[Devel] Re: [PATCH] namespaces: fix race at task exit

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> "Serge E. Hallyn" <serue at us.ibm.com> writes:
> 
> > In do_exit(), the exit_task_namespaces() was placed after
> > exit_notify() because exit_notify ends up using the pid
> > namespace both to access the reaper, and for detaching the
> > pid.  However, this placement allows an nfs server to reap
> > the task before exit_task_namespaces() completes.
> >
> > This patch moves the exit_task_namespaces() into release_task,
> > below release_thread() which puts the pids(), and just above
> > the call_rcu(delayed_put_task_struct).  I believe this should
> > solve both problems.
> 
> 
> For the pid namespace this seems to be correct placement.
> For the mount namespace this would seem to exacerbate the problem
> because it now gets called after the task has been reaped!
> 
> I'd love to be convinced otherwise but I do not believe we
> can safely exit both the mount and the pid namespace at the
> same location in the code.
> 
> The NFS unmount currently wants a killable thread as it
> uses interruptible sleeps.  How does starting that process
> after the process in which it lives aid this?

I should have mentioned I'm unable to reproduce the original
oops myself, so i wanted confirmation about whether this fixed
the problem.

I had thought the mount problem was that the nfs server causes
the task_struct to be freed before exit_task_namespaces() completes,
so that exit_task_namespaces() dereferences a bad pointer.  If
that were the case, this would fix it by not putting the final
reference to the task_struct (with delayed_put_task_struct())
until after exit_task_namespaces().  It sounds like I misunderstood
the nfs server problem though.

> But thanks for remembering this.  This is a real problem we
> do need to solve.

If it is confirmed that my patch is wrong, then I guess we simply
need a two-stage namespace exit, where the first stage happens
above exit_notify() and exits the mounts namespace, and the second
stage can happen in the location I used in this patch.

-serge
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