CVE-2026-89835

In the Linux kernel, the following vulnerability has been resolved:

f2fs: avoid NULL checkpoint thread access in sysfs

checkpoint_merge can be enabled even when no checkpoint merge thread is

running. A read-only mount is one case: f2fs does not start

f2fs_issue_ckpt there, but ckpt_thread_ioprio is still writable through

sysfs.

The ckpt_thread_ioprio store path updates the saved ioprio value and,

when checkpoint_merge is enabled, calls set_task_ioprio() for the

checkpoint thread. If cprc->f2fs_issue_ckpt is NULL, that dereferences a

NULL task pointer.

Protect ckpt_thread_ioprio sysfs writes with s_umount as well, so the

checkpoint thread cannot disappear under the store path while updating

its ioprio.