CVE-2026-68173
HighCVSS 7.1Exploitation Probability (EPSS)
Low risk5th percentile - higher than 5% of all known CVEs
Summary
In the Linux kernel, a vulnerability in the ublk subsystem was found where waiting on ub->completion instead of the actual device readiness (ublk_dev_ready()) can lead to a deadlock. If the ublk server sends END_USER_RECOVERY without a successful START_USER_RECOVERY, the device may be marked LIVE and the requeue list not kicked again, causing requests (e.g., flush_rq) to be stranded and blocking fsync and teardown.
Risk Assessment
The organization may face filesystem unavailability (e.g., ext4 on ublk) after a ublk daemon crash, with processes stuck in uninterruptible sleep and inability to cleanly shut down. A local attacker could intentionally trigger this scenario, leading to DoS.
Recommendation
Apply the official Linux kernel patch that replaces waiting on ub->completion with the new ublk_wait_dev_ready_and_lock() mechanism. Reboot after the update to avoid leaving old vulnerable versions in memory.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: ublk: wait on ublk_dev_ready() instead of ub->completion ub->completion is only re-armed by a successful START_USER_RECOVERY. If the ublk server sends END_USER_RECOVERY without one - e.g. its START failed with -EBUSY and the error was ignored - the wait is satisfied by the stale completion of the previous recovery cycle, and the device is marked LIVE and the requeue list kicked while the FETCH stream is still running and ubq->canceling is still set. The kick redispatches a previously requeued request, __ublk_queue_rq_common() sees ->canceling and parks it again via __ublk_abort_rq(), and after the last FETCH clears ->canceling nothing ever kicks the requeue list again: the request is stranded there while holding its tag. If it is the flush machinery's flush_rq, every subsequent fsync piles up in uninterruptible sleep and teardown hangs on tag draining. This matches a report of a lost PREFLUSH with ext4 on top of ublk after daemon crash recovery. ub->completion is an edge-triggered latch used as a proxy for the level condition "every queue has fetched all I/O commands", which can regress (F_BATCH's UNPREP, daemon death) and whose re-arm can be skipped. Drop it and wait on the real condition instead: the new helper ublk_wait_dev_ready_and_lock() waits on ublk_dev_ready() via wait_var_event_interruptible(), woken from ublk_mark_io_ready(), then re-checks it under ub->mutex, waiting again on regression, and returns with the mutex held and readiness guaranteed. Readiness becomes true in the same ub->mutex critical section that clears the last queue's ->canceling, so END_USER_RECOVERY marks the device LIVE and kicks the requeue list strictly after ->canceling clears. The wait stays interruptible, so a server whose daemon died can still be signalled out. For ublk_ctrl_start_dev() this replaces the fail-fast -EINVAL on an F_BATCH ready->UNPREP regression with waiting until the device is ready again.