netfilter: nf_nat: also check reverse tuple to obtain clashing entry · Pull Request !5422 · anolis/cloud-kernel

1. netfilter: nf_nat: also check reverse tuple to obtain clashing entry
The logic added in the blamed commit was supposed to only omit nat source
port allocation if neither the existing nor the new entry are subject to
NAT.

However, its not enough to lookup the conntrack based on the proposed
tuple, we must also check the reverse direction.

Otherwise there are esoteric cases where the collision is in the reverse
direction because that colliding connection has a port rewrite, but the
new entry doesn't.  In this case, we only check the new entry and then
erronously conclude that no clash exists anymore.

The existing (udp) tuple is:
  a:p -> b:P, with nat translation to s:P, i.e. pure daddr rewrite,
  reverse tuple in conntrack table is s:P -> a:p.

When another UDP packet is sent directly to s, i.e. a:p->s:P, this is
correctly detected as a colliding entry: tuple is taken by existing reply
tuple in reverse direction.

But the colliding conntrack is only searched for with unreversed
direction, and we can't find such entry matching a:p->s:P.

The incorrect conclusion is that the clashing entry has timed out and
that no port address translation is required.

Such conntrack will then be discarded at nf_confirm time because the
proposed reverse direction clashes with an existing mapping in the
conntrack table.

Search for the reverse tuple too, this will then check the NAT bits of
the colliding entry and triggers port reallocation.

Followp patch extends nft_nat.sh selftest to cover this scenario.

The IPS_SEQ_ADJUST change is also a bug fix:
Instead of checking for SEQ_ADJ this tested for SEEN_REPLY and ASSURED
by accident -- _BIT is only for use with the test_bit() API.

This bug has little consequence in practice, because the sequence number
adjustments are only useful for TCP which doesn't support clash resolution.

The existing test case (conntrack_reverse_clash.sh) exercise a race
condition path (parallel conntrack creation on different CPUs), so
the colliding entries have neither SEEN_REPLY nor ASSURED set.

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