In cases where the header straight after the tunnel header was
another ethernet header (TEB), instead of the network header,
the ECN decapsulation code would treat the ethernet header as if
it was an IP header, resulting in mishandling and possible
wrong drops or corruption of the IP header.

In this case, ECT(1) is sent, so IP_ECN_decapsulate tries to copy it to the
inner IPv4 header, and correct its checksum.

The offset of the ECT bits in an IPv4 header corresponds to the
lower 2 bits of the second octet of the destination MAC address
in the ethernet header.
The IPv4 checksum corresponds to end of the source address.

In order to reproduce:

    $ ip netns add A
    $ ip netns add B
    $ ip -n A link add _v0 type veth peer name _v1 netns B
    $ ip -n A link set _v0 up
    $ ip -n A addr add dev _v0 10.254.3.1/24
    $ ip -n A route add default dev _v0 scope global
    $ ip -n B link set _v1 up
    $ ip -n B addr add dev _v1 10.254.1.6/24
    $ ip -n B route add default dev _v1 scope global
    $ ip -n B link add gre1 type gretap local 10.254.1.6 remote 10.254.3.1 key 0x49000000
    $ ip -n B link set gre1 up

    # Now send an IPv4/GRE/Eth/IPv4 frame where the outer header has ECT(1),
    # and the inner header has no ECT bits set:

    $ cat send_pkt.py
        #!/usr/bin/env python3
        from scapy.all import *

        pkt = IP(b'E\x01\x00\xa7\x00\x00\x00\x00@/`%\n\xfe\x03\x01\n\xfe\x01\x06 \x00eXI\x00'
                 b'\x00\x00\x18\xbe\x92\xa0\xee&\x18\xb0\x92\xa0l&\x08\x00E\x00\x00}\x8b\x85'
                 b'@\x00\x01\x01\xe4\xf2\x82\x82\x82\x01\x82\x82\x82\x02\x08\x00d\x11\xa6\xeb'
                 b'3\x1e\x1e\\xf3\\xf7`\x00\x00\x00\x00ZN\x00\x00\x00\x00\x00\x00\x10\x11\x12'
                 b'\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-./01234'
                 b'56789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ')

        send(pkt)
    $ sudo ip netns exec B tcpdump -neqlllvi gre1 icmp & ; sleep 1
    $ sudo ip netns exec A python3 send_pkt.py

In the original packet, the source/destinatio MAC addresses are
dst=18:be:92:a0:ee:26 src=18:b0:92:a0:6c:26

In the received packet, they are
dst=18:bd:92:a0:ee:26 src=18:b0:92:a0:6c:27

Thanks to Lahav Schlesinger <lschlesinger@drivenets.com> and Isaac Garzon <isaac@speed.io>
for helping me pinpoint the origin.

Fixes: b7237487 ("tunnel: Propagate ECT(1) when decapsulating as recommended by RFC6040")
Cc: David S. Miller <davem@davemloft.net>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Gilad Naaman <gnaaman@drivenets.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>