On Fri, Dec 16, 2022 at 4:20 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> On Fri, Dec 16, 2022 at 3:59 PM Chia-I Wu <olvaffe@xxxxxxxxx> wrote:
> >
> > On Fri, Dec 16, 2022 at 3:34 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
> > >
> > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > >
> > > Relying on an unreturned handle to hold a reference to an object we
> > > dereference is not safe. Userspace can guess the handle and race us
> > > by closing the handle from another thread. The _create_with_handle()
> > > that returns an object ptr is pretty much a pattern to avoid. And
> > > ideally creating the handle would be done after any needed dererencing.
> > > But in this case creation of the mapping is tied to the handle creation.
> > > Fortunately the mapping is refcnt'd and holds a reference to the object,
> > > so we can drop the handle's reference once we hold a mapping reference.
> > >
> > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > ---
> > > drivers/gpu/drm/panfrost/panfrost_drv.c | 7 +++++++
> > > drivers/gpu/drm/panfrost/panfrost_gem.c | 10 +++++++---
> > > 2 files changed, 14 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> > > index 2fa5afe21288..aa5848de647c 100644
> > > --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> > > +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> > > @@ -98,6 +98,13 @@ static int panfrost_ioctl_create_bo(struct drm_device *dev, void *data,
> > > return PTR_ERR(bo);
> > >
> > > mapping = panfrost_gem_mapping_get(bo, priv);
> > > +
> > > + /*
> > > + * Now that the mapping holds a reference to the bo until we no longer
> > > + * need it, we can safely drop the handle's reference.
> > > + */
> > Not too familiar with panfrost, but I don't see
> > panfrost_gem_mapping_get hold a reference to the bo?
>
> It doesn't directly, but the mapping already holds a reference, taken
> before the handle reference is dropped
>
> It is all a bit too subtle for my taste.
Ack.
> > > + drm_gem_object_put(&bo->base.base);
> > > +
> > > if (!mapping) {
> > > drm_gem_object_put(&bo->base.base);
> > > return -EINVAL;
> > > diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> > > index 293e799e2fe8..e3e21c500d24 100644
> > > --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> > > +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> > > @@ -234,6 +234,10 @@ struct drm_gem_object *panfrost_gem_create_object(struct drm_device *dev, size_t
> > > return &obj->base.base;
> > > }
> > >
> > > +/*
> > > + * NOTE: if this succeeds, both the handle and the returned object have
> > > + * an outstanding reference.
> > > + */
> > I might suggest dropping the "_with_handle" suffix.
>
> Maybe _and_handle would be a better name (for this and other cases
> that return both a handle and obj)?
Sounds good. We will want that for panfrost, qxl, and vmwgfx.
The other drivers and helpers only need the handle. Their
_with_handle should be fixed to not return the pointer.
>
> > The naming convention is used in several drivers. I think we should
> > make it the case that the _with_handle variants always return the
> > handle without the pointer. (And with the change, it immediately
> > becomes clear that qxl and vmwgfx also have similar issues).
>
> ugg, yeah, qxl does have the issue in the qxl_mode_dumb_create path.
> I overlooked that it returns an obj pointer by reference.
>
> on the surface vmwgfx looked ok, but I could have overlooked something.
>
> BR,
> -R
>
> > > struct panfrost_gem_object *
> > > panfrost_gem_create_with_handle(struct drm_file *file_priv,
> > > struct drm_device *dev, size_t size,
> > > @@ -261,10 +265,10 @@ panfrost_gem_create_with_handle(struct drm_file *file_priv,
> > > * and handle has the id what user can see.
> > > */
> > > ret = drm_gem_handle_create(file_priv, &shmem->base, handle);
> > > - /* drop reference from allocate - handle holds it now. */
> > > - drm_gem_object_put(&shmem->base);
> > > - if (ret)
> > > + if (ret) {
> > > + drm_gem_object_put(&shmem->base);
> > > return ERR_PTR(ret);
> > > + }
> > >
> > > return bo;
> > > }
> > > --
> > > 2.38.1
> > >
