net: loopback: Avoid sending IP packets without an Ethernet header

net: loopback: Avoid sending IP packets without an Ethernet header

After commit 22600596b675 ("ipv4: give an IPv4 dev to blackhole_netdev")
IPv4 neighbors can be constructed on the blackhole net device, but they
are constructed with an output function (neigh_direct_output()) that
simply calls dev_queue_xmit(). The latter will transmit packets via
'skb->dev' which might not be the blackhole net device if dst_dev_put()
switched 'dst->dev' to the blackhole net device while another CPU was
using the dst entry in ip_output(), but after it already initialized
'skb->dev' from 'dst->dev'.

Specifically, the following can happen:

    CPU1                                      CPU2

udp_sendmsg(sk1)                          udp_sendmsg(sk2)
udp_send_skb()                            [...]
ip_output()
    skb->dev = skb_dst(skb)->dev
                                          dst_dev_put()
                                              dst->dev = blackhole_netdev
ip_finish_output2()
    resolves neigh on dst->dev
neigh_output()
neigh_direct_output()
dev_queue_xmit()

This will result in IPv4 packets being sent without an Ethernet header
via a valid net device:

tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on enp9s0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
22:07:02.329668 20:00:40:11:18:fb > 45:00:00:44:f4:94, ethertype Unknown
(0x58c6), length 68:
        0x0000:  8dda 74ca f1ae ca6c ca6c 0098 969c 0400  ..t....l.l......
        0x0010:  0000 4730 3f18 6800 0000 0000 0000 9971  ..G0?.h........q
        0x0020:  c4c9 9055 a157 0a70 9ead bf83 38ca ab38  ...U.W.p....8..8
        0x0030:  8add ab96 e052                           .....R

Fix by making sure that neighbors are constructed on top of the
blackhole net device with an output function that simply consumes the
packets, in a similar fashion to dst_discard_out() and
blackhole_netdev_xmit().

Fixes: 8d7017fd621d ("blackhole_netdev: use blackhole_netdev to invalidate dst entries")
Fixes: 22600596b675 ("ipv4: give an IPv4 dev to blackhole_netdev")
Reported-by: Florian Meister <fmei@sfs.com>
Closes: https://lore.kernel.org/netdev/20250210084931.23a5c2e4@hermes.local/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250220072559.782296-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>