netdev - Re: [PATCH bpf-next v6 1/2] Add skb_store_bytes() for BPF_PROG_TYPE_CGROUP

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Date:   Wed, 12 Jan 2022 18:14:12 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Tyler Wear <quic_twear@...cinc.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Maciej Żenczykowski <maze@...gle.com>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <song@...nel.org>
Subject: Re: [PATCH bpf-next v6 1/2] Add skb_store_bytes() for BPF_PROG_TYPE_CGROUP_SKB

On Wed, Jan 12, 2022 at 5:15 PM Tyler Wear <quic_twear@...cinc.com> wrote:
>
> Need to modify the ds field to support upcoming Wifi QoS Alliance spec.
> Instead of adding generic function for just modifying the ds field,
> add skb_store_bytes for BPF_PROG_TYPE_CGROUP_SKB.
> This allows other fields in the network and transport header to be
> modified in the future.
>
> Checksum API's also need to be added for completeness.
>
> It is not possible to use CGROUP_(SET|GET)SOCKOPT since
> the policy may change during runtime and would result
> in a large number of entries with wildcards.
>
> V4 patch fixes warnings and errors from checkpatch.
>
> The existing check for bpf_try_make_writable() should mean that
> skb_share_check() is not needed.
>
> Signed-off-by: Tyler Wear <quic_twear@...cinc.com>
> ---
>  net/core/filter.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 6102f093d59a..f30d939cb4cf 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -7299,6 +7299,18 @@ cg_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>                 return &bpf_sk_storage_delete_proto;
>         case BPF_FUNC_perf_event_output:
>                 return &bpf_skb_event_output_proto;
> +       case BPF_FUNC_skb_store_bytes:
> +               return &bpf_skb_store_bytes_proto;
> +       case BPF_FUNC_csum_update:
> +               return &bpf_csum_update_proto;
> +       case BPF_FUNC_csum_level:
> +               return &bpf_csum_level_proto;
> +       case BPF_FUNC_l3_csum_replace:
> +               return &bpf_l3_csum_replace_proto;
> +       case BPF_FUNC_l4_csum_replace:
> +               return &bpf_l4_csum_replace_proto;
> +       case BPF_FUNC_csum_diff:
> +               return &bpf_csum_diff_proto;

This is wrong.
CGROUP_INET_EGRESS bpf prog cannot arbitrary change packet data.
The networking stack populated the IP header at that point.
If the prog changes it to something else it will be
confusing other layers of stack. neigh(L2) will be wrong, etc.
We can still change certain things in the packet, but not arbitrary bytes.

We cannot change the DS field directly in the packet either.
It can only be changed by changing its value in the socket.

TC layer is where packet modifications are allowed.