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

netfilter: nf_conncount: fix zone comparison in tuple dedup

The "already exists" dedup logic in __nf_conncount_add() decides
whether a connection has already been counted and can be skipped instead
of incrementing the connlimit count.  It compares the conntrack zone of a
list entry with the zone of the connection being added using
nf_ct_zone_id() and nf_ct_zone_equal(), passing conn->zone.dir or
zone->dir as the direction argument.

Those helpers take enum ip_conntrack_dir values: IP_CT_DIR_ORIGINAL is 0
and IP_CT_DIR_REPLY is 1.  However, zone->dir is a u8 bitmask:
NF_CT_ZONE_DIR_ORIG is 1, NF_CT_ZONE_DIR_REPL is 2 and
NF_CT_DEFAULT_ZONE_DIR is 3.  Passing that bitmask as the enum direction
shifts the meaning of every non-zero value.  An ORIG-only zone passes 1
and is tested as REPLY, while REPL-only and default zones pass 2 or 3 and
test bits beyond the valid direction range.  In those cases
nf_ct_zone_id() can fall back to NF_CT_DEFAULT_ZONE_ID instead of using
the real zone id, so different zones can be treated as equal and dedup
collapses to tuple equality alone.

nf_conncount stores and compares the original-direction tuple for a
connection.  If an skb already has an attached conntrack entry,
get_ct_or_tuple_from_skb() explicitly copies
ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple, regardless of the packet's
ctinfo.  Therefore the zone comparison in the tuple dedup path must use
IP_CT_DIR_ORIGINAL as well; the zone direction bitmask describes where a
zone id applies, not which direction this conncount tuple represents.

Fix the two dedup comparisons by passing IP_CT_DIR_ORIGINAL directly.
Do not special-case NF_CT_DEFAULT_ZONE_DIR and do not compare raw zone
ids: using the existing helpers with IP_CT_DIR_ORIGINAL preserves the
direction-aware NF_CT_DEFAULT_ZONE_ID fallback.  A default bidirectional
zone contains the ORIG bit, so it naturally returns the real zone id;
reply-only zones continue to fall back for original-direction tuple
comparisons.