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

[Al Viro]

On Fri, May 03, 2024 at 10:11:09PM +0100, Al Viro wrote:
> On Fri, May 03, 2024 at 01:28:37PM -0700, Kees Cook wrote:
> > 
> > Is this the right approach? It still feels to me like get_file() needs
> > to happen much earlier...
> 
> I don't believe it needs to happen at all.  The problem is not that
> ->release() can be called during ->poll() - it can't and it doesn't.
> It's that this instance of ->poll() is trying to extend the lifetime
> of that struct file, when it might very well be past the point of no
> return.
> 
> 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.
> 
> As it is, dma_buf ->poll() is very suspicious regardless of that
> mess - it can grab reference to file for unspecified interval.
> Have that happen shortly before reboot and you are asking for failing
> umount.
> 
> ->poll() must be refcount-neutral wrt file passed to it.  I'm seriously
> tempted to make ->poll() take const struct file * and see if there's
> anything else that would fall out.

... the last part is no-go - poll_wait() must be able to grab a reference
(well, the callback in it must)