net/sched: act_ife: Only operate on Ethernet frames

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

net/sched: act_ife: Only operate on Ethernet frames

act_ife encapsulates/decapsulates the original Ethernet header and uses
skb->dev->hard_header_len as the length of that header. That is only
correct for Ethernet devices: on a device where hard_header_len does not
match the L2 header that was actually pulled (PPP reports PPP_HDRLEN
while nothing is stripped on ingress), the ingress skb_push()/skb_pull()
use the wrong length and can hit skb_under_panic when headroom is tight.

IFE is Ethernet-only by design - it builds an outer ethhdr, rewrites
h_source/h_dest/h_proto, and calls eth_type_trans() on decode - so
instead of trying to make the offsets work for arbitrary link types,
simply drop packets that do not carry an Ethernet header.

Checking skb->dev->type alone is not enough. We have to cater for a
corner case where mirred can redirect an skb from a non-Ethernet device
to an Ethernet one, and skb->dev then says nothing about the framing the
skb actually has: an skb redirected from ppp0 reaches the target's ingress
hook with mac_len 0 and no Ethernet header at all. So at ingress also
require mac_len to be ETH_HLEN. On egress mac_len is not maintained, so
the device type is all we have; a bogus redirect there yields a malformed
frame rather than an out-of-bounds push, and it would be malformed with or
without IFE.

That corner case is not theoretical - redirecting from ppp0 into a veth
that has an ife encode action on its ingress hook panics without this
patch:

skbuff: skb_under_panic: len:98 put:14 head:ffff88800e410000
data:ffff88800e40fff5 tail:0x57 end:0x640 dev:veth3
kernel BUG at net/core/skbuff.c:214!
Call Trace:
skb_push (net/core/skbuff.c:224 net/core/skbuff.c:2657)
tcf_ife_act (net/sched/act_ife.c:829 net/sched/act_ife.c:874)
tc_run (net/core/dev.c:4463)
netif_receive_skb (net/core/dev.c:6463 net/core/dev.c:6522)
tcf_mirred_to_dev (net/sched/act_mirred.c:248 net/sched/act_mirred.c:328)
tcf_mirred_act (net/sched/act_mirred.c:489)
tc_run (net/core/dev.c:4463)
process_backlog (net/core/dev.c:6728)

With Ethernet framing guaranteed, use ETH_HLEN instead of
hard_header_len.