ipv6: seg6: clear IPv4 control block on IPIP decapsulation

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

ipv6: seg6: clear IPv4 control block on IPIP decapsulation

End.DX4 and End.DT4 decapsulate an IPv4 packet through
decap_and_validate() and send it directly to IPv4 routing. The inner
packet therefore bypasses ip_rcv_core(), which normally clears IPCB
before IPv4 interprets skb->cb.

The skb instead retains IP6CB data from the outer packet. IP6CB and
IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps
IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and
ts.

The sender can make the stale optlen byte nonzero with a valid outer
extension-header chain. The reproducers put an eight-byte Destination
Options header immediately after the 40-byte IPv6 header and before the
Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled
Destination Options offset in both lastopt and nhoff, setting them to
40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees
optlen = 40 and rr = 40.

Both tcp_v4_save_options() and __ip_options_echo() skip option copying
when optlen is zero. Here optlen is 40, so the TCP SYN path allocates
room for 40 bytes of option data and calls __ip_options_echo(). The
stale rr value makes that function read inner packet byte 41 as the
Record Route option length. The reproducers set that sender-controlled
byte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte
option-data area.

Separate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5
kernel both produced:

BUG: KASAN: slab-out-of-bounds in __ip_options_echo()
Write of size 255

The relevant End.DX4 call path is:

__ip_options_echo
tcp_v4_route_req
tcp_conn_request
tcp_v4_conn_request
tcp_rcv_state_process
tcp_v4_do_rcv
tcp_v4_rcv
ip_protocol_deliver_rcu
ip_local_deliver_finish
ip_local_deliver
input_action_end_dx4_finish
input_action_end_dx4

The relevant End.DT4 call path is:

__ip_options_echo
tcp_v4_route_req
tcp_conn_request
tcp_v4_conn_request
tcp_rcv_state_process
tcp_v4_do_rcv
tcp_v4_rcv
ip_protocol_deliver_rcu
ip_local_deliver_finish
ip_local_deliver
input_action_end_dt4

tcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so
it does not appear as a separate frame.

When decap_and_validate() handles IPPROTO_IPIP, save the ingress
interface from IP6CB, clear IPCB, and restore the saved value. Doing
this in the common decapsulation path covers End.DX4, End.DT4, and
End.DT46's IPv4 arm.

Use IP6CB(skb)->iif rather than skb->skb_iif. These actions run after
l3mdev processing, which can replace skb_iif with the L3 master;
IP6CB iif still records the receiving interface set at IPv6 ingress.