vlan: require the MAC header to be present in __vlan_insert_inner_tag()

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

vlan: require the MAC header to be present in __vlan_insert_inner_tag()

__vlan_insert_inner_tag() only guarantees head room via skb_cow_head(),
never that mac_len bytes of MAC header are present. Its ETH_HLEN
wrappers - __vlan_insert_tag() under skb_vlan_push(), and
vlan_insert_tag() under validate_xmit_vlan() on the generic transmit
path - therefore rewrite the first 16 bytes at skb->data: a 12-byte
memmove plus two 2-byte stores at +12 and +14. No caller supplies the
bound, while the pop helpers use skb_ensure_writable()/pskb_may_pull().

An IFF_TUN device has hard_header_len == 0, so packet_snd() accepts a
one-byte AF_PACKET/SOCK_RAW frame. The first vlan push only sets a
hwaccel tag; the next - clsact "action vlan push" or
bpf_skb_vlan_push() - enters the helper with skb->len still 1. The
head comes from skbuff_small_head without __GFP_ZERO, so each push
drags bytes from beyond skb->tail into the frame. After three the
one-byte send leaves as 13 bytes carrying 11 bytes of uninitialised
slab:

0000: 5a b3 62 12 80 88 ff ff 00 b3 62 12 81
`------------------------------'
only 0x5a was sent; the rest is slab, here the top 56 bits of a
linear-map address

Require the MAC header the helper rewrites to be present, so such a
frame is dropped rather than transmitted.