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CVE-2026-43418 Linux CVE debrief

CVE-2026-43418 describes a Linux kernel concurrency bug in the sched/mmcid path. A newly forked task could be counted as an MMCID user before it became visible in the thread and task lists. Under concurrent fork activity, this race could prevent already allocated CIDs from being fixed up, and a later schedule-in could fail to obtain a transitional CID, stalling the machine. The fix reorders the fork handling so the task is visible before sched_mm_cid_fork runs, making the behavior consistent with exit() cleanup.

Vendor
Linux
Product
Unknown
CVSS
Unknown
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-08
Original CVE updated
2026-05-12
Advisory published
2026-05-08
Advisory updated
2026-05-12

Who should care

Linux kernel maintainers, distro security teams, and operators of systems running kernels that include the sched/mmcid code path should care, especially where availability and stall resistance are important.

Technical summary

The issue is a race between task visibility and MMCID accounting during fork(). sched_mm_cid_fork() incremented mm_cid_users and assigned a CID before the new task was inserted into the thread and global task lists. If a concurrent fork triggered mm_cid_fixup_tasks_to_cpus() while the first task was still invisible, that task could be missed during CID fixup. The result could be a later scheduling attempt that cannot acquire a transitional CID, leading to a system stall. The reported fix moves sched_mm_cid_fork() after the new task is visible.

Defensive priority

High for any environment using affected Linux kernels: this is an availability issue that can stall the machine, so patching should be prioritized once vendor or stable kernel updates are available.

Recommended defensive actions

  • Apply the kernel stable updates linked in the supplied references.
  • Check whether your distribution kernel includes the sched/mmcid code path and whether a backport is available.
  • Review release notes or changelogs for mentions of sched/mmcid, MMCID, or concurrent-fork CID fixups.
  • Treat unexplained kernel stalls or hangs on unpatched systems as a potential sign of exposure until the fix is deployed.
  • Reboot into the patched kernel after updating so the corrected fork ordering is in effect.

Evidence notes

The supplied CVE record shows publication on 2026-05-08T15:16:53.803Z and modification on 2026-05-12T14:10:27.343Z. The description explicitly states that concurrent forks could leave a newly forked task invisible during CID fixup, causing a schedule-in failure and machine stall. In the supplied corpus, NVD status is 'Undergoing Analysis'; no CVSS score, severity, or KEV entry is provided.

Official resources

Publicly published on 2026-05-08T15:16:53.803Z and last modified on 2026-05-12T14:10:27.343Z. No KEV listing was provided in the supplied data.