Comment 7 for bug 843892

The attach patch fixes the issue.

The oops is called from cleanup_net when the namespace is destroyed. conntrack iterates through outstanding events and calls death_by_timeout on each of them, which in turn produces a call to ctnetlink_conntrack_event. This calls nf_netlink_has_listeners, which oopses because net->nfnl is NULL.

I made the container through (essentially) 'unshare -n'; I didn't explicitly set up netlink, but I presume it was set up else net->nfnl would have been NULL earlier (i.e. when an earlier connection timed out). This would thus suggest that net->nfnl is made NULL during the destruction of the container, which I think is done by nfnetlink_net_exit_batch.

I can see that the various subsystems are deinitialised in the opposite order to which the relevant register_pernet_subsys calls are called, and both nf_conntrack and nfnetlink_net_ops register their relevant subsystems. If nfnetlink_net_ops registered later than nfconntrack, then its exit routine would have been called first, which would cause the oops described. I am not sure there is anything to prevent this happening in a container environment.

Whilst there's perhaps a more complex problem revolving around ordering of subsystem deinit, it seems to me that missing a netlink event on a container that is dying is not a disaster. An early check for net->nfnl being non-NULL in ctnetlink_conntrack_event appears to fix this. There may remain a potential race condition if it becomes NULL immediately after being checked (I am not sure any lock is held at this point or how synchronisation for subsystem deinitialization works).

This patch should apply on everything from 2.6.26 (if not before) onwards; it appears to be a problem on all kernels. This was taken against Ubuntu-3.0.0-11.17. I have torture-tested it with the above perl script for 15 minutes or so; the perl script hung the machine within 20 seconds without this patch.