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 02:42:22PM -0700, Linus Torvalds wrote:
> On Fri, 3 May 2024 at 14:36, Al Viro <viro at 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.

Not really.  Note that select's __pollwait() does *NOT* leave a reference
at the mercy of driver - it's stuck into poll_table_entry->filp and
the poll_freewait() knows how to take those out.


dmabuf does something very different - it grabs the damn thing into
its private data structures and for all we know it could keep it for
a few hours, until some even materializes.