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PatchSiren cyber security CVE debrief

CVE-2025-68823 Linux CVE debrief

A local deadlock vulnerability in the Linux kernel's ublk (userspace block device) subsystem can cause system hangs when processes attempt to read partition tables from ublk block devices. The deadlock occurs when bdev_open() acquires disk->open_mutex, issues I/O to the ublk backend, and completion handling triggers a deferred fput() that attempts to reacquire the same mutex via blkdev_release(). The fix disables bottom halves during request completion to force deferred release processing into kernel work-queue context, breaking the circular wait condition.

Vendor
Linux
Product
Unknown
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-01-13
Original CVE updated
2026-06-01
Advisory published
2026-01-13
Advisory updated
2026-06-01

Who should care

Linux system administrators running kernels with ublk support; virtualization and container platform operators using userspace block devices; embedded systems leveraging ublk for storage virtualization

Technical summary

The vulnerability exists in the ublk driver's request completion path. When __ublk_complete_rq() calls blk_update_request() or blk_mq_end_request(), bio completion callbacks may trigger fput() on the ublk block device file descriptor. The fput() implementation can defer to the current task's task work, which then executes blkdev_release() in the same task context. Since bdev_open() already holds disk->open_mutex, the reentrant attempt to acquire it in blkdev_release() causes a self-deadlock. The fix runs these completion routines with local bottom halves disabled, which prevents task_work from running in the current context and instead schedules the release to a kernel work-queue, allowing the original mutex holder to complete and release it.

Defensive priority

medium

Recommended defensive actions

  • Apply stable kernel patches for affected versions (6.0.x through 6.6.124, 6.7.x through 6.12.70, 6.13.x through 6.18.3, and 6.19-rc1) from the Linux kernel stable tree
  • Upgrade to patched kernel versions: 6.6.125+, 6.12.71+, 6.18.4+, or later stable releases
  • Monitor systems using ublk (userspace block devices) for hung task warnings or processes stuck in D-state during partition table reads
  • If patching is not immediately feasible, consider avoiding concurrent ublk device open/release operations that trigger partition table reads
  • Review system logs for ublk-related deadlock traces as indicators of exploitation or trigger conditions

Evidence notes

CVE published 2026-01-13; modified 2026-06-01. NVD records CVSS 3.1 score 5.5 (MEDIUM) with vector AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. CPE criteria indicate affected Linux kernel versions: 6.0 through 6.6.124, 6.7 through 6.12.70, 6.13 through 6.18.3, and 6.19-rc1. CWE-667 (Improper Locking) classified. Five kernel.org stable branch commits provided as patches.

Official resources

2026-01-13