Linux vulnerability

What happened

In the Linux kernel, the following vulnerability has been resolved: comedi: fix race between polling and detaching syzbot reports a use-after-free in comedi in the below link, which is due to comedi gladly removing the allocated async area even though poll requests are still active on the wait_queue_head inside of it. This can cause a use-after-free when the poll entries are later triggered or removed, as the memory for the wait_queue_head has been freed. We need to check there are no tasks queued on any of the subdevices' wait queues before allowing the device to be detached by the `COMEDI_DEVCONFIG` ioctl. Tasks will read-lock `dev->attach_lock` before adding themselves to the subdevice wait queue, so fix the problem in the `COMEDI_DEVCONFIG` ioctl handler by write-locking `dev->attach_lock` before checking that all of the subdevices are safe to be deleted. This includes testing for any sleepers on the subdevices' wait queues. It remains locked until the device has been detached. This requires the `comedi_device_detach()` function to be refactored slightly, moving the bulk of it into new function `comedi_device_detach_locked()`. Note that the refactor of `comedi_device_detach()` results in `comedi_device_cancel_all()` now being called while `dev->attach_lock` is write-locked, which wasn't the case previously, but that does not matter. Thanks to Jens Axboe for diagnosing the problem and co-developing this patch.

Affected versions

Linux: 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before fe67122ba781df44a1a9716eb1dfd751321ab512 (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before cd4286123d6948ff638ea9cd5818ae4796d5d252 (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before d85fac8729c9acfd72368faff1d576ec585e5c8f (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before 0f989f9d05492028afd2bded4b42023c57d8a76e (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before 5c4a2ffcbd052c69bbf4680677d4c4eaa5a252d4 (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before 017198079551a2a5cf61eae966af3c4b145e1f3b (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before 71ca60d2e631cf9c63bcbc7017961c61ff04e419 (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before 5724e82df4f9a4be62908362c97d522d25de75dd (git); 2f3fdcd7ce935f6f2899ceab57dc8fe5286db3e1 through before 35b6fc51c666fc96355be5cd633ed0fe4ccf68b2 (git); 3.14; before V5.0 (custom) Fixed: See vendor advisory.

Why it matters

Review the vendor advisory and exposure of the affected product to determine operational impact.

Detection & mitigation

• Apply vendor-provided updates or mitigations.
• Review affected product exposure and access logs.