Date:   Thu, 19 Jan 2023 18:52:53 -0800
From:   Munehisa Kamata <kamatam@...zon.com>
To:     <surenb@...gle.com>
CC:     <ebiggers@...nel.org>, <hannes@...xchg.org>, <hdanton@...a.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <mengcc@...zon.com>, <kamatam@...zon.com>, <tj@...nel.org>
Subject: Re: another use-after-free in ep_remove_wait_queue()

On Fri, 2023-01-20 02:46:13 +0000, Munehisa Kamata <kamatam@...zon.com> wrote:
>
> On Fri, 2023-01-20 01:37:11 +0000, Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> > On Thu, Jan 19, 2023 at 5:31 PM Hillf Danton <hdanton@...a.com> wrote:
> > >
> > > On Thu, 19 Jan 2023 13:01:42 -0800 Suren Baghdasaryan <surenb@...gle.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.
>  
> But pwq->whead is still pointing the freed head, then we just hit the same
> issue earlier?
 
Ah nevermind, that was just a hypothetical case if cgroup_file_release()
was tied to file's lifetime and assuming trigger destruction that frees    
the queue and clears pwq->whead would happen later in f_op->release();     
there is no such an implementation today.                                  
                                                                           
Sorry for noise.

> > > 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.
> > >
> > 
> > 
> 
> 

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