Re: [PATCH v2] drm/vmwgfx: Do not drop the reference to the handle too soon

["Martin Krastev (VMware)" <martinkrastev768@xxxxxxxxx>]

From: Martin Krastev <krastevm@xxxxxxxxxx>


LGTM
Reviewed-by: Martin Krastev <krastevm@xxxxxxxxxx>


Regards,
Martin

On 9.02.23 г. 14:37 ч., Zack Rusin wrote:
> From: Zack Rusin <zackr@xxxxxxxxxx>
>
> v2: Update the commit message to include note describing why the second
> usag of vmw_gem_object_create_with_handle in vmwgfx_surface.c wasn't
> changed
>
> It is possible for userspace to predict the next buffer handle and
> to destroy the buffer while it's still used by the kernel. Delay
> dropping the internal reference on the buffers until kernel is done
> with them.
>
> Also fixes the second usage of vmw_gem_object_create_with_handle in
> vmwgfx_surface.c which wasn't grabbing an explicit reference
> to the gem object which could have been destroyed by the userspace
> on the owning surface at any point.
>
> Signed-off-by: Zack Rusin <zackr@xxxxxxxxxx>
> Fixes: 8afa13a0583f ("drm/vmwgfx: Implement DRIVER_GEM")
> Cc: <stable@xxxxxxxxxxxxxxx> # v5.17+
> ---
>   drivers/gpu/drm/vmwgfx/vmwgfx_bo.c      | 3 ++-
>   drivers/gpu/drm/vmwgfx/vmwgfx_gem.c     | 4 ++--
>   drivers/gpu/drm/vmwgfx/vmwgfx_surface.c | 1 -
>   3 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
> index 43ffa5c7acbd..65bd88c8fef9 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
> @@ -708,7 +708,8 @@ int vmw_dumb_create(struct drm_file *file_priv,
>   	ret = vmw_gem_object_create_with_handle(dev_priv, file_priv,
>   						args->size, &args->handle,
>   						&vbo);
> -
> +	/* drop reference from allocate - handle holds it now */
> +	drm_gem_object_put(&vbo->tbo.base);
>   	return ret;
>   }
>   
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c b/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
> index 51bd1f8c5cc4..d6baf73a6458 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
> @@ -133,8 +133,6 @@ int vmw_gem_object_create_with_handle(struct vmw_private *dev_priv,
>   	(*p_vbo)->tbo.base.funcs = &vmw_gem_object_funcs;
>   
>   	ret = drm_gem_handle_create(filp, &(*p_vbo)->tbo.base, handle);
> -	/* drop reference from allocate - handle holds it now */
> -	drm_gem_object_put(&(*p_vbo)->tbo.base);
>   out_no_bo:
>   	return ret;
>   }
> @@ -161,6 +159,8 @@ int vmw_gem_object_create_ioctl(struct drm_device *dev, void *data,
>   	rep->map_handle = drm_vma_node_offset_addr(&vbo->tbo.base.vma_node);
>   	rep->cur_gmr_id = handle;
>   	rep->cur_gmr_offset = 0;
> +	/* drop reference from allocate - handle holds it now */
> +	drm_gem_object_put(&vbo->tbo.base);
>   out_no_bo:
>   	return ret;
>   }
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
> index 9d4ae9623a00..d18fec953fa7 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
> @@ -867,7 +867,6 @@ int vmw_surface_define_ioctl(struct drm_device *dev, void *data,
>   			goto out_unlock;
>   		}
>   		vmw_bo_reference(res->guest_memory_bo);
> -		drm_gem_object_get(&res->guest_memory_bo->tbo.base);
>   	}
>   
>   	tmp = vmw_resource_reference(&srf->res);