net: extend IPv6 exthdr detection of tunneled packets

In the Linux kernel, the following vulnerability has been resolved:

net: extend IPv6 exthdr detection of tunneled packets

Commit c4336a07eb6b ("net: correctly handle tunneled traffic on IPV6_CSUM
GSO fallback") split skb_gso_has_extension_hdr() into mutually exclusive
branches on skb->encapsulation. This did not yet address all paths:

1. With skb->encapsulation set, the outer header is not checked. IPv6
tunnels such as ip6_gre and ip6_tunnel add an outer Destination
Options header by default (encap_limit). Their GSO packets skip
software GSO, then skb_csum_hwoffload_help() sees the outer extension
header and calls skb_checksum_help() on the GSO skb, which warns and
drops it.

2. Tunnels over IPv6 without an outer transport header, such as
ip6_tunnel, leave skb->transport_header at the inner transport
header. skb_network_header_len() then spans the outer IPv6, tunnel
and inner IP headers, a false positive.

3. UDP tunnels without an inner network header, such as SCTP-in-UDP or
PSP, have no inner IPv6 header to check. Decide on the outer header
alone.

4. Directly dereferencing inner_ip_hdr(skb)->version without
skb_header_pointer() is unsafe.

Instead, check ipv6_ext_hdr(nexthdr) on the outer IPv6 header and, if
set, on the inner IPv6 header. Read the headers with skb_header_pointer().
Keep the skb_network_header_len() check when !skb->encapsulation to also
catch encapsulated packets without skb->encapsulation (e.g., virtio).

Use the same helper in skb_csum_hwoffload_help(). Its open coded test
has the false positive of (2) and ignores the inner header.

Background: checksum offload of tunneled packets invariants:

Non-GSO skb:
- If the inner packet is CHECKSUM_PARTIAL, Local Checksum Offload computes
the outer checksum in software and the device offloads only the inner L4
checksum.
- If the inner packet is CHECKSUM_NONE (e.g., SCTP-in-UDP, ESP-in-UDP,
Remote Checksum Offload), the device offloads the outer UDP or GRE
checksum instead.

GSO skb:
- A device with NETIF_F_GSO_UDP_TUNNEL_CSUM or NETIF_F_GSO_GRE_CSUM
computes both inner and outer checksums per segment.
The outer checksum is seeded with only the pseudo-header checksum.

A NETIF_F_IPV6_CSUM device must parse through the outer headers to
reach the inner ones.