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)
