CVE-2026-43417
Undergoing Analysis Undergoing Analysis - In Progress
Linux Kernel Scheduling Loop Vulnerability in mmcid

Publication date: 2026-05-08

Last updated on: 2026-05-22

Assigner: kernel.org

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.
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Meta Information
Published
2026-05-08
Last Modified
2026-05-22
Generated
2026-06-19
AI Q&A
2026-05-08
EPSS Evaluated
2026-06-18
NVD
CVE-2026-43417
EUVD
EUVD-2026-28723
Affected Vendors & Products
Showing 4 associated CPEs
Vendor Product Version / Range
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel From 6.19 (inc) to 6.19.9 (exc)
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Exploitability
CWE
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CWE ID Description
CWE-UNKNOWN
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Mitigation Strategies

The vulnerability is resolved in the Linux kernel by fixing the handling of vfork()/CLONE_VM tasks in the scheduler. To mitigate this vulnerability, you should update your Linux kernel to a version that includes this fix.

Executive Summary

This vulnerability exists in the Linux kernel's scheduling code related to handling vfork() and CLONE_VM. Specifically, the logic that manages tasks created by vfork() is broken. When the number of tasks associated with a process is smaller than the number of MMCID users, the kernel attempts to find the vfork()ed task by walking the task list. However, it incorrectly counts already processed tasks, which can cause the walk to stop prematurely without fixing the vfork()ed task's CID (context ID). As a result, when the scheduler tries to schedule in the task, it fails to acquire a necessary transitional CID, causing the machine to stall.

Impact Analysis

The impact of this vulnerability is that the affected Linux system can stall or hang during task scheduling. This happens because the scheduler fails to acquire a required context ID for a vfork()ed task, leading to an endless loop or stall in the kernel. Such a stall can cause system unresponsiveness, potentially requiring a reboot or manual intervention to recover.

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