Re: bcachefs: suspicious mm pointer in struct dio_write

[Jens Axboe <axboe@kernel.dk>]

On Wed, Nov 27, 2024 at 2:27?PM Kent Overstreet <kent.overstreet@linux.dev> wrote:
>
> 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.

Not trying to place blame.

> > > 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.

That sounds good - and FMODE_NOWAIT will be a good addition. It'll make
RWF_NOWAIT work, and things like io_uring will also work better as it
won't need to needlessly punt to an io-wq worker to complete this IO.

> 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.

Nope, I do have an updated branch since then, but it's still sitting
waiting on getting a bit more love. I suspect it'll be done for 6.14.

-- 
Jens Axboe