CVE-2026-53303: Linux Kernel f2fs Race Condition Information Disclosure

Technical summary

CVE-2026-53303 involves a synchronization flaw in f2fs_sbi_show() within the f2fs filesystem driver. The function reads extension_list, extension_count, and hot_ext_count without holding the sbi->sb_lock mutex, creating a time-of-check-time-of-use (TOCTOU) condition. Concurrent modification via f2fs_update_extension_list() can cause the show path to read a count that does not match the current array state, leading to out-of-bounds reads or information disclosure. The fix adds sb_lock protection around the entire extension list read and formatting operation to ensure consistent state during access.

Business impact

This vulnerability can lead to information disclosure or denial of service on affected Linux systems, particularly those with untrusted local user access or multi-tenant environments. Successful exploitation allows a local attacker to leak sensitive filesystem metadata or crash processes, impacting system availability and confidentiality. Organizations running f2fs on production systems with local user access should prioritize remediation to prevent potential data exposure or system instability.

Affected systems

Linux systems running affected versions of the Linux kernel with f2fs filesystem support enabled. The vulnerability requires local code execution capability to trigger the race condition. Systems where sysfs is world-readable and f2fs extensions are in active use face elevated risk. Embedded systems, containers, and multi-user environments are particularly vulnerable due to the local attack requirement being more easily satisfied.

Exploitability

Exploitation requires local access and the ability to trigger concurrent reads and writes to the f2fs extension list via sysfs. An attacker would need to synchronize thread execution to consistently trigger the race condition, making it non-trivial but achievable in multi-threaded scenarios. The vulnerability is not remotely exploitable and does not require elevated privileges to attempt. However, reliable exploitation may require timing precision or multiple attempts.

Remediation

Apply kernel patches that add sb_lock synchronization to f2fs_sbi_show(). Verify the patched version against your Linux distributor's security advisories. As a temporary mitigation, restrict sysfs access to trusted users where feasible, though this does not eliminate the underlying race condition. Systems should be updated to the next stable or long-term support kernel version containing the fix.

Patch guidance

Check your Linux distributor's security advisory portal for the specific patched kernel version addressing CVE-2026-53303. Kernel updates for f2fs fixes are typically delivered as part of routine kernel security releases. For LTS kernels, patches may be backported to stable branches. Verify patch application by confirming the presence of sb_lock protection in the f2fs_sbi_show() function or by checking the kernel version against the vendor's published advisory. Test in a staging environment before deploying to production systems relying on f2fs.

Detection guidance

Monitor system logs for unexpected crashes or panics in f2fs-related code paths. Check for repeated access patterns to /sys/fs/f2fs/ while concurrent filesystem modifications occur, which may indicate race condition exploitation attempts. Kernel debugging tools like kprobes can be used to instrument f2fs_sbi_show() and f2fs_update_extension_list() to detect concurrent access without lock protection. Systems with memory protection features enabled may halt on out-of-bounds access attempts, providing clear indicators of exploitation.

Why prioritize this

This vulnerability merits prioritization because it affects core filesystem functionality in the Linux kernel and can result in information disclosure or denial of service. The local-only attack vector limits impact in certain environments, but the HIGH CVSS score (7.1) reflects the combination of high confidentiality and availability impact. Organizations running f2fs in multi-user or container environments should prioritize patches, while single-user systems face lower risk. The race condition nature means it may be difficult to exploit reliably in production, but determined attackers with local access can achieve consistent triggering.

Risk score, explained

The CVSS 3.1 score of 7.1 (HIGH) reflects the vulnerability's characteristics: local attack vector (AV:L) with low complexity (AC:L), requiring low privilege (PR:L), high confidentiality impact (C:H), and high availability impact (A:I). No integrity impact is documented. The score appropriately captures the risk in multi-user systems where untrusted local users may trigger the race condition, but excludes network and no-privilege scenarios that would increase severity. Organizations should adjust their internal risk scoring based on the prevalence of f2fs use and local user access policies.

Frequently asked questions