CVE-2026-23111

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

netfilter: nf_tables: fix inverted genmask check in nft_map_catchall_activate()

nft_map_catchall_activate() has an inverted element activity check

compared to its non-catchall counterpart nft_mapelem_activate() and

compared to what is logically required.

nft_map_catchall_activate() is called from the abort path to re-activate

catchall map elements that were deactivated during a failed transaction.

It should skip elements that are already active (they don't need

re-activation) and process elements that are inactive (they need to be

restored). Instead, the current code does the opposite: it skips inactive

elements and processes active ones.

Compare the non-catchall activate callback, which is correct:

nft_mapelem_activate():

if (nft_set_elem_active(ext, iter->genmask))

return 0; /* skip active, process inactive */

With the buggy catchall version:

nft_map_catchall_activate():

if (!nft_set_elem_active(ext, genmask))

continue; /* skip inactive, process active */

The consequence is that when a DELSET operation is aborted,

nft_setelem_data_activate() is never called for the catchall element.

For NFT_GOTO verdict elements, this means nft_data_hold() is never

called to restore the chain->use reference count. Each abort cycle

permanently decrements chain->use. Once chain->use reaches zero,

DELCHAIN succeeds and frees the chain while catchall verdict elements

still reference it, resulting in a use-after-free.

This is exploitable for local privilege escalation from an unprivileged

user via user namespaces + nftables on distributions that enable

CONFIG_USER_NS and CONFIG_NF_TABLES.

Fix by removing the negation so the check matches nft_mapelem_activate():

skip active elements, process inactive ones.