CVE-2026-43418

Undergoing Analysis Undergoing Analysis - In Progress

Race Condition in Linux Kernel Sched MMCID Fork

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: Prevent CID stalls due to concurrent forks A newly forked task is accounted as MMCID user before the task is visible in the process' thread list and the global task list. This creates the following problem: CPU1 CPU2 fork() sched_mm_cid_fork(tnew1) tnew1->mm.mm_cid_users++; tnew1->mm_cid.cid = getcid() -> preemption fork() sched_mm_cid_fork(tnew2) tnew2->mm.mm_cid_users++; // Reaches the per CPU threshold mm_cid_fixup_tasks_to_cpus() for_each_other(current, p) .... As tnew1 is not visible yet, this fails to fix up the already allocated CID of tnew1. As a consequence a subsequent schedule in might fail to acquire a (transitional) CID and the machine stalls. Move the invocation of sched_mm_cid_fork() after the new task becomes visible in the thread and the task list to prevent this. This also makes it symmetrical vs. exit() where the task is removed as CID user before the task is removed from the thread and task lists.

CVSS Scores

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Meta Information

Published

2026-05-08

Last Modified

2026-05-22

Generated

2026-07-09

AI Q&A

2026-05-08

EPSS Evaluated

2026-07-08

NVD

CVE-2026-43418

EUVD

EUVD-2026-28724

Affected Vendors & Products

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)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-UNKNOWN

Attack-Flow Graph

Executive Summary

This vulnerability occurs in the Linux kernel's scheduling subsystem related to MMCID (Memory Management Context ID) handling during task forking.

When a new task is forked, it is accounted as an MMCID user before it becomes visible in the process's thread list and the global task list. This timing issue can cause problems when multiple forks happen concurrently on different CPUs.

Specifically, if one task (tnew1) is not yet visible and another task (tnew2) forks and reaches a per-CPU threshold for MMCID users, the system tries to fix up allocated CIDs for other tasks. However, since tnew1 is not visible yet, the fix-up fails for tnew1's CID.

As a result, a subsequent scheduling operation might fail to acquire a transitional CID, causing the machine to stall.

The fix involves moving the invocation of sched_mm_cid_fork() to after the new task becomes visible in the thread and task lists, preventing this race condition and making the behavior symmetrical with task exit handling.

Impact Analysis

This vulnerability can cause the machine to stall during task scheduling due to failure in acquiring a transitional CID.

In practical terms, this means that under certain concurrent fork conditions, the system could become unresponsive or hang, impacting availability and stability.

Mitigation Strategies

The vulnerability is resolved by moving the invocation of sched_mm_cid_fork() after the new task becomes visible in the thread and task list. This prevents CID stalls caused by concurrent forks.

To mitigate this vulnerability immediately, you should update your Linux kernel to a version that includes this fix.

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