Bug 2535339 (CVE-2026-89835)

Summary: CVE-2026-89835 kernel: f2fs: avoid NULL checkpoint thread access in sysfs
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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Doc Text:
A flaw was found in the Linux kernel's f2fs filesystem. This vulnerability occurs when the `checkpoint_merge` feature is enabled without an active checkpoint merge thread, such as during a read-only mount. A local attacker with write access to `ckpt_thread_ioprio` through sysfs could trigger a null pointer dereference, leading to a system crash and a denial of service.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Description OSIDB Bzimport 2026-09-16 12:09:48 UTC 
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.
Comment 1 gkamathe 2026-09-17 11:29:06 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89835.mbox