[CVE-2026-64017][IMPORTANT] blk-mq: pop cached request if it is usable [ Upstream

[CVE-2026-64017][IMPORTANT] blk-mq: pop cached request if it is usable [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64017
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: blk-mq: pop cached request if it is usable [ Upstream
Commit: 388468f7e7d1eab092cf2a39fdfb502e52019ec6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=388468f7e7d1eab092cf2a39fdfb502e52019ec6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64017
Analysis date: Sun, 19 Jul 2026 15:17:34 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in blk-mq cached request handling can free a request during plug flush after it was peeked but before it was popped, leading to use-after-free in local block I/O submission and possible kernel crash or memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-64017 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64017

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: IMPORTANT
Manual review required: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64017	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in blk mq bio submission because blk_mq_submit_bio() first peeked a cached request and then could block in rq_qos_throttle() before removing it from plug cached_rqs. If the task sleeps in that window, plug flushing can free the cached request and later code may continue using the stale request pointer. For the CVSS the PR:L is used because a local user or process capable of issuing block I/O may be able to reach this path without full administrative privileges, although reliable triggering depends on queue and rq qos conditions. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the stale request object is used after free in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	LOCK INIT RACE UAF  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Local processes capable of issuing block I/O may reach blk_mq_submit_bio().
* Generic memory corruption, strong corruption primitive: +2. The commit explicitly describes a use-after-free of struct request, and the stale request is later used in the submit path.
* Real lifetime corruption: +1. The cached request can be freed by plug flushing while the submit path still keeps a pointer to it.
* Reliable kernel crash / strong DoS: +1. A freed request reused in blk-mq can plausibly crash the kernel.
* Important filesystem/storage path: +1. blk-mq is a core block I/O submission path.
* Broad/default/common subsystem: +1. The block layer is widely used on normal systems.
* Hard or unreliable race / special timing required: -1. Triggering depends on sleeping in the window after peeking the cached request and before popping it.

Conservative total: 6

Paranoid additional signal:

* Weak LPE concern: +1. The stale request pointer is not only read. The path can write request fields after the possible free, creating a post-free write concern, but no attacker-controlled reclaim or replacement primitive is demonstrated.

Paranoid total: 7

## 3. Reachability analysis

A local user or local process that can generate block I/O may be able to reach the affected path. Reliable triggering depends on having a cached request in the plug and causing rq_qos_throttle() or a similar path to sleep before the request is popped. This is not network reachable.

Namespaces and containers may matter if untrusted workloads can issue block I/O through host-backed storage, but the bug is still local in CVSS and triage terms. It does not require CAP_SYS_ADMIN merely to submit I/O, although specific rq qos and storage configuration may affect reproducibility.

Call-site confidence: high. The patch includes the relevant call path in blk_mq_submit_bio(), the cached request helper, the blocking throttle call, and the cleanup path.

## 4. Severity interpretation

This is above ordinary Moderate because the patch explicitly fixes a real use-after-free in a core block-layer path. The realistic impact is at least local kernel crash or storage I/O instability. The paranoid interpretation treats it as an Important candidate because stale struct request use after free may become memory corruption beyond DoS, especially due to post-free request field updates.

There is no demonstrated arbitrary write, attacker-controlled reclaim, type confusion, or working LPE chain in the provided material. Therefore the conservative score remains Actionable Moderate, while the paranoid score reaches Strong Important candidate for manual review sensitivity.

## 5. One-sentence report phrase

A race in blk-mq cached request handling can free a request during plug flush after it was peeked but before it was popped, leading to use-after-free in local block I/O submission and possible kernel crash or memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because it is an explicit UAF in a common block I/O path, with post-free use of struct request and plausible impact beyond simple DoS even though a reliable privilege-escalation primitive is not demonstrated.

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

Patch: blk-mq: pop cached request if it is usable [ Upstream
Commit: 388468f7e7d1eab092cf2a39fdfb502e52019ec6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=388468f7e7d1eab092cf2a39fdfb502e52019ec6

Changed files:
  block/blk-mq.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=388468f7e7d1eab092cf2a39fdfb502e52019ec6

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

The original Linux kernel CVE announcement for CVE-2026-64017 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64017

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).