From 8244a3bc27b3efd057da154b8d7e414670d5044f Mon Sep 17 00:00:00 2001
From: Anand K Mistry <amistry@google.com>
Date: Thu, 30 Sep 2021 09:00:07 +1000
Subject: drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap
Git-commit: 8244a3bc27b3efd057da154b8d7e414670d5044f
Patch-mainline: v5.16-rc1
References: bsc#1152472

drm_gem_ttm_mmap() drops a reference to the gem object on success. If
the gem object's refcount == 1 on entry to drm_gem_prime_mmap(), that
drop will free the gem object, and the subsequent drm_gem_object_get()
will be a UAF. Fix by grabbing a reference before calling the mmap
helper.

This issue was forseen when the reference dropping was adding in
commit 9786b65bc61ac ("drm/ttm: fix mmap refcounting"):
  "For that to work properly the drm_gem_object_get() call in
  drm_gem_ttm_mmap() must be moved so it happens before calling
  obj->funcs->mmap(), otherwise the gem refcount would go down
  to zero."

Signed-off-by: Anand K Mistry <amistry@google.com>
Fixes: 9786b65bc61a ("drm/ttm: fix mmap refcounting")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.5+
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930085932.1.I8043d61cc238e0168e2f4ca5f4783223434aa587@changeid
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/drm_prime.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -719,11 +719,13 @@ int drm_gem_prime_mmap(struct drm_gem_ob
 	vma->vm_pgoff += drm_vma_node_start(&obj->vma_node);
 
 	if (obj->funcs && obj->funcs->mmap) {
+		drm_gem_object_get(obj);
 		ret = obj->funcs->mmap(obj, vma);
-		if (ret)
-			return ret;
+		if (ret) {
+			drm_gem_object_put(obj);
+ 			return ret;
+		}
 		vma->vm_private_data = obj;
-		drm_gem_object_get(obj);
 		return 0;
 	}