CVE-2025-38687

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 wasnt the case previously, but that does not matter.

Thanks to Jens Axboe for diagnosing the problem and co-developing this patch.

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/017198079551a2a5cf61eae966af3c4b145e1f3b
• https://git.kernel.org/stable/c/0f989f9d05492028afd2bded4b42023c57d8a76e
• https://git.kernel.org/stable/c/35b6fc51c666fc96355be5cd633ed0fe4ccf68b2
• https://git.kernel.org/stable/c/5724e82df4f9a4be62908362c97d522d25de75dd
• https://git.kernel.org/stable/c/5c4a2ffcbd052c69bbf4680677d4c4eaa5a252d4
• https://git.kernel.org/stable/c/71ca60d2e631cf9c63bcbc7017961c61ff04e419
• https://git.kernel.org/stable/c/cd4286123d6948ff638ea9cd5818ae4796d5d252
• https://git.kernel.org/stable/c/d85fac8729c9acfd72368faff1d576ec585e5c8f
• https://git.kernel.org/stable/c/fe67122ba781df44a1a9716eb1dfd751321ab512
• https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html
• https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html