drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

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. For example, 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.

To fix this bug, move mapping of imported dma-bufs to drm_gem_mmap_obj().
Now mmaping of imported dma-bufs works properly for all DRM drivers.

Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

drivers/gpu/drm/drm_gem.c

@@ -1038,6 +1038,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
 	if (obj_size < vma->vm_end - vma->vm_start)
 		return -EINVAL;
 
+	if (obj->import_attach)
+		return dma_buf_mmap(obj->dma_buf, vma, 0);
+
 	/* Take a ref for this mapping of the object, so that the fault
 	 * handler can dereference the mmap offset's pointer to the object.
 	 * This reference is cleaned up by the corresponding vm_close

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

drivers/gpu/drm/tegra/gem.c

@@ -564,6 +564,10 @@ int __tegra_gem_mmap(struct drm_gem_object *gem, struct vm_area_struct *vma)
 {
 	struct tegra_bo *bo = to_tegra_bo(gem);
 
+	/* imported dmu-buf is mapped by drm_gem_mmap_obj()  */
+	if (gem->import_attach)
+		return 0;
+
 	if (!bo->pages) {
 		unsigned long vm_pgoff = vma->vm_pgoff;
 		int err;