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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 19, 2023 at 5:31 PM Hillf Danton <hdanton@sina.com> wrote:
>
> 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
>

Yes but fput() calls eventpoll_release() *before* f_op->release(), so
waitqueue_head would be removed before trigger destruction.

> Different roads run into the same Roma city.

You butchered the phrase :)

>
> > 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.
>