Re: [PATCH 2/2] aio: fix use-after-free due to missing POLLFREE handling

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Dec 6, 2021 at 11:54 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> It could be fixed by converting signalfd and binder to use something like this,
> right?
>
>         #define wake_up_pollfree(x)  \
>                __wake_up(x, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE))

Yeah, that looks better to me.

That said, maybe it would be even better to then make it more explicit
about what it does, and not make it look like just another wakeup with
an odd parameter.

IOW, maybe that "pollfree()" function itself could very much do the
waitqueue entry removal on each entry using list_del_init(), and not
expect the wakeup code for the entry to do so.

I think that kind of explicit "this removes all entries from the wait
list head" is better than "let's do a wakeup and expect all entries to
magically implicitly remove themselves".

After all, that "implicitly remove themselves" was what didn't happen,
and caused the bug in the first place.

And all the normal cases, that don't care about POLLFREE at all,
because their waitqueues don't go away from under them, wouldn't care,
because "list_del_init()" still  leaves a valid self-pointing list in
place, so if they do list_del() afterwards, nothing happens.

I dunno. But yes, that wake_up_pollfree() of yours certainly looks
better than what we have now.

> As for eliminating POLLFREE entirely, that would require that the waitqueue
> heads be moved to a location which has a longer lifetime.

Yeah, the problem with aio and epoll is exactly that they end up using
waitqueue heads without knowing what they are.

I'm not at all convinced that there aren't other situations where the
thing the waitqueue head is embedded might not have other lifetimes.

The *common* situation is obviously that it's associated with a file,
and the file pointer ends up holding the reference to whatever device
or something (global list in a loadable module, or whatever) it is.

Hmm. The poll_wait() callback function actually does get the 'struct
file *' that the wait is associated with. I wonder if epoll queueing
could actually increment the file ref when it creates its own wait
entry, and release it at ep_remove_wait_queue()?

Maybe epoll could avoid having to remove entries entirely that way -
simply by virtue of having a ref to the files - and remove the need
for having the ->whead pointer entirely (and remove the need for
POLLFREE handling)?

And maybe the signalfd case can do the same - instead of expecting
exit() to clean up the list when sighand->count goes to zero, maybe
the signalfd filp can just hold a ref to that 'struct sighand_struct',
and it gets free'd whenever there are no signalfd users left?

That would involve making signalfd_ctx actually tied to one particular
'struct signal', but that might be the right thing to do regardless
(instead of making it always act on 'current' like it does now).

So maybe with some re-organization, we could get rid of the need for
POLLFREE entirely.. Anybody?

But your patches are certainly simpler in that they just fix the status quo.

             Linus