Use-after-free in Linux kernel netfilter nf_tables flowtable error handling

CVE-2026-23392 is a use-after-free vulnerability in the Linux kernel netfilter nf_tables subsystem. The flaw occurs in flowtable teardown on a rare error path: after hooks are unregistered, the flowtable can be released before an RCU grace period has elapsed, even though a hook may already hold a reference to that flowtable from the packet-processing path or the nfnetlink_hook control plane. The upstream fix adds synchronize_rcu() after hook unregistration in the error path so the flowtable is only released after pre-existing RCU readers have quiesced. The issue was uncovered by KASAN, which reported a use-after-free in the nfnetlink_hook path while dumping hooks. According to the fix description, the vulnerable path is typically reached only when the maximum number of hooks is hit, hardware offload setup fails, or a late EEXIST condition occurs because device hooks are already used by a different flowtable.