get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove)

[Linus Torvalds]

On Fri, 3 May 2024 at 14:11, Al Viro <viro at zeniv.linux.org.uk> wrote:
>
> What we need is
>         * promise that ep_item_poll() won't happen after eventpoll_release_file().
> AFAICS, we do have that.
>         * ->poll() not playing silly buggers.

No. That is not enough at all.

Because even with perfectly normal "->poll()", and even with the
ep_item_poll() happening *before* eventpoll_release_file(), you have
this trivial race:

  ep_item_poll()
     ->poll()

and *between* those two operations, another CPU does "close()", and
that causes eventpoll_release_file() to be called, and now f_count
goes down to zero while ->poll() is running.

So you do need to increment the file count around the ->poll() call, I feel.

Or, alternatively, you'd need to serialize with
eventpoll_release_file(), but that would need to be some sleeping lock
held over the ->poll() call.

> As it is, dma_buf ->poll() is very suspicious regardless of that
> mess - it can grab reference to file for unspecified interval.

I think that's actually much preferable to what epoll does, which is
to keep using files without having reference counts to them (and then
relying on magically not racing with eventpoll_release_file().

                Linus