drm/vkms: Fix null-ptr-deref in vkms_release()

In the Linux kernel, the following vulnerability has been resolved:

drm/vkms: Fix null-ptr-deref in vkms_release()

A null-ptr-deref is triggered when it tries to destroy the workqueue in

vkms->output.composer_workq in vkms_release().

KASAN: null-ptr-deref in range [0x0000000000000118-0x000000000000011f]

CPU: 5 PID: 17193 Comm: modprobe Not tainted 6.0.0-11331-gd465bff130bf #24

RIP: 0010:destroy_workqueue+0x2f/0x710

...

Call Trace:

<TASK>

? vkms_config_debugfs_init+0x50/0x50 [vkms]

__devm_drm_dev_alloc+0x15a/0x1c0 [drm]

vkms_init+0x245/0x1000 [vkms]

do_one_initcall+0xd0/0x4f0

do_init_module+0x1a4/0x680

load_module+0x6249/0x7110

__do_sys_finit_module+0x140/0x200

do_syscall_64+0x35/0x80

entry_SYSCALL_64_after_hwframe+0x46/0xb0

The reason is that an OOM happened which triggers the destroy of the

workqueue, however, the workqueue is alloced in the later process,

thus a null-ptr-deref happened. A simple call graph is shown as below:

vkms_init()

vkms_create()

devm_drm_dev_alloc()

__devm_drm_dev_alloc()

devm_drm_dev_init()

devm_add_action_or_reset()

devm_add_action() # an error happened

devm_drm_dev_init_release()

drm_dev_put()

kref_put()

drm_dev_release()

vkms_release()

destroy_workqueue() # null-ptr-deref happened

vkms_modeset_init()

vkms_output_init()

vkms_crtc_init() # where the workqueue get allocated

Fix this by checking if composer_workq is NULL before passing it to

the destroy_workqueue() in vkms_release().