CVE-2026-98538 writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes
A local attacker can cause a denial of service via soft lockups in the Linux kernel writeback subsystem by triggering quadratic time complexity during inode cleanup.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98538?
CVE-2026-98538 is a denial of service vulnerability in Linux, published on 18 Oct 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:
writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes
cleanup_offline_cgwb() prepares at most WB_MAX_INODES_PER_ISW inodes per call and is called again until the dying wb is drained, but every call walks wb->b_attached and then wb->b_dirty_time from the same end. Inodes already prepared (they stay on the list with I_WB_SWITCH set until the switch worker runs) and inodes that cannot be switched (I_FREEING, I_WILL_FREE, !SB_ACTIVE, DAX, already on the target wb) stay where they are, so each pass rescans a growing run of them under wb->list_lock and a full drain is quadratic in the number of inodes on the list. With ~17M inodes attached to one dying cgwb we saw this end in soft lockups, with CPUs reported stuck for 21-48s.
Walk both lists from the oldest end and move every scanned inode to the newest end, so the next pass starts where the previous one stopped and the drain becomes linear. b_attached is unordered, so nobody sees the reorder there. b_dirty_time is ordered by dirtied_when, but the oldest unscanned inode stays at the end move_expired_inodes() picks from, sync takes the whole list regardless of order, and prepared inodes leave the list as soon as the switch work runs and get a new dirtied_time_when on the new wb anyway, so the only inodes left out of order are the ones that can never switch (DAX), and only on the dying wb.
Which versions are affected by CVE-2026-98538?
How do I fix CVE-2026-98538?
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-2026-98538 being exploited?
There is no known exploitation of CVE-2026-98538 in the wild and no public exploit code.
Sources
Also known as AZL-108635, BELL-CVE-2026-98538, DEBIAN-CVE-2026-98538, GHSA-m444-q4wx-jpmh, UBUNTU-CVE-2026-98538.