[CVE-2025-38087][MODERATE 7.0] net/sched: fix use-after-free in taprio_dev_notifier

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2025-38087
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: fix use-after-free in taprio_dev_notifier
Commit: 8c5713ce1ced75f9e9ed5c642ea3d2ba06ead69c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c5713ce1ced75f9e9ed5c642ea3d2ba06ead69c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38087
Analysis date: Mon, 06 Jul 2026 18:35:21 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in `taprio_dev_notifier()` can dereference `oper_sched` or `admin_sched` without RCU read-side protection while `advance_sched()` may free the same schedule object, causing a local kernel use-after-free and likely DoS, with further impact requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2025-38087 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38087

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2025-38087	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-672;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 race exists in taprio_dev_notifier because q oper_sched and q admin_sched can be dereferenced without an RCU read-side critical section while advance_sched may replace or free the same schedule objects. A local actor able to control taprio qdisc state or trigger relevant netdev notifier paths can race schedule advancement with notifier handling, causing access to freed kernel memory. For the CVSS the PR:L is selected for the paranoid score because CAP_NET_ADMIN or an equivalent delegated network administration capability is commonly sufficient in practical deployments, including containers or service contexts where this is not full host administrator control. The issue is not directly network reachable through packet traffic and is triggered through local control plane operations. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel use-after-free memory corruption. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4) 	YES	REMOTE LOCK RACE UAF NETWORK LINUS  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	YES	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1
  The patch and commit explicitly describe a use-after-free race, but there is no demonstrated reclaim, type confusion, callback control, write-after-free, or arbitrary write primitive.
* Real lifetime corruption: +1
  Missing RCU read-side protection allows `taprio_dev_notifier()` to observe `oper_sched` or `admin_sched` while `advance_sched()` may replace or free the same schedule object.
* Reliable kernel crash / strong DoS concern: +1
  A kernel UAF in `net/sched` can realistically crash the host, even if practical exploitability beyond DoS is not demonstrated.
* Broad kernel networking scheduler exposure: +1
  `net/sched` is a common kernel subsystem, although `taprio` itself is a more specialized qdisc.
* Hard or unreliable race: -1
  Triggering requires a race between notifier handling and schedule advancement.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1
  The likely trigger path involves qdisc or netdev control. This is not fully unprivileged, but CAP_NET_ADMIN may be delegated to containers or service contexts, so using -1 is more appropriate than full host-root -2 for practical triage.

Paranoid interpretation:

* Same UAF and RCU lifetime signals apply.
* The CAP_NET_ADMIN penalty is not applied in the paranoid score because delegated network administration can make this reachable by reduced-privilege actors.
* Availability impact is treated as host-wide and actionable.
* The score is capped at 5 under the race-UAF downgrade rule because no controlled reclaim, stale callback, write primitive, or type confusion is shown.

## 3. Reachability analysis

The bug is locally triggerable through control-plane operations around `taprio` qdisc state and netdev notifier activity. It is not directly network reachable through packet traffic. Typical triggering requires CAP_NET_ADMIN or equivalent control over the affected network device or qdisc. Namespaces and containers matter because CAP_NET_ADMIN can be delegated without giving full host-root privileges, so practical PR can be lower than a strict administrator-only interpretation. The affected path is not a default generic packet receive path, and `taprio` is mainly used in TSN or traffic scheduling deployments, but the bug is still inside a real kernel networking lifetime path.

Call-site confidence: medium. The patch shows the vulnerable dereference site and the missing RCU critical section, but not the full lifetime/free path of `advance_sched()`.

## 4. Severity interpretation

This behaves above an ordinary Moderate because the patch fixes an explicit kernel use-after-free caused by missing RCU lifetime protection. Realistic demonstrated impact is most likely local DoS via kernel crash. Privilege escalation is theoretically possible for kernel UAF bugs, but the patch does not show controlled reclaim, object replacement, write-after-free, callback dispatch, or type confusion. Therefore the conservative interpretation is borderline manual-review Moderate, while the paranoid interpretation is Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

A race in `taprio_dev_notifier()` can dereference `oper_sched` or `admin_sched` without RCU read-side protection while `advance_sched()` may free the same schedule object, causing a local kernel use-after-free and likely DoS, with further impact requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit kernel UAF plus RCU lifetime bug in `net/sched`. Even though no strong LPE primitive is demonstrated, this class should not be auto-closed because exploitability beyond DoS depends on object lifetime and reclaim behavior that requires manual source review.

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

Patch: net/sched: fix use-after-free in taprio_dev_notifier
Commit: 8c5713ce1ced75f9e9ed5c642ea3d2ba06ead69c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c5713ce1ced75f9e9ed5c642ea3d2ba06ead69c

Commit description:

Since taprio’s taprio_dev_notifier() isn’t protected by an
RCU read-side critical section, a race with advance_sched()
can lead to a use-after-free.

Adding rcu_read_lock() inside taprio_dev_notifier() prevents this.

Fixes: fed87cc ("net/sched: taprio: automatically calculate queueMaxSDU based on TC gate durations")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/aEzIYYxt0is9upYG@v4bel-B760M-AORUS-ELITE-AX
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/sch_taprio.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=8c5713ce1ced75f9e9ed5c642ea3d2ba06ead69c

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

The original Linux kernel CVE announcement for CVE-2025-38087 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38087

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: 3
  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: NO
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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).