[PATCH 2/2] drm/vkms: Fix null-ptr-deref in vkms_release()

[Melissa Wen]

On 11/01, Yuan Can wrote:
> 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().
> 
> Fixes: 6c234fe37c57 ("drm/vkms: Implement CRC debugfs API")
> Signed-off-by: Yuan Can <yuancan at huawei.com>
> ---
>  drivers/gpu/drm/vkms/vkms_drv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> index dfe983eaa07f..f716c5796f5f 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.c
> +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> @@ -57,7 +57,8 @@ static void vkms_release(struct drm_device *dev)
>  {
>  	struct vkms_device *vkms = drm_device_to_vkms_device(dev);
>  
> -	destroy_workqueue(vkms->output.composer_workq);
> +	if (vkms->output.composer_workq)
> +		destroy_workqueue(vkms->output.composer_workq);

Reviewed-by: Melissa Wen <mwen at igalia.com>
>  }
>  
>  static void vkms_atomic_commit_tail(struct drm_atomic_state *old_state)
> -- 
> 2.17.1
> 
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