Revert "drm/gem-framebuffer: Use dma_buf from GEM object instance"

This reverts commit cce16fc.

The dma_buf field in struct drm_gem_object is not stable over the
object instance's lifetime. The field becomes NULL when user space
releases the final GEM handle on the buffer object. This resulted
in a NULL-pointer deref.

Workarounds in commit 5307dce ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9a ("drm/framebuffer:
Acquire internal references on GEM handles") only solved the problem
partially. They especially don't work for buffer objects without a DRM
framebuffer associated.

Hence, this revert to going back to using .import_attach->dmabuf.

v3:
- cc stable

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Simona Vetter <[email protected]>
Acked-by: Christian König <[email protected]>
Acked-by: Zack Rusin <[email protected]>
Cc: <[email protected]> # v6.15+
Link: https://lore.kernel.org/r/[email protected]

Author: Thomas Zimmermann

drivers/gpu/drm/drm_gem_framebuffer_helper.c

@@ -419,6 +419,7 @@ EXPORT_SYMBOL(drm_gem_fb_vunmap);
 static void __drm_gem_fb_end_cpu_access(struct drm_framebuffer *fb, enum dma_data_direction dir,
 					unsigned int num_planes)
 {
+	struct dma_buf_attachment *import_attach;
 	struct drm_gem_object *obj;
 	int ret;
 
@@ -427,9 +428,10 @@ static void __drm_gem_fb_end_cpu_access(struct drm_framebuffer *fb, enum dma_dat
 		obj = drm_gem_fb_get_obj(fb, num_planes);
 		if (!obj)
 			continue;
+		import_attach = obj->import_attach;
 		if (!drm_gem_is_imported(obj))
 			continue;
-		ret = dma_buf_end_cpu_access(obj->dma_buf, dir);
+		ret = dma_buf_end_cpu_access(import_attach->dmabuf, dir);
 		if (ret)
 			drm_err(fb->dev, "dma_buf_end_cpu_access(%u, %d) failed: %d\n",
 				ret, num_planes, dir);
@@ -452,6 +454,7 @@ static void __drm_gem_fb_end_cpu_access(struct drm_framebuffer *fb, enum dma_dat
  */
 int drm_gem_fb_begin_cpu_access(struct drm_framebuffer *fb, enum dma_data_direction dir)
 {
+	struct dma_buf_attachment *import_attach;
 	struct drm_gem_object *obj;
 	unsigned int i;
 	int ret;
@@ -462,9 +465,10 @@ int drm_gem_fb_begin_cpu_access(struct drm_framebuffer *fb, enum dma_data_direct
 			ret = -EINVAL;
 			goto err___drm_gem_fb_end_cpu_access;
 		}
+		import_attach = obj->import_attach;
 		if (!drm_gem_is_imported(obj))
 			continue;
-		ret = dma_buf_begin_cpu_access(obj->dma_buf, dir);
+		ret = dma_buf_begin_cpu_access(import_attach->dmabuf, dir);
 		if (ret)
 			goto err___drm_gem_fb_end_cpu_access;
 	}