AZL-78650

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

netfilter: nftables: fix use-after-free in nftables_addchain()

nftablesaddchain() publishes the chain to table->chains via listaddtailrcu() (in nftchainadd()) before registering hooks. If nftablesregisterhook() then fails, the error path calls nftchaindel() (listdelrcu()) followed by nftableschain_destroy() with no RCU grace period in between.

This creates two use-after-free conditions:

1) Control-plane: nftablesdumpchains() traverses table->chains under rcuread_lock(). A concurrent dump can still be walking the chain when the error path frees it.

2) Packet path: for NFPROTOINET, nfregisternethook() briefly installs the IPv4 hook before IPv6 registration fails. Packets entering nftdochain() via the transient IPv4 hook can still be dereferencing chain->blobgenX when the error path frees the chain.

Add synchronizercu() between nftchain_del() and the chain destroy so that all RCU readers -- both dump threads and in-flight packet evaluation -- have finished before the chain is freed.