[CVE-2026-68173][MODERATE REGULAR] ublk: wait on ublk_dev_ready() instead of ub->completion

[CVE-2026-68173][MODERATE REGULAR] ublk: wait on ublk_dev_ready() instead of ub->completion

[AL-KERNEL]

CVE: CVE-2026-68173
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ublk: wait on ublk_dev_ready() instead of ub->completion
Commit: 7dd26adf7e7d482af524e3a0cca4a81ef7c159d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7dd26adf7e7d482af524e3a0cca4a81ef7c159d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68173
Analysis date: Mon, 10 Aug 2026 16:18:29 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A stale completion in ublk recovery can mark a device LIVE before all queues are ready, stranding requeued requests and causing persistent local I/O hangs or teardown DoS when the ublk userspace recovery flow is misordered.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-68173 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68173

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-68173	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-667;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'ublk user recovery can use a stale completion as a proxy for device readiness, so END_USER_RECOVERY may mark the device LIVE before all queues have fetched their I/O commands. This can redispatch a requeued request while queue canceling is still active, causing the request to be parked again and then stranded after canceling is cleared. If the stranded request is the flush request, later fsync operations can pile up in uninterruptible sleep and device teardown can hang while draining tags. For the CVSS the PR:L is used because triggering requires local ability to run or control the ublk userspace server or recovery flow, but not necessarily full administrator privileges in deployments that allow ublk user daemons. The issue is not network reachable. Impact is denial of service through blocked I/O and hung teardown, with no evidence of confidentiality impact, integrity impact, UAF, OOB access, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)  SKIP CVE-2026-68173 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 728cbac5fe219d3b8a21a0688a08f2b7f8aeda2b YES  NO NO unknown 	MAYBE	LOCK DANGER DISK INIT DMAorINTERRUPT HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Local reduced privilege trigger: +1. Triggering requires local control of the ublk userspace server or recovery flow. This is not network reachable, but it may not require full host root in deployments that allow ublk user daemons.
* Reliable kernel strong DoS: +1. A stale completion can mark the device LIVE too early, strand a requeued request, and make later fsync operations block in uninterruptible sleep.
* Availability impact realistic: +1. The impact can persist as blocked I/O and teardown hang on tag draining, not only a transient warning.
* Important filesystem/storage path: +1. The bug affects ublk block I/O recovery and was observed with ext4 PREFLUSH behavior.
* Special recovery sequence required: -1. The trigger depends on daemon crash or recovery, failed START_USER_RECOVERY, ignored error, and END_USER_RECOVERY ordering.

Paranoid difference:

* The -1 special sequence penalty is not applied if the local attacker controls the ublk daemon and can reliably drive the recovery sequence.

Not counted:

* No remote reachable signal.
* No generic memory corruption signal.
* No real lifetime corruption signal.
* No privilege escalation signal.
* No confidentiality or integrity impact signal.
* No Dirty Pipe like or page cache corruption primitive.

Call-site confidence: high. The patch includes the relevant start recovery, end recovery, readiness, and wakeup paths.

## 3. Reachability analysis

The bug is locally reachable through ublk device recovery control, especially by the userspace ublk server or a process controlling that server. It is not triggered by network traffic. Namespace and container impact depends on whether the product permits untrusted users or container workloads to create or manage ublk devices. If ublk control is restricted to full host root, practical risk is lower. If unprivileged or reduced privilege ublk daemons are allowed, PR:L is a reasonable practical interpretation.

## 4. Severity interpretation

This behaves like a storage subsystem denial of service rather than an Important class memory corruption issue. The realistic impact is blocked I/O, fsync hangs, and teardown hangs after recovery state desynchronization. There is no evidence in the patch of UAF, OOB access, arbitrary write, stale callback, refcount takeover, or privilege escalation. Because the affected path is storage and the DoS can be persistent for the affected device, it should not be treated as a trivial cleanup, but it remains below Actionable Moderate unless the deployment exposes ublk recovery control to untrusted local users.

## 5. One-sentence report phrase

A stale completion in ublk recovery can mark a device LIVE before all queues are ready, stranding requeued requests and causing persistent local I/O hangs or teardown DoS when the ublk userspace recovery flow is misordered.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review should confirm whether affected systems allow untrusted or reduced privilege users to run ublk daemons or control ublk recovery. If only full host root can trigger the path, this can likely be handled as ordinary Moderate or lower.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: ublk: wait on ublk_dev_ready() instead of ub->completion
Commit: 7dd26adf7e7d482af524e3a0cca4a81ef7c159d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7dd26adf7e7d482af524e3a0cca4a81ef7c159d0

Commit description:

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.

Reported-by: George Salisbury <gsalisbury@apnic.net>
Fixes: 728cbac ("ublk: move device reset into ublk_ch_release()")
Cc: stable@vger.kernel.org
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260719134540.120269-1-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

[ 6.18.y tracks readiness per fetched I/O command in ->nr_io_ready rather
  than per queue in ->nr_queue_ready, so wait and wake on that counter;
  ->canceling is cleared by ublk_reset_io_flags(), which stays in
  ublk_mark_io_ready(). UBLK_F_BATCH_IO does not exist here, so
  ublk_ctrl_start_dev() has no F_BATCH readiness re-check to replace, and
  readiness can only regress via ublk_reset_ch_dev() on daemon death. ]

Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  drivers/block/ublk_drv.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7dd26adf7e7d482af524e3a0cca4a81ef7c159d0

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-68173 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68173

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 3
  Paranoid ActionableScore: 4

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).