CVE-2025-21667

What it actually does

On 32-bit Linux kernels, iomap_write_delalloc_scan() truncates 64-bit file offsets returned by folio_next_index() to 32 bits, causing an infinite loop during delayed allocation scanning on XFS filesystems.

Why it matters to the business

File write operations on affected 32-bit systems using XFS can hang indefinitely, resulting in denial of service, system unresponsiveness, and potential data loss or corruption for workloads relying on that filesystem.

How an attack unfolds

An attacker with local access or the ability to trigger file writes on an XFS filesystem on a 32-bit kernel repeatedly invokes the delalloc scan path; the truncated offset prevents loop termination, locking the kernel thread and freezing the system. No named threat actors or ransomware campaigns are associated with this issue.

How to fix it

Upgrade the Linux kernel to a fixed version such as 6.1.127 or apply commit 7ca4bd6b754913910151acce00be093f03642725 from the stable git tree. Reboot after installation and verify the running kernel version.

How to confirm the fix worked

Confirm the installed kernel version is at or above the listed fixed releases; inspect the iomap source for the corrected 64-bit handling in iomap_write_delalloc_scan(); test file write workloads on XFS to ensure no hang occurs.

Typical attack path

1. Obtain ability to perform or trigger writes to an XFS filesystem on a 32-bit kernel
2. Force repeated execution of iomap_write_delalloc_scan() via sustained file I/O
3. Exploit 32-bit truncation of the position variable to enter and sustain an infinite loop
4. Cause kernel thread hang leading to denial of service

How to detect it

Monitor kernel logs and system traces for repeated calls or stalls in iomap-related functions during XFS write operations; watch for sustained high CPU usage in kernel threads without progress on file I/O; review dmesg or syslog for signs of hung tasks on 32-bit systems.

What your security team should do

• Prioritize kernel upgrade on all 32-bit systems using XFS within the next maintenance window
• Temporarily restrict untrusted local users from writing to XFS volumes if immediate patching is not feasible
• Include kernel version checks in vulnerability scanning and asset inventory processes