Re: [PATCH 13/13] vfio/type1: Mark follow_pfn as unsafe

[Daniel Vetter]

On Wed, Oct 7, 2020 at 7:39 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Wed, Oct 07, 2020 at 06:44:26PM +0200, Daniel Vetter wrote:
> > The code seems to stuff these pfns into iommu pts (or something like
> > that, I didn't follow), but there's no mmu_notifier to ensure that
> > access is synchronized with pte updates.
> >
> > Hence mark these as unsafe. This means that with
> > CONFIG_STRICT_FOLLOW_PFN, these will be rejected.
> >
> > Real fix is to wire up an mmu_notifier ... somehow. Probably means any
> > invalidate is a fatal fault for this vfio device, but then this
> > shouldn't ever happen if userspace is reasonable.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
> > Cc: linux-media@xxxxxxxxxxxxxxx
> > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> > Cc: kvm@xxxxxxxxxxxxxxx
> > ---
> >  drivers/vfio/vfio_iommu_type1.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 5fbf0c1f7433..a4d53f3d0a35 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -421,7 +421,7 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> >  {
> >       int ret;
> >
> > -     ret = follow_pfn(vma, vaddr, pfn);
> > +     ret = unsafe_follow_pfn(vma, vaddr, pfn);
> >       if (ret) {
> >               bool unlocked = false;
> >
> > @@ -435,7 +435,7 @@ static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> >               if (ret)
> >                       return ret;
> >
> > -             ret = follow_pfn(vma, vaddr, pfn);
> > +             ret = unsafe_follow_pfn(vma, vaddr, pfn);
> >       }
>
> This is actually being commonly used, so it needs fixing.
>
> When I talked to Alex about this last we had worked out a patch series
> that adds a test on vm_ops that the vma came from vfio in the first
> place. The VMA's created by VFIO are 'safe' as the PTEs are never changed.

Hm, but wouldn't need that the semi-nasty vma_open trick to make sure
that vma doesn't untimely disappear? Or is the idea to look up the
underlying vfio object, and refcount that directly?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch