CVE-2026-43418 sched/mmcid: Prevent CID stalls due to concurrent forks
A race condition in the Linux kernel's MM CID handling during concurrent forks can cause a system stall, 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-2026-43418?
CVE-2026-43418 is a medium-severity 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: 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.
Which versions are affected by CVE-2026-43418?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-43418?
Apply the vendor's patch.
Is CVE-2026-43418 being exploited?
There is no known exploitation of CVE-2026-43418 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as DEBIAN-CVE-2026-43418, GHSA-5r8w-mfmh-79h7, UBUNTU-CVE-2026-43418.