CVE-2026-53349: netfilter: nf_conntrack: destroy stale expectfn expectations on unregister

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

netfilter: nfconntrack: destroy stale expectfn expectations on unregister

NAT helpers such as nfnath323 store a raw pointer to module text in exp->expectfn (e.g. ipnatq931expect). nfcthelperexpectfnunregister() only unlinks the callback descriptor and never walks the expectation table, so an expectation pending at module removal survives with a dangling exp->expectfn into freed module text.

When the expected connection arrives, initconntrack() invokes exp->expectfn(), now a stale pointer into the unloaded module. Reproduced on a KASAN build by loading the H.323 helpers, creating a Q.931 expectation, unloading nfnath323, then connecting to the expected port:

Oops: int3: 0000 [#1] SMP KASAN NOPTI RIP: 0010:0xffffffffa06102d1 initconntrack.isra.0 (net/netfilter/nfconntrackcore.c:1862) nfconntrackin (net/netfilter/nfconntrackcore.c:2049) ipv4conntracklocal (net/netfilter/nfconntrackproto.c:223) nfhookslow (net/netfilter/core.c:619) iplocalout (net/ipv4/ipoutput.c:120) tcptransmitskb (net/ipv4/tcpoutput.c:1715) tcpconnect (net/ipv4/tcpoutput.c:4374) tcpv4connect (net/ipv4/tcpipv4.c:345) sysconnect (net/socket.c:2167) Modules linked in: nfconntrackh323 [last unloaded: nfnath323]

Reaching the dangling state requires CAPSYSMODULE in the initial user namespace to remove a NAT helper that still has live expectations, so this is a robustness fix; leaving an expectation pointing at freed text is wrong regardless.

Add nfcthelperexpectfndestroy(), which walks the expectation table and drops every expectation whose ->expectfn matches the descriptor being torn down. Call it from each NAT helper's exit path after the existing RCU grace period, so no expectation outlives the code it points at and no extra synchronizercu() is introduced. With the fix, the same reproducer runs to completion without the Oops.