net: lwtunnel: Drop skb metadata before LWT encapsulation

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

net: lwtunnel: Drop skb metadata before LWT encapsulation

skb metadata is meant for passing information between XDP and TC. It lives
in the skb headroom, immediately before skb->data. LWT programs cannot
access the __sk_buff->data_meta pseudo-pointer to metadata.

However, LWT encapsulation prepends outer headers, moving skb->data back
over the headroom where the metadata sits. On an RX-originated (forwarded)
packet that still carries XDP metadata this goes wrong in two different
ways, depending on the encap type:

1. Non-BPF LWT encaps (mpls, seg6, ioam6 ...) call skb_push()/skb_pull()
and silently overwrite the metadata that sits in the headroom.

2) BPF LWT xmit calls bpf_skb_change_head(), which uses skb_data_move().
That helper expects metadata immediately before skb->data. But since
the IP output path runs LWT xmit before neighbour output has built
the outgoing L2 header, for forwarded packets skb->data points at the
L3 header while skb_mac_header() still points at the old L2 header.
skb_data_move() sees metadata ending at skb_mac_header(), not before
skb->data, warns and clears metadata:

WARNING: CPU: 21 PID: 454557 at include/linux/skbuff.h:4609 skb_data_move+0x47/0x90
CPU: 21 UID: 0 PID: 454557 Comm: napi/iconduit-g Tainted: G O 6.18.21 #1
RIP: 0010:skb_data_move+0x47/0x90
Call Trace:
<IRQ>
bpf_skb_change_head+0xe6/0x1a0
bpf_prog_...+0x213/0x2e3
run_lwt_bpf.isra.0+0x1d3/0x360
bpf_xmit+0x46/0xe0
lwtunnel_xmit+0xa1/0xf0
ip_finish_output2+0x1e7/0x5e0
ip_output+0x63/0x100
__netif_receive_skb_one_core+0x85/0xa0
process_backlog+0x9c/0x150
__napi_poll+0x2b/0x190
net_rx_action+0x40b/0x7f0
handle_softirqs+0xd2/0x270
do_softirq+0x3f/0x60
</IRQ>

That is what happens, as for how to fix it - a received packet that
carries metadata can reach an encap through any of the three LWT
redirect modes:

LWTUNNEL_STATE_INPUT_REDIRECT
ip6_rcv_finish
dst_input
lwtunnel_input

LWTUNNEL_STATE_OUTPUT_REDIRECT
ip6_rcv_finish
dst_input
ip6_forward
ip6_forward_finish
dst_output
lwtunnel_output

LWTUNNEL_STATE_XMIT_REDIRECT
ip6_rcv_finish
dst_input
ip6_forward
ip6_forward_finish
dst_output
ip6_output
ip6_finish_output
ip6_finish_output2
lwtunnel_xmit

Every encap funnels through the three LWT dispatch helpers, so drop the
metadata there, right before handing the skb to the encap op. This
single chokepoint covers all encap types and all three redirect modes:

- lwtunnel_input(): seg6, rpl, ila, seg6_local
- lwtunnel_output(): ioam6
- lwtunnel_xmit(): mpls, LWT BPF xmit

Alternatively, we could clear the metadata right after TC ingress hook.
That would require a compromise, however. Metadata would become
inaccessible from TC egress (in setups where it actually reaches the
hook it tact, that is without any L2 tunnels on path).