CVE-2026-43323 sched/fair: Fix zero_vruntime tracking fix
A flaw in the Linux kernel scheduler allows local attackers to cause system instability or denial of service via specific task yielding patterns.
- 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-2026-43323?
CVE-2026-43323 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/fair: Fix zero_vruntime tracking fix
John reported that stress-ng-yield could make his machine unhappy and managed to bisect it to commit b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking").
The combination of yield and that commit was specific enough to hypothesize the following scenario:
Suppose we have 2 runnable tasks, both doing yield. Then one will be eligible and one will not be, because the average position must be in between these two entities.
Therefore, the runnable task will be eligible, and be promoted a full slice (all the tasks do is yield after all). This causes it to jump over the other task and now the other task is eligible and current is no longer. So we schedule.
Since we are runnable, there is no {de,en}queue. All we have is the __{en,de}queue_entity() from {put_prev,set_next}_task(). But per the fingered commit, those two no longer move zero_vruntime.
All that moves zero_vruntime are tick and full {de,en}queue.
This means, that if the two tasks playing leapfrog can reach the critical speed to reach the overflow point inside one tick's worth of time, we're up a creek.
Additionally, when multiple cgroups are involved, there is no guarantee the tick will in fact hit every cgroup in a timely manner. Statistically speaking it will, but that same statistics does not rule out the possibility of one cgroup not getting a tick for a significant amount of time -- however unlikely.
Therefore, just like with the yield() case, force an update at the end of every slice. This ensures the update is never more than a single slice behind and the whole thing is within 2 lag bounds as per the comment on entity_key().
Which versions are affected by CVE-2026-43323?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-43323?
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-2026-43323 being exploited?
There is no known exploitation of CVE-2026-43323 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-43323, DEBIAN-CVE-2026-43323, GHSA-549j-xvh5-g4r6, UBUNTU-CVE-2026-43323.