Re: bcachefs: suspicious mm pointer in struct dio_write

[Kent Overstreet <kent.overstreet@linux.dev>]

On Wed, Nov 27, 2024 at 02:16:24PM -0700, Jens Axboe wrote:
> I'd argue the fact that you are using an mm from a different process
> without grabbing a reference is the wrinkle. I just don't think it's a
> problem right now, but it could be... aio is tied to the mm because of
> how it does completions, potentially, and hence needs this exit_aio()
> hack because of that. aio also doesn't care, because it doesn't care
> about blocking - it'll happily block during issue.

I'm not trying to debate who's bug it is, I'm just checking if I need to
backport a security fix.

> > Jens, is it really FMODE_NOWAIT that controls whether we can hit this? A
> > very cursory glance leads me to suspect "no", it seems like this is a
> > bug if io_uring is allowed on bcachefs at all.
> 
> I just looked at bcachefs dio writes, which look to be the only case of
> this. And yes, for writes, if FMODE_NOWAIT isn't set, then io-wq is
> always involved for the IO.

Ok, sounds like we're in the clear. I already started writing the patch,
so it'll just be a "now we can turn on FMODE_NOWAIT" instead of a
bugfix.

By the way, did the lifetime issue that was causing umount/remount to
fail ever get resolved? I've currently got no test coverage for
io_uring, would be nice to flip that back on.