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 dont 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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/1444ff890b4653add12f734ffeffc173d42862dd
• https://git.kernel.org/stable/c/42c574c1504aa089a0a142e4c13859327570473d
• https://git.kernel.org/stable/c/8b68a45f9722f2babe9e7bad00aa74638addf081
• https://git.kernel.org/stable/c/8c760ba4e36c750379d13569f23f5a6e185333f5
• https://git.kernel.org/stable/c/b9b6573421de51829f7ec1cce76d85f5f6fbbd7f
• https://git.kernel.org/stable/c/f41c5d151078c5348271ffaf8e7410d96f2d82f8