CVE-2026-53303: Linux Kernel Use-After-Free Vulnerability

CVE-2026-53303 Overview

CVE-2026-53303 is a race condition vulnerability in the Linux kernel's Flash-Friendly File System (f2fs) implementation. The flaw resides in the f2fs_sbi_show() function, which reads extension_list, extension_count, and hot_ext_count without acquiring the sbi->sb_lock spinlock. A concurrent sysfs store operation invoking f2fs_update_extension_list() can modify the extension list while the show path is reading it. This produces inconsistent count values and array contents, potentially causing out-of-bounds access or exposing stale data through the sysfs interface. Kernel maintainers resolved the issue by holding sb_lock around the entire extension list read and format operation.

Critical Impact
Concurrent sysfs read and update operations against the f2fs extension list can trigger out-of-bounds memory access in the kernel, leading to potential information disclosure or system instability.

Affected Products

Linux kernel with f2fs (Flash-Friendly File System) support enabled
Multiple stable kernel branches (see referenced git commits for exact ranges)
Systems exposing f2fs sysfs attributes for extension list configuration

Discovery Timeline

2026-06-26 - CVE-2026-53303 published to NVD
2026-06-30 - Last updated in NVD database

Technical Details for CVE-2026-53303

Vulnerability Analysis

The vulnerability is a race condition [CWE-362] between the sysfs show handler and the sysfs store handler in the f2fs subsystem. The f2fs_sbi_show() function iterates over sbi->raw_super->extension_list using extension_count and hot_ext_count as loop bounds. Because the function does not acquire sbi->sb_lock, another thread executing f2fs_update_extension_list() can concurrently modify these values.

If the count values increase between the loop bound check and the array access, the show path reads beyond the valid array bounds. This constitutes an out-of-bounds read [CWE-125]. If the extension list is being reordered or shrunk during the read, the show path returns torn or stale data that does not reflect any consistent state of the filesystem configuration.

Root Cause

The root cause is missing synchronization on a shared data structure. The extension_list array and its associated counters are protected by sbi->sb_lock during write operations but not during the sysfs read path in f2fs_sbi_show(). This asymmetric locking discipline created the window for concurrent access.

Attack Vector

Exploitation requires a local unprivileged or privileged actor capable of triggering concurrent reads of the f2fs sysfs attributes and writes to extension_list via the sysfs store interface. An attacker with write access to the relevant sysfs nodes typically requires CAP_SYS_ADMIN, which limits practical exploitation to local privileged contexts or containers with elevated filesystem capabilities. The immediate impact is a kernel out-of-bounds read rather than direct code execution.

No verified proof-of-concept code has been published. See the kernel stable commit references for the exact source-level changes applied by the maintainers.

Detection Methods for CVE-2026-53303

Indicators of Compromise

Kernel warnings or KASAN reports referencing f2fs_sbi_show and out-of-bounds access on extension_list
Unexpected sysfs output from /sys/fs/f2fs/<device>/extension_list containing malformed or truncated entries
Repeated concurrent access patterns to f2fs sysfs nodes from unprivileged workloads

Detection Strategies

Enable CONFIG_KASAN in test kernels to catch out-of-bounds reads originating from the f2fs sysfs path
Audit kernel version banners across the fleet and compare against the fixed stable kernel commits listed in the CVE references
Review sysfs access logs and auditd records for processes writing to f2fs extension_list attributes

Monitoring Recommendations

Monitor kernel ring buffer output (dmesg) for f2fs subsystem warnings and KASAN splats
Track sysfs write events targeting /sys/fs/f2fs/*/extension_list using the Linux audit subsystem
Alert on unexpected kernel crashes or oopses on hosts using f2fs as a primary or secondary filesystem

How to Mitigate CVE-2026-53303

Immediate Actions Required

Identify hosts running kernels with f2fs support and inventory kernel versions against the fixed stable releases
Apply the upstream stable kernel updates that include the sb_lock protection fix in f2fs_sbi_show()
Restrict write access to f2fs sysfs attributes to trusted administrative accounts only

Patch Information

The fix wraps the extension list read and format operation in f2fs_sbi_show() with sbi->sb_lock, ensuring atomicity relative to f2fs_update_extension_list(). The corrective changes are available in the following stable kernel commits: 4b3a1bf4c2ff, 5909bedbed38, cea15f66b7b6, d0e877810baf, d3ff0c121bba, and ea3ab43a1f3c. Consult your distribution vendor for backported packages that include these commits.

Workarounds

Where patching is not immediately possible, restrict sysfs permissions on f2fs attributes to prevent unprivileged concurrent writes
Avoid deploying f2fs on multi-tenant hosts until the patched kernel is installed
Disable dynamic modification of the extension list at runtime in workflows that do not require it

bash

# Verify running kernel version and f2fs sysfs exposure
uname -r
ls -l /sys/fs/f2fs/
chmod 600 /sys/fs/f2fs/*/extension_list