Re: [PATCH] drm/panfrost: Fix GEM handle creation ref-counting

[Rob Clark]

On Mon, Dec 19, 2022 at 6:02 AM Steven Price <steven.price@xxxxxxx> wrote:
>
> panfrost_gem_create_with_handle() previously returned a BO but with the
> only reference being from the handle, which user space could in theory
> guess and release, causing a use-after-free. Additionally if the call to
> panfrost_gem_mapping_get() in panfrost_ioctl_create_bo() failed then
> a(nother) reference on the BO was dropped.
>
> The _create_with_handle() is a problematic pattern, so ditch it and
> instead create the handle in panfrost_ioctl_create_bo(). If the call to
> panfrost_gem_mapping_get() fails then this means that user space has
> indeed gone behind our back and freed the handle. In which case just
> return an error code.
>
> Reported-by: Rob Clark <robdclark@xxxxxxxxxxxx>

Yeah, I like getting rid of the _create_with_handle() pattern, the
only place where that pattern works is if you immediately return the
handle to userspace (and don't otherwise touch the obj)

Reviewed-by: Rob Clark <robdclark@xxxxxxxxx>

> Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")
> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> ---
>  drivers/gpu/drm/panfrost/panfrost_drv.c | 27 ++++++++++++++++---------
>  drivers/gpu/drm/panfrost/panfrost_gem.c | 16 +--------------
>  drivers/gpu/drm/panfrost/panfrost_gem.h |  5 +----
>  3 files changed, 20 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index fa619fe72086..abb0dadd8f63 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -82,6 +82,7 @@ static int panfrost_ioctl_create_bo(struct drm_device *dev, void *data,
>         struct panfrost_gem_object *bo;
>         struct drm_panfrost_create_bo *args = data;
>         struct panfrost_gem_mapping *mapping;
> +       int ret;
>
>         if (!args->size || args->pad ||
>             (args->flags & ~(PANFROST_BO_NOEXEC | PANFROST_BO_HEAP)))
> @@ -92,21 +93,29 @@ static int panfrost_ioctl_create_bo(struct drm_device *dev, void *data,
>             !(args->flags & PANFROST_BO_NOEXEC))
>                 return -EINVAL;
>
> -       bo = panfrost_gem_create_with_handle(file, dev, args->size, args->flags,
> -                                            &args->handle);
> +       bo = panfrost_gem_create(dev, args->size, args->flags);
>         if (IS_ERR(bo))
>                 return PTR_ERR(bo);
>
> +       ret = drm_gem_handle_create(file, &bo->base.base, &args->handle);
> +       if (ret)
> +               goto out;
> +
>         mapping = panfrost_gem_mapping_get(bo, priv);
> -       if (!mapping) {
> -               drm_gem_object_put(&bo->base.base);
> -               return -EINVAL;
> +       if (mapping) {
> +               args->offset = mapping->mmnode.start << PAGE_SHIFT;
> +               panfrost_gem_mapping_put(mapping);
> +       } else {
> +               /* This can only happen if the handle from
> +                * drm_gem_handle_create() has already been guessed and freed
> +                * by user space
> +                */
> +               ret = -EINVAL;
>         }
>
> -       args->offset = mapping->mmnode.start << PAGE_SHIFT;
> -       panfrost_gem_mapping_put(mapping);
> -
> -       return 0;
> +out:
> +       drm_gem_object_put(&bo->base.base);
> +       return ret;
>  }
>
>  /**
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> index 293e799e2fe8..3c812fbd126f 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -235,12 +235,8 @@ struct drm_gem_object *panfrost_gem_create_object(struct drm_device *dev, size_t
>  }
>
>  struct panfrost_gem_object *
> -panfrost_gem_create_with_handle(struct drm_file *file_priv,
> -                               struct drm_device *dev, size_t size,
> -                               u32 flags,
> -                               uint32_t *handle)
> +panfrost_gem_create(struct drm_device *dev, size_t size, u32 flags)
>  {
> -       int ret;
>         struct drm_gem_shmem_object *shmem;
>         struct panfrost_gem_object *bo;
>
> @@ -256,16 +252,6 @@ panfrost_gem_create_with_handle(struct drm_file *file_priv,
>         bo->noexec = !!(flags & PANFROST_BO_NOEXEC);
>         bo->is_heap = !!(flags & PANFROST_BO_HEAP);
>
> -       /*
> -        * Allocate an id of idr table where the obj is registered
> -        * 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)
> -               return ERR_PTR(ret);
> -
>         return bo;
>  }
>
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.h b/drivers/gpu/drm/panfrost/panfrost_gem.h
> index 8088d5fd8480..ad2877eeeccd 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.h
> @@ -69,10 +69,7 @@ panfrost_gem_prime_import_sg_table(struct drm_device *dev,
>                                    struct sg_table *sgt);
>
>  struct panfrost_gem_object *
> -panfrost_gem_create_with_handle(struct drm_file *file_priv,
> -                               struct drm_device *dev, size_t size,
> -                               u32 flags,
> -                               uint32_t *handle);
> +panfrost_gem_create(struct drm_device *dev, size_t size, u32 flags);
>
>  int panfrost_gem_open(struct drm_gem_object *obj, struct drm_file *file_priv);
>  void panfrost_gem_close(struct drm_gem_object *obj,
> --
> 2.34.1
>