netfilter: validate L4 headers after userspace packet writes

NFQUEUE and nft_payload can hand packet data modified by userspace back
to the stack. Recent restrictions keep link and network headers stable,
but transport header fields can still be changed.

A packet can therefore keep the same network header and conntrack entry
while changing the transport header layout. For TCP, increasing doff can
make later helper or NAT code use a different transport-header base than
the parser used, and can make offsets point past skb->tail.

Extend NFQUEUE payload validation to check the final L4 protocol and
known base headers after IPv4 options or IPv6 extension headers. Reject
packets whose L4 protocol no longer matches an attached non-template
conntrack entry, and reject IP fragments that already have such a
conntrack entry before trying to validate transport headers. Unknown L4
protocols are left to their normal protocol handlers.

For nft payload writes, reject transport-header stores that overlap TCP
doff. nft_nh_write_ok() already rejects network-header protocol changes,
so keeping doff stable prevents nft payload writes from changing the TCP
header length underneath conntrack and helper users.

This patch is a follow up to commit df07998dfd40 ("netfilter: nftables:
restrict linklayer and network header writes") and commit 54f34607d184
("netfilter: nfnetlink_queue: restrict writes to network header").

Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

0 files changed, 0 insertions, 0 deletions