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

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, May 03, 2024 at 02:24:45PM -0700, Linus Torvalds wrote:
> 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().

eventpoll_release_file() calling __ep_remove() while ep_item_poll()
is something we need to avoid anyway - having epi freed under
ep_item_poll() would be a problem regardless of struct file
lifetime issues.