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

JIRA: https://issues.redhat.com/browse/RHEL-106712

commit 1918e79
Author: Thomas Zimmermann <tzimmermann@suse.de>
Date:   Tue Jul 15 17:58:17 2025 +0200

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

    This reverts commit e8afa15.

    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 <tzimmermann@suse.de>
    Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
    Acked-by: Christian König <christian.koenig@amd.com>
    Acked-by: Zack Rusin <zack.rusin@broadcom.com>
    Cc: <stable@vger.kernel.org> # v6.15+
    Link: https://lore.kernel.org/r/20250715155934.150656-8-tzimmermann@suse.de

Signed-off-by: José Expósito <jexposit@redhat.com>

Author: jexposit

drivers/gpu/drm/drm_gem_dma_helper.c

@@ -230,7 +230,7 @@ void drm_gem_dma_free(struct drm_gem_dma_object *dma_obj)
 
 	if (drm_gem_is_imported(gem_obj)) {
 		if (dma_obj->vaddr)
-			dma_buf_vunmap_unlocked(gem_obj->dma_buf, &map);
+			dma_buf_vunmap_unlocked(gem_obj->import_attach->dmabuf, &map);
 		drm_prime_gem_destroy(gem_obj, dma_obj->sgt);
 	} else if (dma_obj->vaddr) {
 		if (dma_obj->map_noncoherent)