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

[Rob Clark]

On Wed, Aug 10, 2022 at 4:47 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Wed, Jul 06, 2022 at 10:02:07AM +0300, Dmitry Osipenko wrote:
> > On 7/6/22 00:48, Rob Clark wrote:
> > > On Tue, Jul 5, 2022 at 4:51 AM Christian König <christian.koenig@xxxxxxx> 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@xxxxxxxxxxxxxxx>
> > >>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> > >>
> > >> 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?
> >
> > Given that that only drivers which use DRM-SHMEM potentially could've
> > map imported dma-bufs (Panfrost, Lima) and they already don't allow to
> > do that, I think we're good.
>
> So can I have an ack from Rob here or are there still questions that this
> might go boom?
>
> Dmitry, since you have a bunch of patches merged now I think would also be
> good to get commit rights so you can drive this more yourself. I've asked
> Daniel Stone to help you out with getting that.

I *think* we'd be ok with this on msm, mostly just by dumb luck.
Because the dma-buf's we import will be self-import.  I'm less sure
about panfrost (src/panfrost/lib/pan_bo.c doesn't seem to have a
special path for imported dma-bufs either, and in that case they won't
be self-imports.. but I guess no one has tried to run android cts on
panfrost).

What about something less drastic to start, like (apologies for
hand-edited patch):

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;

+       WARN_ON_ONCE(obj->import_attach);
+
        /* 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);
+               return -EINVAL;
        }

        ret = drm_gem_shmem_get_pages(shmem);
        if (ret) {
                drm_gem_vm_close(vma);
--
2.36.1