CVE-2026-43116: netfilter: ctnetlink: ensure safe access to master conntrack

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

netfilter: ctnetlink: ensure safe access to master conntrack

Holding reference on the expectation is not sufficient, the master conntrack object can just go away, making exp->master invalid.

To access exp->master safely:

- Grab the nfconntrackexpectlock, this gets serialized with cleanfromlists() which also holds this lock when the master conntrack goes away.

- Hold reference on master conntrack via nfconntrackfindget(). Not so easy since the master tuple to look up for the master conntrack is not available in the existing problematic paths.

This patch goes for extending the nfconntrackexpectlock section to address this issue for simplicity, in the cases that are described below this is just slightly extending the lock section.

The add expectation command already holds a reference to the master conntrack from ctnetlinkcreateexpect().

However, the delete expectation command needs to grab the spinlock before looking up for the expectation. Expand the existing spinlock section to address this to cover the expectation lookup. Note that, the nfctexpectiteratenet() calls already grabs the spinlock while iterating over the expectation table, which is correct.

The get expectation command needs to grab the spinlock to ensure master conntrack does not go away. This also expands the existing spinlock section to cover the expectation lookup too. I needed to move the netlink skb allocation out of the spinlock to keep it GFPKERNEL.

For the expectation events, the IPEXPDESTROY event is already delivered under the spinlock, just move the delivery of IPEXPNEW under the spinlock too because the master conntrack event cache is reached through exp->master.

While at it, add lockdep notations to help identify what codepaths need to grab the spinlock.