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

[Chenyuan Yang <chenyuan0y@gmail.com>]

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);
``` 

Please kindly correct us if we missed any key information. Looking forward to your response!

Best,
Chenyuan