CVE-2026-31418 — netfilter: ipset: drop logically empty buckets in mtype_del

Affected: Linux, Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP, Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP, Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP

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

netfilter: ipset: drop logically empty buckets in mtype_del

mtype_del() counts empty slots below n->pos in k, but it only drops the bucket when both n->pos and k are zero. This misses buckets whose live entries have all been removed while n->pos still points past deleted slots.

Treat a bucket as empty when all positions below n->pos are unused and release it directly instead of shrinking it further.

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CVE-2026-31418 — netfilter: ipset: drop logically empty buckets in mtype_del