* [CVE-2025-37890][MODERATE 7.0] net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc [ Upstream
@ 2026-10-13 0:44 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-13 0:44 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2025-37890
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc [ Upstream
Commit: e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-37890
Analysis date: Mon, 12 Oct 2026 20:44:56 -0400
ActionableScore: 5
ActionableScore lower bound: 4
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-2025-37890 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-37890
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-2025-37890 MODERATE https://www.kernelcve.org/list/?q=CVE-2025-37890 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 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-664;CWE-670;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An HFSC class with a netem child qdisc can be activated more than once because an empty child queue does not guarantee that the class is absent from the vttree or eltree. Netem packet duplication can create a reentrant enqueue path where the same active class is inserted into a scheduling tree again. This can corrupt scheduler tree state and result in a use-after-free when the class or tree entries are later traversed or released. Impact is at least a local denial of service via kernel crash and in the worst case may include confidentiality and integrity impact or privilege escalation through kernel memory corruption. For the CVSS the PR:L reflects that reliable triggering may be available to a local user, container root, or service with delegated CAP_NET_ADMIN rather than requiring unrestricted host root privileges. AV:L and AC:H reflect the need for a specific HFSC and netem configuration and control over the reentrant traffic condition. Network packets may enter an already configured qdisc, but the available evidence does not establish a reliable unauthenticated remote attack path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE REMOTE UAF NETWORK LINUS QDISC KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor: 3 -> 4 under REAL UAF FLOOR 4. This is an active structured object used beyond an ordinary cleanup fault, and host-admin-equivalent privilege is not positively established.
* No paranoid floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF and lifetime violation. The commit explicitly identifies a UAF caused by inserting the same active HFSC class into `vttree` or `eltree` twice.
* +1 Reliable kernel crash or strong DoS. Corrupted scheduler-tree membership followed by traversal or release can realistically produce a fatal stale-object access.
* Total raw conservative score: 3.
Additional paranoid signals:
* +1 Structured stale-object exploitation leverage. The stale state belongs to an active HFSC class, and later scheduler operations consume its tree nodes and scheduling state rather than merely passing it to a shallow cleanup helper.
* +1 Attacker-influenced repeated active-state transition. An actor able to configure HFSC with a netem duplicate child and supply traffic can repeatedly drive the reentrant enqueue and class activation sequence.
* Total raw paranoid score: 5.
Not awarded:
* No +2 strong exploitation leverage. Controlled reclaim, attacker-shaped replacement contents, callback control, arbitrary write, and refcount takeover are not demonstrated.
* No confidentiality or integrity points. A practical disclosure, protected-state modification, or privilege escalation chain is not established.
* No remote-reachability points. Network traffic may exercise an existing configuration, but the supplied evidence does not prove a complete unauthenticated remote trigger.
* No local-unprivileged bonus. Below-host-admin access to the complete qdisc configuration and trigger sequence is unresolved.
* No race penalty. The defect is a reentrant state transition associated with netem duplication, not an established narrow or uncontrollable timing race.
* No extra Availability point. The same potential kernel crash is counted only once.
* No repeated release or refcount point. The patch does not establish a first lifetime-ending put followed by a second put on the exact same dangling reference.
* No B5-R classification. There is no retry-accumulated heap OOB write.
* No B3-PENDING classification. No shared-backing write is involved.
* No B4-PENDING classification. No authorization decision changes from DENY to ALLOW.
Repeated-release checks:
* First release can end lifetime: not established.
* Exact same reference survives a first release: not established.
* Second lifetime or refcount mutation: not established.
* Concrete repeated release/refcount +1: not awarded.
* Paranoid repeated-refcount +1: not awarded.
* FLOOR 5 from repeated release: not applied.
3. Reachability analysis
Configuring the HFSC hierarchy and its netem child is performed through the traffic-control rtnetlink interface and requires `CAP_NET_ADMIN` relative to the relevant network namespace. The supplied material does not establish whether the complete setup is limited to host-owned devices or can be performed entirely in an attacker-created user and network namespace. Therefore, both host-admin requirement and complete namespace-scoped reachability remain unknown.
An actor who already controls the qdisc configuration can select HFSC, attach netem with duplication, and generate traffic that reaches `hfsc_enqueue()`. Once such a hierarchy exists, other traffic sources might exercise it, but the evidence does not show that packet traffic alone reliably creates the vulnerable state without suitable prior configuration.
No special hardware, boot option, or sysctl is identified. The combination is runtime-configurable and should not receive a rare-subsystem penalty. The trigger is reentrant and traffic-driven rather than a demonstrated one-shot timing race.
4. Primitive and severity interpretation
The exact primitive is duplicate insertion of an already active HFSC class into active scheduling trees. The old `first` test relied on the child queue length, but netem duplication can make that condition true while `cl_nactive` shows that the class is already active. Duplicate tree membership can leave scheduler state referring to a class after its valid lifetime and produces the UAF identified by the commit.
This is an active structured-object UAF, not fixed stale storage and not a cleanup-only pointer fault. The affected class and its tree nodes participate in later scheduling, traversal, removal, and lifetime operations. However, the supplied patch does not demonstrate controlled allocation reuse, attacker-controlled post-free contents, a directed write, callback consumption, or a completed privilege-escalation chain.
PrimitiveClass is CLASS_B. The demonstrated practical consequence is kernel corruption with plausible crash or DoS. Stronger exploitation remains unresolved rather than established. ImportantCandidate is YES because an active scheduler object can remain in corrupted tree state and later be consumed, making automatic crash-only classification unsafe. AutoDowngradeAllowed is NO because this unresolved active-object lifetime primitive would otherwise be discarded without review.
The likely operational severity is Actionable Moderate at minimum, with Important classification dependent on confirming namespace reachability and useful post-free behavior.
5. One-sentence report phrase
A reentrant netem duplicate enqueue can insert an already active HFSC class into its scheduling trees twice, causing an active-object use-after-free and potential kernel crash while leaving stronger exploitation leverage unresolved.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Confirm whether the full HFSC plus netem trigger works in an ordinary attacker-created network namespace and inspect subsequent tree removal and traversal paths for reclaim, write, or pointer-consumption leverage.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc [ Upstream
Commit: e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
Changed files:
net/sched/sch_hfsc.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=e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2025-37890 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-37890
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: 4
Paranoid ActionableScore: 5
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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