Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

[Rob Clark <robdclark@gmail.com>]

On Tue, Jul 5, 2022 at 4:51 AM Christian König <christian.koenig@amd.com> wrote:
>
> Am 01.07.22 um 11:02 schrieb Dmitry Osipenko:
> > Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
> > handle imported dma-bufs properly, which results in mapping of something
> > else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
> > userspace writes to the memory mapping of a dma-buf that was imported into
> > Tegra's DRM GEM.
> >
> > Majority of DRM drivers prohibit mapping of the imported GEM objects.
> > Mapping of imported GEMs require special care from userspace since it
> > should sync dma-buf because mapping coherency of the exporter device may
> > not match the DRM device. Let's prohibit the mapping for all DRM drivers
> > for consistency.
> >
> > Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>
> I'm pretty sure that this is the right approach, but it's certainly more
> than possible that somebody abused this already.

I suspect that this is abused if you run deqp cts on android.. ie. all
winsys buffers are dma-buf imports from gralloc.  And then when you
hit readpix...

You might only hit this in scenarios with separate gpu and display (or
dGPU+iGPU) because self-imports are handled differently in
drm_gem_prime_import_dev().. and maybe not in cases where you end up
with a blit from tiled/compressed to linear.. maybe that narrows the
scope enough to just fix it in userspace?

BR,
-R

> Anyway patch is Reviewed-by: Christian König <christian.koenig@amd.com>
> since you are really fixing a major stability problem here.
>
> Regards,
> Christian.
>
> > ---
> >   drivers/gpu/drm/drm_gem.c              | 4 ++++
> >   drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
> >   2 files changed, 4 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index 86d670c71286..fc9ec42fa0ab 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -1034,6 +1034,10 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
> >   {
> >       int ret;
> >
> > +     /* Don't allow imported objects to be mapped */
> > +     if (obj->import_attach)
> > +             return -EINVAL;
> > +
> >       /* Check for valid size. */
> >       if (obj_size < vma->vm_end - vma->vm_start)
> >               return -EINVAL;
> > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > index 8ad0e02991ca..6190f5018986 100644
> > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > @@ -609,17 +609,8 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_vm_ops);
> >    */
> >   int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct *vma)
> >   {
> > -     struct drm_gem_object *obj = &shmem->base;
> >       int ret;
> >
> > -     if (obj->import_attach) {
> > -             /* Drop the reference drm_gem_mmap_obj() acquired.*/
> > -             drm_gem_object_put(obj);
> > -             vma->vm_private_data = NULL;
> > -
> > -             return dma_buf_mmap(obj->dma_buf, vma, 0);
> > -     }
> > -
> >       ret = drm_gem_shmem_get_pages(shmem);
> >       if (ret) {
> >               drm_gem_vm_close(vma);
>