netfilter: nft_payload: incorrect arithmetics when fetching VLAN header bits

netfilter: nft_payload: incorrect arithmetics when fetching VLAN header bits

Origin: https://git.kernel.org/linus/696e1a48b1a1b01edad542a1ef293665864a4dd0
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2023-0179

If the offset + length goes over the ethernet + vlan header, then the
length is adjusted to copy the bytes that are within the boundaries of
the vlan_ethhdr scratchpad area. The remaining bytes beyond ethernet +
vlan header are copied directly from the skbuff data area.

Fix incorrect arithmetic operator: subtract, not add, the size of the
vlan header in case of double-tagged packets to adjust the length
accordingly to address CVE-2023-0179.

Reported-by: Davide Ornaghi <d.ornaghi97@gmail.com>
Fixes: f6ae9f120dad ("netfilter: nft_payload: add C-VLAN support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name netfilter-nft_payload-incorrect-arithmetics-when-fet.patch