skb,
>                 skb_walk_frags(skb, frag_iter)
>                         pinfo->gso_segs++;
>
> +               pinfo->gso_type &= ~SKB_GSO_DODGY;
> +
>                 segs = NULL;
>                 goto out;
>         }
> --
> 2.30.2
>