drm/vgem: Close use-after-free race in vgem_gem_create

commit 4b848f20eda5974020f043ca14bacf7a7e634fc8 upstream.

There's two references floating around here (for the object reference,
not the handle_count reference, that's a different thing):

- The temporary reference held by vgem_gem_create, acquired by
  creating the object and released by calling

- The reference held by the object handle, created by
  drm_gem_handle_create. This one generally outlives the function,
  except if a 2nd thread races with a GEM_CLOSE ioctl call.

So usually everything is correct, except in that race case, where the
access to gem_object->size could be looking at freed data already.
Which again isn't a real problem (userspace shot its feet off already
with the race, we could return garbage), but maybe someone can exploit
this as an information leak.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Hillf Danton <hdanton@sina.com>
Reported-by: syzbot+0dc4444774d419e916c8@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Anholt <eric@anholt.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Rob Clark <robdclark@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200202132133.1891846-1-daniel.vetter@ffwll.ch
[OP: backport to 4.19: adjusted DRM_DEBUG() -> DRM_DEBUG_DRIVER()]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
index 1c1a435..56ed771 100644
--- a/drivers/gpu/drm/vgem/vgem_drv.c
+++ b/drivers/gpu/drm/vgem/vgem_drv.c
@@ -189,9 +189,10 @@
 		return ERR_CAST(obj);
 	ret = drm_gem_handle_create(file, &obj->base, handle);
-	drm_gem_object_put_unlocked(&obj->base);
-	if (ret)
+	if (ret) {
+		drm_gem_object_put_unlocked(&obj->base);
 		return ERR_PTR(ret);
+	}
 	return &obj->base;
@@ -214,7 +215,9 @@
 	args->size = gem_object->size;
 	args->pitch = pitch;
-	DRM_DEBUG_DRIVER("Created object of size %lld\n", size);
+	drm_gem_object_put_unlocked(gem_object);
+	DRM_DEBUG_DRIVER("Created object of size %llu\n", args->size);
 	return 0;