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-…

Overview

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.

Affected

• linux_kernel >= 4.19.316, < 4.20
• linux_kernel >= 5.4.262, < 5.5
• linux_kernel >= 5.10.188, < 5.11
• linux_kernel >= 5.15.121, < 5.15.200
• linux_kernel >= 6.1.36, < 6.1.163
• linux_kernel >= 6.3.10, < 6.4
• linux_kernel >= 6.4.1, < 6.6.124
• linux_kernel >= 6.7, < 6.12.70
• linux_kernel >= 6.13, < 6.18.10
• linux_kernel = 6.4
• linux_kernel = 6.19

Remediation

Upgrade past the affected range:

• linux_kernel 6.18.10