Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn

[Tomasz Figa]

Hi Daniel,

On Fri, Oct 9, 2020 at 7:52 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
>
> On Fri, Oct 9, 2020 at 2:48 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Fri, Oct 09, 2020 at 02:37:23PM +0200, Mauro Carvalho Chehab wrote:
> >
> > > I'm not a mm/ expert, but, from what I understood from Daniel's patch
> > > description is that this is unsafe *only if*  __GFP_MOVABLE is used.
> >
> > No, it is unconditionally unsafe. The CMA movable mappings are
> > specific VMAs that will have bad issues here, but there are other
> > types too.
> >
> > The only way to do something at a VMA level is to have a list of OK
> > VMAs, eg because they were creatd via a special mmap helper from the
> > media subsystem.
> >
> > > Well, no drivers inside the media subsystem uses such flag, although
> > > they may rely on some infrastructure that could be using it behind
> > > the bars.
> >
> > It doesn't matter, nothing prevents the user from calling media APIs
> > on mmaps it gets from other subsystems.
>
> I think a good first step would be to disable userptr of non struct
> page backed storage going forward for any new hw support. Even on
> existing drivers. dma-buf sharing has been around for long enough now
> that this shouldn't be a problem. Unfortunately right now this doesn't
> seem to exist, so the entire problem keeps getting perpetuated.
>
> > > If this is the case, the proper fix seems to have a GFP_NOT_MOVABLE
> > > flag that it would be denying the core mm code to set __GFP_MOVABLE.
> >
> > We can't tell from the VMA these kinds of details..
> >
> > It has to go the other direction, evey mmap that might be used as a
> > userptr here has to be found and the VMA specially created to allow
> > its use. At least that is a kernel only change, but will need people
> > with the HW to do this work.
>
> I think the only reasonable way to keep this working is:
> - add a struct dma_buf *vma_tryget_dma_buf(struct vm_area_struct *vma);
> - add dma-buf export support to fbdev and v4l

I assume you mean V4L2 and not the obsolete V4L that is emulated in
the userspace by libv4l. If so, every video device that uses videobuf2
gets DMA-buf export for free and there is nothing needed to enable it.

We probably still have a few legacy drivers using videobuf (non-2),
but IMHO those should be safe to put behind some disabled-by-default
Kconfig symbol or even completely drop, as the legacy framework has
been deprecated for many years already.

> - roll this out everywhere we still need it.
>
> Realistically this just isn't going to happen. And anything else just
> reimplements half of dma-buf, which is kinda pointless (you need
> minimally refcounting and some way to get at a promise of a permanent
> sg list for dma. Plus probably the vmap for kernel cpu access.
>
> > > Please let address the issue on this way, instead of broken an
> > > userspace API that it is there since 1991.
> >
> > It has happened before :( It took 4 years for RDMA to undo the uAPI
> > breakage caused by a security fix for something that was a 15 years
> > old bug.
>
> Yeah we have a bunch of these on the drm side too. Some of them are
> really just "you have to upgrade userspace", and there's no real fix
> for the security nightmare without that.

I think we need to phase out such userspace indeed. The Kconfig symbol
allows enabling the unsafe functionality for anyone who still needs
it, so I think it's not entirely a breakage.

Best regards,
Tomasz