Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Apr 24, 2023 at 09:28:07AM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 24, 2023 at 11:17:55AM +0100, Lorenzo Stoakes wrote:
> > On Mon, Apr 24, 2023 at 02:43:56AM -0700, Christoph Hellwig wrote:
> > > I'm pretty sure DIRECT I/O reads that write into file backed mappings
> > > are out there in the wild.
>
> I wonder if that is really the case? I know people tried this with
> RDMA and it didn't get very far before testing uncovered data
> corruption and kernel crashes.. Maybe O_DIRECT has a much smaller race
> window so people can get away with it?
>
> > I know Jason is keen on fixing this at a fundamental level and this flag is
> > ultimately his suggestion, so it certainly doesn't stand in the way of this
> > work moving forward.
>
> Yeah, the point is to close it off, because while we wish it was
> fixed properly, it isn't. We are still who knows how far away from it.
>
> In the mean time this is a fairly simple way to oops the kernel,
> especially with cases like io_uring and RDMA. So, I view it as a
> security problem.
>
> My general dislike was that io_uring protected itself from the
> security problem and we left all the rest of the GUP users out to dry.
>
> So, my suggestion was to mark the places where we want to allow this,
> eg O_DIRECT, and block everwhere else. Lorenzo, I would significantly
> par back the list you have.

I was being fairly conservative in that list, though we certainly need to
set the flag for /proc/$pid/mem and ptrace to avoid breaking this
functionality (I observed breakpoints breaking without it which obviously
is a no go :). I'm not sure if there's a more general way we could check
for this though?

A perhaps slightly unpleasant solution might be to not enforce this when
FOLL_FORCE is specified which is mostly a ptrace + friends thing then we
could drop all those exceptions.

I wouldn't be totally opposed to dropping it for RDMA too, because I
suspect accessing file-backed mappings for that is pretty iffy.

Do you have a sense of which in the list you feel could be pared back?

>
> I also suggest we force block it at some kernel lockdown level..
>
> Alternatively, perhaps we abuse FOLL_LONGTERM and prevent it from
> working with filebacked pages since, I think, the ease of triggering a
> bug goes up the longer the pages are pinned.
>

This would solve the io_uring case and it is certainly more of a concern
when the pin is intended to be kept around, though it feels a bit icky as a
non-FOLL_LONGTERM pin could surely be problematic too?

> Jason