xen/netfront: drop RX packets with a short Ethernet header
In the Linux kernel, the following vulnerability has been resolved:
xen/netfront: drop RX packets with a short Ethernet header
handle_incoming_queue() pulls pull_to bytes into the head before
calling eth_type_trans(). pull_to is the length of the first RX slot,
capped at RX_COPY_THRESHOLD, and that length comes from the backend.
Nothing checks it against ETH_HLEN.
If the first slot is shorter than ETH_HLEN and more slots follow, the
head ends up shorter than an Ethernet header while skb->len is longer,
and eth_type_trans() BUG()s in __skb_pull(). If the whole packet is
shorter than ETH_HLEN, eth_type_trans() reads the header past the end
of the data instead.
Pull at least ETH_HLEN, and drop the packet if that fails, which also
drops packets too short to hold an Ethernet header. This also checks
the return value of the pull, which was ignored.
