Re: another use-after-free in ep_remove_wait_queue()

[Hillf Danton <hdanton@sina.com>]

On Thu, 19 Jan 2023 13:01:42 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
> 
> Hi Folks,
> I spent some more time digging into the details and this is what's
> happening. When we call rmdir to delete the cgroup with the pressure
> file being epoll'ed, roughly the following call chain happens in the
> context of the shell process:
> 
> do_rmdir
>   cgroup_rmdir
>     kernfs_drain_open_files
>       cgroup_file_release
>         cgroup_pressure_release
>           psi_trigger_destroy
> 
> Later on in the context of our reproducer, the last fput() is called
> causing wait queue removal:
> 
> fput
>   ep_eventpoll_release
>     ep_free
>       ep_remove_wait_queue
>         remove_wait_queue
> 
> By this time psi_trigger_destroy() already destroyed the trigger's
> waitqueue head and we hit UAF.
> I think the conceptual problem here (or maybe that's by design?) is
> that cgroup_file_release() is not really tied to the file's real
> lifetime (when the last fput() is issued). Otherwise fput() would call
> eventpoll_release() before f_op->release() and the order would be fine
> (we would remove the wait queue first in eventpoll_release() and then
> f_op->release() would cause trigger's destruction).

  eventpoll_release
    eventpoll_release_file
      ep_remove
        ep_unregister_pollwait
	  ep_remove_wait_queue

Different roads run into the same Roma city.

> Considering these findings, I think we can use the wake_up_pollfree()
> without contradicting the comment at
> https://elixir.bootlin.com/linux/latest/source/include/linux/wait.h#L253
> because indeed, cgroup_file_release() and therefore
> psi_trigger_destroy() are not tied to the file's lifetime.
> 
> I'm CC'ing Tejun to check if this makes sense to him and
> cgroup_file_release() is working as expected in this case.
> 
> Munehisha, if Tejun confirms this is all valid, could you please post
> a patch replacing wake_up_interruptible() with wake_up_pollfree()? We
> don't need to worry about wake_up_all() because we have a limitation
> of one trigger per file descriptor:
> https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L1419,
> so there can be only one waiter.
> Thanks,
> Suren.