drm/vkms: call drm_atomic_helper_shutdown before drm_dev_put()

commit 73a82b22963d ("drm/atomic: Fix potential use-after-free
in nonblocking commits") introduced drm_dev_get/put() to
drm_atomic_helper_shutdown(). And this cause problem in vkms driver exit
process.

vkms_exit()
  drm_dev_put()
    vkms_release()
      drm_atomic_helper_shutdown()
        drm_dev_get()
        drm_dev_put()
          vkms_release()    ------ use after free

Using 5.4 stable x86 image on qemu, below stacktrace can be triggered by
load and unload vkms.ko.

root:~ # insmod vkms.ko
[   76.957802] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[   76.961490] [drm] Driver supports precise vblank timestamp query.
[   76.964416] [drm] Initialized vkms 1.0.0 20180514 for vkms on minor 0
root:~ # rmmod vkms.ko
[   79.650202] refcount_t: addition on 0; use-after-free.
[   79.650249] WARNING: CPU: 2 PID: 3533 at ../lib/refcount.c:25 refcount_warn_saturate+0xcf/0xf0
[   79.654241] Modules linked in: vkms(-)
[   79.654249] CPU: 2 PID: 3533 Comm: rmmod Not tainted 5.4.273 #4
[   79.654251] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[   79.654262] RIP: 0010:refcount_warn_saturate+0xcf/0xf0
...
[   79.654296] Call Trace:
[   79.654462]  ? __warn+0x80/0xd0
[   79.654473]  ? refcount_warn_saturate+0xcf/0xf0
[   79.654481]  ? report_bug+0xb6/0x130
[   79.654484]  ? refcount_warn_saturate+0xcf/0xf0
[   79.654489]  ? fixup_bug.part.12+0x13/0x30
[   79.654492]  ? do_error_trap+0x90/0xb0
[   79.654495]  ? do_invalid_op+0x31/0x40
[   79.654497]  ? refcount_warn_saturate+0xcf/0xf0
[   79.654504]  ? invalid_op+0x1e/0x30
[   79.654508]  ? refcount_warn_saturate+0xcf/0xf0
[   79.654516]  drm_atomic_state_init+0x68/0xb0
[   79.654543]  drm_atomic_state_alloc+0x43/0x60
[   79.654551]  drm_atomic_helper_disable_all+0x13/0x180
[   79.654562]  drm_atomic_helper_shutdown+0x5f/0xb0
[   79.654571]  vkms_release+0x18/0x40 [vkms]
[   79.654575]  vkms_exit+0x29/0xc00 [vkms]
[   79.654582]  __x64_sys_delete_module+0x155/0x220
[   79.654592]  do_syscall_64+0x43/0x120
[   79.654603]  entry_SYSCALL_64_after_hwframe+0x5c/0xc1
[   79.654619] ---[ end trace ce0c02f57ea6bf73 ]---

It seems that the proper unload sequence is:
	drm_atomic_helper_shutdown();
	drm_dev_put();

Just put drm_atomic_helper_shutdown() before drm_dev_put()
should solve the problem.

Note that vkms exit code is refactored by commit 53d77aaa3f76
("drm/vkms: Use devm_drm_dev_alloc") in tags/v5.10-rc1.

So this bug only exists on 4.19 and 5.4.

Fixes: 380c7ceabdde ("drm/atomic: Fix potential use-after-free in nonblocking commits")
Fixes: 2ead1be54b22 ("drm/vkms: Fix connector leak at the module removal")
Signed-off-by: Guo Mengqi <guomengqi3@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
index b1201c18d3eb..d32e08f17427 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.c
+++ b/drivers/gpu/drm/vkms/vkms_drv.c
@@ -39,7 +39,6 @@ static void vkms_release(struct drm_device *dev)
 	struct vkms_device *vkms = container_of(dev, struct vkms_device, drm);
 
 	platform_device_unregister(vkms->platform);
-	drm_atomic_helper_shutdown(&vkms->drm);
 	drm_mode_config_cleanup(&vkms->drm);
 	drm_dev_fini(&vkms->drm);
 }
@@ -137,6 +136,7 @@ static void __exit vkms_exit(void)
 	}
 
 	drm_dev_unregister(&vkms_device->drm);
+	drm_atomic_helper_shutdown(&vkms_device->drm);
 	drm_dev_put(&vkms_device->drm);
 
 	kfree(vkms_device);