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

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

On Fri, 3 May 2024 at 14:36, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> ... the last part is no-go - poll_wait() must be able to grab a reference
> (well, the callback in it must)

Yeah. I really think that *poll* itself is doing everything right. It
knows that it's called with a file pointer with a reference, and it
adds its own references as needed.

And I think that's all fine - both for dmabuf in particular, but for
poll in general. That's how things are *supposed* to work. You can
keep references to other things in your 'struct file *', knowing that
files are properly refcounted, and won't go away while you are dealing
with them.

The problem, of course, is that then epoll violates that "called with
reference" part.  epoll very much by design does *not* take references
to the files it keeps track of, and then tears them down at close()
time.

Now, epoll has its reasons for doing that. They are even good reasons.
But that does mean that when epoll needs to deal with that hackery.

I wish we could remove epoll entirely, but that isn't an option, so we
need to just make sure that when it accesses the ffd.file pointer, it
does so more carefully.

              Linus