Re: [PATCH] drm/panfrost: Fix GEM handle creation UAF

[Rob Clark]

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.

> > +       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)?

> 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
> >