* [CVE-2026-90325][MODERATE 7.0] Revert "blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()" This reverts commit 81c1188 which is
@ 2026-09-17 17:11 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-17 17:11 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90325
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Revert "blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()" This reverts commit 81c1188 which is
Commit: b5dae1cd0d8368b4338430ff93403df67f0b8bcc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b5dae1cd0d8368b4338430ff93403df67f0b8bcc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90325
Analysis date: Thu, 17 Sep 2026 13:11:05 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90325 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90325
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 7.0
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-2026-90325 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90325 CHECK WITH IMPACT FROM ORIG NN MODERATE 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-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in blk-cgroup can occur when blkcg_activate_policy() walks blkgs while a concurrent blkcg deletion is destroying the same blkg. The dying object can be referenced again after its refcount reached zero, which can lead to a second release path and a use-after-free in rcu_accelerate_cbs. For the CVSS the PR:L value is used for the paranoid score because some deployments delegate cgroup or block control operations to local service accounts or container management components rather than only full host root. The issue is local and not directly network reachable. Impact is at least denial of service via kernel crash. Because this is a kernel UAF in an RCU and refcount lifecycle path, worst case impact may include confidentiality and integrity compromise and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) YES LOCK INIT RACE UAF HARDWARE KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: FLOOR 5 applied because the patch establishes a concrete repeated release/refcount lifetime mutation on the same potentially dangling blkg reference.
* Raw paranoid score: 6
* Paranoid score: 6
* Applied paranoid floor/cap: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Positive signals, conservative:
* +2 Real UAF / active lifetime violation. The commit explicitly describes blkg refcount reaching zero, blkg_release scheduling RCU release, and later blkcg_activate_policy() taking a reference to the same dying blkg.
* +1 Reliable kernel crash / strong DoS. The stated consequence is use-after-free in rcu_accelerate_cbs.
* +1 Concrete repeated release/refcount operation. The same blkg reference can be resurrected from refcount 0 to 1 by blkg_get(), then dropped again by blkg_put(), causing a second blkg_release and call_rcu path.
Positive signals, paranoid:
* +1 Attacker-influenced repeatable lifetime manipulation. A local actor able to drive IO scheduler switching and blkcg deletion can repeatedly exercise both sides of the lifetime race.
* +1 Paranoid repeated-refcount leverage. The second lifetime operation is not merely a stale read. It performs a write-like refcount/lifetime mutation through storage whose original object lifetime may already have ended.
Non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary kernel read/write is not demonstrated.
* Callback or function-pointer control is not demonstrated.
* Confidentiality and integrity points are not awarded directly. The higher paranoid score is based on unresolved post-lifetime refcount corruption leverage, not on proven C/I impact.
* Remote reachability is not established.
* Extra Availability is not awarded because the same crash is already counted once.
* Weak LPE is not separately awarded because the same fact is already counted as concrete repeated release/refcount mutation.
* Structured stale-object leverage is not separately awarded because no distinct post-reuse field-consumption mechanism beyond the second refcount/lifetime mutation is established.
* No race penalty is applied. The bug is a race, but the supplied evidence does not prove a narrow one-shot externally uncontrollable timing condition.
Repeated-release checks:
* First release can end lifetime: yes. blkg_destroy kills blkg->refcnt from 1 to 0 and blkg_release schedules call_rcu.
* Exact same reference survives: yes. blkcg_activate_policy() can still encounter the same blkg while iterating.
* Second operation mutates lifetime/refcount: yes. blkg_get() and later blkg_put() mutate the refcount and can invoke release again.
* +1 Concrete repeated release/refcount awarded: yes.
* Paranoid repeated-refcount leverage +1 awarded: yes.
* FLOOR 5 applied: yes.
3. Reachability analysis
The trigger is local, not network-reachable. The described path involves switching IO schedulers through elv_iosched_store/elevator_switch while blkcg deletion is happening concurrently.
The complete privilege boundary is not fully resolved from the supplied patch alone. Switching an IO scheduler for a block device is commonly restricted to host administrative control, while blkcg and cgroup lifecycle operations may also be controlled by privileged or delegated management components. Because the evidence does not fully establish whether all required operations must be host-admin-equivalent in every relevant deployment, HostAdminRequired is UNKNOWN rather than YES.
NamespaceScopedPrivilege is UNKNOWN. The input does not establish that the complete sequence, including block-device scheduler switching and blkcg deletion, is performable entirely with attacker-obtained namespace-scoped privileges.
Prior setup may include an active block device with blk-cgroup policy support and concurrent cgroup deletion. No special hardware-only or rare physical-device prerequisite is established beyond normal block/cgroup functionality. The race appears repeatable by repeatedly driving scheduler switches and blkcg deletion, but no direct ordinary unprivileged trigger is established.
4. Primitive and severity interpretation
The exact primitive is a real kernel lifetime/refcount UAF involving struct blkcg_gq / blkg. This is an active structured kernel object, not fixed stale storage. The important distinction is not merely a stale cleanup dereference. The patch-supported sequence shows a second lifetime/refcount mutation through a potentially dangling object after a prior release may already have ended its lifetime.
This is CLASS_B. It is not CLASS_A because controlled reclaim, directed corruption, arbitrary read/write, callback control, or demonstrated privilege escalation is not established. It is stronger than CLASS_C because the stale path performs concrete lifetime/refcount mutation rather than only causing an immediate invalid dereference.
Demonstrated consequence is kernel crash / DoS. Speculative consequence is broader post-lifetime corruption from refcount/lifetime mutation if the freed storage is reused or if the second release corrupts lifetime state. That speculative consequence is technically defensible enough for manual review, but not enough to claim proven LPE.
ImportantCandidate is YES because a concrete repeated release/refcount UAF leaves meaningful Important-vs-Moderate exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically closing this as ordinary crash-only DoS would discard a real repeated lifetime mutation primitive.
Likely operational severity is Actionable Moderate at minimum, with Strong Important candidate review justified.
5. One-sentence report phrase
A race in blk-cgroup can resurrect a dying blkg during blkcg_activate_policy(), causing a second refcount release path and a UAF in RCU callback handling, with at least local kernel crash impact and unresolved post-lifetime corruption leverage.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a concrete repeated release/refcount UAF on the same potentially dangling kernel object, and post-lifetime exploitation leverage remains unresolved even though controlled reclaim or LPE is not demonstrated.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: Revert "blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()" This reverts commit 81c1188 which is
Commit: b5dae1cd0d8368b4338430ff93403df67f0b8bcc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b5dae1cd0d8368b4338430ff93403df67f0b8bcc
Commit description:
It is reported to cause problems, as only 2 of the 3 patch series were
applied to the stable branches.
Reported-by: Mike Cloaked <[email protected]>
Reported-by: Eric Biggers <[email protected]>
Cc: Yu Kuai <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Sasha Levin <[email protected]>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217174
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
block/blk-cgroup.c
include/linux/blkdev.h
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=b5dae1cd0d8368b4338430ff93403df67f0b8bcc
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90325 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90325
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:H/UI:N/S:U/C:N/I:N/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: 5
Paranoid ActionableScore: 6
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: YES
Published priority for this report (same as in Subject): MODERATE 7.0
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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