CVE-2025-38687 comedi: fix race between polling and detaching

What is CVE-2025-38687?

CVE-2025-38687 is a high-severity use after free vulnerability in Debian Debian Linux, published on 4 Sep 2025. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

Official description

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.

Weakness: CWE-362

Which versions are affected by CVE-2025-38687?

It affects 2 products.

Products include Debian Debian Linux, Linux kernel.

How do I fix CVE-2025-38687?

Apply the vendor's patch.

Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.

Is CVE-2025-38687 being exploited?

There is no known exploitation of CVE-2025-38687 in the wild and no public exploit code.

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Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-66860, BELL-CVE-2025-38687, DEBIAN-CVE-2025-38687, GHSA-gmr8-hqwc-6phj, ROOT-OS-DEBIAN-11-CVE-2025-38687, ROOT-OS-DEBIAN-12-CVE-2025-38687 and 3 more.