Re: [PATCH bpf v2] bpf: check BPF_F_ADJ_ROOM_FIXED_GSO when upgrading mss in 6 to 4

[Willem de Bruijn]

On Tue, May 11, 2021 at 2:51 AM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote:
>
> In the forwarding path GRO -> BPF 6 to 4 -> GSO for TCP traffic, the
> coalesced packet payload can be > MSS, but < MSS + 20.
> bpf_skb_proto_6_to_4 will increase the MSS and it can be > the payload
> length. After then tcp_gso_segment checks for the payload length if it
> is <= MSS. The condition is causing the packet to be dropped.
>
> tcp_gso_segment():
>         [...]
>         mss = skb_shinfo(skb)->gso_size;
>         if (unlikely(skb->len <= mss))
>                 goto out;
>         [...]
>
> Allow to increase MSS when BPF_F_ADJ_ROOM_FIXED_GSO is not set.
>
> Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper)
> Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
>
> ---

Thanks. Note that this feature does not preclude the alternatives
discussed, of converting the packet to non-TSO (by clearing gso_size)
or optionally modifying MSS (but that should get okay from TCP
experts).

I would target this for bpf-next and drop the Fixes. But that is
admittedly debatable.

>  net/core/filter.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> v2:
> per Willem de Bruijn request,
> checked the flag instead of a generic approach.
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index cae56d0..a98b28d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3276,7 +3276,7 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
>         return 0;
>  }
>
> -static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
> +static int bpf_skb_proto_6_to_4(struct sk_buff *skb, u64 flags)
>  {
>         const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);
>         u32 off = skb_mac_header_len(skb);
> @@ -3305,7 +3305,8 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
>                 }
>
>                 /* Due to IPv4 header, MSS can be upgraded. */
> -               skb_increase_gso_size(shinfo, len_diff);
> +               if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
> +                       skb_increase_gso_size(shinfo, len_diff);
>                 /* Header must be checked, and gso_segs recomputed. */
>                 shinfo->gso_type |= SKB_GSO_DODGY;
>                 shinfo->gso_segs = 0;
> @@ -3317,7 +3318,7 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
>         return 0;
>  }
>
> -static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)
> +static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto, u64 flags)
>  {
>         __be16 from_proto = skb->protocol;
>
> @@ -3327,7 +3328,7 @@ static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)
>
>         if (from_proto == htons(ETH_P_IPV6) &&
>               to_proto == htons(ETH_P_IP))
> -               return bpf_skb_proto_6_to_4(skb);
> +               return bpf_skb_proto_6_to_4(skb, flags);
>
>         return -ENOTSUPP;
>  }
> @@ -3337,7 +3338,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
>  {
>         int ret;
>
> -       if (unlikely(flags))
> +       if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO)))
>                 return -EINVAL;

Once allowing this flag, please immediately support it for both
bpf_skb_proto_6_to_4 and bpf_skb_4_to_6.

We cannot do that later if we ignore the second case now.


>         /* General idea is that this helper does the basic groundwork
> @@ -3357,7 +3358,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
>          * that. For offloads, we mark packet as dodgy, so that headers
>          * need to be verified first.
>          */
> -       ret = bpf_skb_proto_xlat(skb, proto);
> +       ret = bpf_skb_proto_xlat(skb, proto, flags);
>         bpf_compute_data_pointers(skb);
>         return ret;
>  }
> --
> 2.7.4
>