Re: [net/sched] Question about possible misuse checksum in tcf_csum_ipv6_icmp()

[Eric Dumazet <edumazet@google.com>]

On Mon, Mar 18, 2024 at 8:31 PM Chenyuan Yang <chenyuan0y@gmail.com> wrote:
>
> Dear TC subsystem maintainers,
>
> We are curious whether the function `tcf_csum_ipv6_icmp()` would have a misuse of `csum_partial()` leading to an out-of-bounds access.
>
> The function `tcf_csum_ipv6_icmp` is https://elixir.bootlin.com/linux/v6.8/source/net/sched/act_csum.c#L183 and the relevant code is
> ```
> static int tcf_csum_ipv6_icmp(struct sk_buff *skb, unsigned int ihl,
>                               unsigned int ipl)
> {
>   ...
>         ip6h = ipv6_hdr(skb);
>         icmp6h->icmp6_cksum = 0;
>         skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
>         icmp6h->icmp6_cksum = csum_ipv6_magic(&ip6h->saddr, &ip6h->daddr,
>                                               ipl - ihl, IPPROTO_ICMPV6,
>                                               skb->csum);
>   ...
> }
> ```
>
> Based on this patch: https://lore.kernel.org/netdev/20240201083817.12774-1-atenart@kernel.org/T/, it seems that the `skb` here for ICMPv6 could be non-linear, and `csum_partial` is not suitable for non-linear SKBs, which could lead to an out-of-bound access. The correct approach is to use `skb_checksum` which properly handles non-linear SKBs.
>
> Based on the above information, a possible fix would be
> ```
> -       skb->csum = csum_partial(icmp6h, ipl - ihl, 0);
> +       skb->csum = skb_checksum(skb, skb_transport_offset(skb), ipl - ihl, 0);
> ```

Why would this code be an issue only in  tcf_csum_ipv6_icmp(), and not
in other functions ?

It seems we have proper pskb_may_pull() calls.

If you have a syzbot/syzkaller trace, please share it.