Re: [PATCH net] net: extend IPv6 exthdr detection of tunneled packets

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- [High] Regression for non-encapsulated IPv6 CHECKSUM_PARTIAL skbs in
  skb_csum_hwoffload_help(), plus a matching gap in…

--

net: extend IPv6 exthdr detection of tunneled packets

This patch replaces skb_gso_has_extension_hdr() with
skb_has_ipv6_extension_hdr(). The new helper runs ipv6_ext_hdr() on the
outer IPv6 nexthdr. For encapsulated skbs that have an inner network
header, it also runs it on the inner IPv6 nexthdr. skb_csum_hwoffload_help()
now uses the same helper instead of its open coded skb_network_header_len()
test.

> diff --git a/net/core/dev.c b/net/core/dev.c
> index f660fccfc0dbc..992b78308c13f 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3818,20 +3818,29 @@ static netdev_features_t dflt_features_check(struct sk_buff *skb,
>  	return vlan_features_check(skb, features);
>  }
>  
> -static bool skb_gso_has_extension_hdr(const struct sk_buff *skb)
> -{
> -	if (!skb->encapsulation)
> -		return ((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
> -			 (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
> -			  vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
> -			skb_transport_header_was_set(skb) &&
> -			skb_network_header_len(skb) != sizeof(struct ipv6hdr));

[ ... ]

> +static bool __skb_has_ipv6_ext_hdr(const struct sk_buff *skb, int nhoff)
> +{
> +	const struct ipv6hdr *ip6h;
> +	struct ipv6hdr _ip6h;
> +
> +	ip6h = skb_header_pointer(skb, nhoff, sizeof(_ip6h), &_ip6h);
> +	return ip6h && ip6h->version == 6 && ipv6_ext_hdr(ip6h->nexthdr);
> +}
> +
> +static bool skb_has_ipv6_extension_hdr(const struct sk_buff *skb)
> +{
> +	if (vlan_get_protocol(skb) == htons(ETH_P_IPV6) &&
> +	    __skb_has_ipv6_ext_hdr(skb, skb_network_offset(skb)))
> +		return true;
> +
> +	/* Tunnels without an inner network header, such as SCTP-in-UDP or
> +	 * PSP, have no inner IP header and thus no inner extension header.
> +	 */
> +	if (skb->encapsulation && skb_inner_network_header_was_set(skb) &&
> +	    __skb_has_ipv6_ext_hdr(skb, skb_inner_network_offset(skb)))
> +		return true;
> +
> +	return false;
>  }

[ ... ]

> @@ -3988,8 +3997,7 @@ int skb_csum_hwoffload_help(struct sk_buff *skb,
>  		return 0;
>  
>  	if (features & (NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM)) {
> -		if (vlan_get_protocol(skb) == htons(ETH_P_IPV6) &&
> -		    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
> +		if (skb_has_ipv6_extension_hdr(skb))
>  			goto sw_checksum;

[Severity: High]
Does this weaken the check for non-encapsulated IPv6 CHECKSUM_PARTIAL
skbs?

When !skb->encapsulation, validate_xmit_skb() sets the transport header to
csum_start just before it calls skb_csum_hwoffload_help():

net/core/dev.c:validate_xmit_skb() {
    ...
		if (skb->ip_summed == CHECKSUM_PARTIAL) {
			if (skb->encapsulation)
				skb_set_inner_transport_header(skb,
							       skb_checksum_start_offset(skb));
			else
				skb_set_transport_header(skb,
							 skb_checksum_start_offset(skb));
			if (skb_csum_hwoffload_help(skb, features))
				goto out_kfree_skb;
    ...
}

That means the removed skb_network_header_len() test checked that
csum_start sits right after the 40 byte IPv6 header.

The new helper only checks whether the outer nexthdr is one of the six
values listed in ipv6_ext_hdr(). It skips the inner check because
encapsulation is 0.

Take IPv6|UDP|VXLAN|Eth|IP|TCP, or IPv6|IPv6|TCP (nexthdr 41), with
csum_start at the inner TCP header and csum_offset 16. Would that packet
now reach the csum_offset switch and return 0?

Such skbs can come from untrusted input. __virtio_net_hdr_to_skb() is
used by tap, vhost-net and AF_PACKET with PACKET_VNET_HDR. It accepts
VIRTIO_NET_HDR_F_NEEDS_CSUM with any csum_start via
skb_partial_csum_set(). It does not look at nexthdr and does not set
skb->encapsulation.

One case: a guest sends IPv6 VXLAN traffic with a CHECKSUM_PARTIAL inner
TCP header (LCO). The host bridges it to a device that has
NETIF_F_IPV6_CSUM but not NETIF_F_HW_CSUM.

On a device that parses headers itself, such as bnxt, bnxt_exthdr_check()
accepts an outer nexthdr of UDP for non-encapsulated skbs. The hardware
would then fill in the outer UDP checksum and leave the inner TCP checksum
holding only the pseudo-header seed. Before this patch, the kernel
computed these checksums in software.

The skbuff.h description of NETIF_F_IPV6_CSUM covers only "unencapsulated
packets of the form IPv6|TCP or IPv6|UDP where the Next Header field in
the IPv6 header is either TCP or UDP."

gso_features_check() has the same gap. For non-encapsulated skbs, the old
branch removed NETIF_F_TSO6 and NETIF_F_GSO_UDP_L4 whenever the transport
header was not at nh + 40. The new code removes them only when the outer
nexthdr is an extension header.

The commit message talks about removing false positives, but it doesn't
mention this relaxation for non-encapsulated skbs.

Could the offset test stay for the !skb->encapsulation case? For example:

	skb_checksum_start_offset(skb) - skb_network_offset(skb) != sizeof(struct ipv6hdr)

Or could the check require nexthdr to be IPPROTO_TCP or IPPROTO_UDP,
instead of testing ipv6_ext_hdr()?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260924192128.1197118-1-willemdebruijn.kernel%40gmail.com