CVE-2026-43417 sched/mmcid: Handle vfork()/CLONE_VM correctly

Medium 5.5 Published 8 May 2026 Updated 11 May 2026

A logic error in the Linux kernel's scheduler causes system stalls when handling vfork() tasks, leading to denial of service.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2026-43417?

CVE-2026-43417 is a medium-severity denial of service vulnerability in Linux kernel, published on 8 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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

sched/mmcid: Handle vfork()/CLONE_VM correctly

Matthieu and Jiri reported stalls where a task endlessly loops in mm_get_cid() when scheduling in.

It turned out that the logic which handles vfork()'ed tasks is broken. It is invoked when the number of tasks associated to a process is smaller than the number of MMCID users. It then walks the task list to find the vfork()'ed task, but accounts all the already processed tasks as well.

If that double processing brings the number of to be handled tasks to 0, the walk stops and the vfork()'ed task's CID is not fixed up. As a consequence a subsequent schedule in fails to acquire a (transitional) CID and the machine stalls.

Cure this by removing the accounting condition and make the fixup always walk the full task list if it could not find the exact number of users in the process' thread list.

Which versions are affected by CVE-2026-43417?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2026-43417?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-43417 being exploited?

There is no known exploitation of CVE-2026-43417 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-43417, GHSA-hjwg-8c2j-jq5p, UBUNTU-CVE-2026-43417.