* [CVE-2025-21726][MODERATE 7.0] padata: avoid UAF for reorder_work [ Upstream
@ 2026-10-13 1:44 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-13 1:44 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2025-21726
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: padata: avoid UAF for reorder_work [ Upstream
Commit: 7000507bb0d2ceb545c0a690e0c707c897d102c2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7000507bb0d2ceb545c0a690e0c707c897d102c2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21726
Analysis date: Mon, 12 Oct 2026 21:44:27 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2025-21726 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21726
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-2025-21726 MODERATE https://www.kernelcve.org/list/?q=CVE-2025-21726 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC) 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:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in padata can leave reorder_work queued after the final outstanding request releases the last reference to its parallel_data object. When the worker later executes invoke_padata_reorder, it obtains pd from the embedded work structure and can access memory that has already been freed. This is a concrete use after free rather than a resource leak and can cause a kernel crash. Reuse of the freed allocation may also create a plausible memory corruption path with potential confidentiality, integrity, or privilege escalation impact. For the CVSS the PR:L value is used for the paranoid score because a local user may be able to submit crypto requests through an exposed crypto interface while algorithm teardown occurs in another context. Reliable triggering still depends on algorithm availability, concurrent teardown, and a narrow race window, so AC:H is appropriate. The issue is not directly network reachable unless a network service exposes the affected crypto request path. Impact is at least a local denial of service and in the worst defensible case may include host compromise through kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) MAYBE DEADLOCK UAF HARDWARE LINUS KPANIC - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 3
* Paranoid score: 4
* Applied conservative REAL UAF FLOOR 4: 3 -> 4
* Applied paranoid REAL UAF FLOOR 4: 3 -> 4
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation. `reorder_work` can remain queued after the final reference to `parallel_data` is released by algorithm teardown. The later worker derives `pd` from the embedded work item and calls `padata_reorder(pd)` after the object may have been freed.
* +1 Realistic kernel crash. Processing a freed `parallel_data` object can dereference invalid list, per-CPU, queue, or instance state and realistically cause an Oops or panic.
Negative signals:
* No negative points. The issue is a race, but the supplied evidence does not establish that attempts are sufficiently narrow, one-shot, or unreliable to justify the timing penalty.
* No host-admin penalty. Algorithm deletion may involve privileged module or crypto-algorithm management, but the complete privilege boundary is not established.
Material signals not awarded:
* No controlled reclaim, attacker-shaped replacement object, arbitrary write, callback control, disclosure, or demonstrated LPE.
* No Structured stale-object leverage point. `parallel_data` is active structured state, but attacker influence over reclaimed contents or security-relevant stale fields is not established.
* No repeatable lifetime-manipulation point. The supplied evidence does not show that one below-host-admin actor controls both request generation and `crypto_del_alg`.
* No concrete repeated release/refcount point. The vulnerable operation is a stale structured-object use, not a second `put` or refcount mutation on the same dangling reference.
* Paranoid repeated-refcount leverage is therefore inapplicable.
* FLOOR 5 was not applied because below-host-admin reachability and meaningful attacker-controlled reuse or post-free state are not established.
* No additional Availability point. The possible kernel crash is already counted.
* No C/I or LPE point. Generic allocator reuse after a UAF is not an independently established exploitation primitive.
* No remote-reachability point. A network service might indirectly submit crypto requests, but that path is not supplied.
* B5-R is not applicable because there is no retry-accumulated heap OOB write.
* B3-PENDING is not applicable because no shared-backing write exists.
* B4-PENDING is not applicable because no authorization decision is changed from DENY to ALLOW.
3. Reachability analysis
The vulnerable sequence requires concurrent crypto requests, queued padata reorder work, completion of the outstanding requests, and `crypto_del_alg` freeing the associated `parallel_data`. A local process may potentially generate requests through AF_ALG or another crypto consumer, but the supplied evidence does not prove access to the affected pcrypt configuration or control over algorithm teardown.
It is also not established that the same attacker can invoke `crypto_del_alg`. That operation may arise from module or algorithm unregister activity controlled by a host administrator or another kernel lifecycle event. The complete trigger is therefore unresolved rather than proven host-admin-only or below-host-admin.
No namespace capability check or complete namespace-scoped path is shown. No special hardware is required. Prior registration and use of the affected pcrypt algorithm are prerequisites, but the actor responsible for that setup is not established. The race involves asynchronous workers and teardown, although the evidence does not quantify it as sufficiently unreliable to receive a timing penalty.
4. Primitive and severity interpretation
The exact primitive is a real post-lifetime use of an active structured `parallel_data` object. The object can be freed after outstanding requests complete while its embedded `reorder_work` remains queued. `invoke_padata_reorder` later reconstructs the freed object pointer with `container_of` and passes it into active reorder processing.
This is broader than a fixed stale buffer and is not merely an ordinary cleanup pointer passed to a destructor. However, it is not a concrete repeated release case. The vulnerable worker reads or traverses stale structured state, but does not itself perform a second demonstrated lifetime mutation on the exact dangling reference.
The appropriate classification is CLASS_B. Kernel crash or memory-safety failure is directly supported. Stronger consequences such as controlled corruption or privilege escalation remain speculative because reclaim control, attacker-shaped post-free contents, useful field consumption after replacement, and complete low-privilege reachability are not established.
ImportantCandidate is NO because the current evidence does not establish meaningful exploitation leverage beyond an uncontrolled active-object UAF and possible crash. Manual review remains recommended because the stale path performs nontrivial processing rather than an immediate fixed-address fault. AutoDowngradeAllowed is YES because no unresolved repeated refcount mutation, controlled reuse, authorization bypass, ownership violation, or other Important-preserving primitive is established.
5. One-sentence report phrase
A race can leave padata reorder work queued after its parallel_data object is freed, causing an active worker to process stale kernel state and potentially crash the host, while controlled reclaim and below-host-admin reachability remain unproven.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm whether an ordinary user can access the affected pcrypt path and independently induce algorithm teardown, and inspect which `parallel_data` fields are consumed before an expected fault.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: padata: avoid UAF for reorder_work [ Upstream
Commit: 7000507bb0d2ceb545c0a690e0c707c897d102c2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7000507bb0d2ceb545c0a690e0c707c897d102c2
Changed files:
kernel/padata.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=7000507bb0d2ceb545c0a690e0c707c897d102c2
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2025-21726 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21726
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: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: unknown
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: 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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