[CVE-2026-72036][MODERATE REGULAR] net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked

[CVE-2026-72036][MODERATE REGULAR] net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked

[AL-KERNEL]

CVE: CVE-2026-72036
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked
Commit: 3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72036
Analysis date: Sun, 16 Aug 2026 05:19:27 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline. Manual review recommended
Manual review required: YES

Summary:
sch_multiq can desynchronize child qdisc state by directly dequeuing after peek, allowing specific traffic-control configurations to trigger a NULL pointer dereference and kernel panic during ordinary egress traffic.

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

The original Linux kernel CVE announcement for CVE-2026-72036 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72036

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 REGULAR
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-2026-72036	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;*CWE-664;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in sch_multiq can occur because multiq_dequeue peeks a child qdisc and then calls the child dequeue path directly instead of consuming the peeked skb. With a non-work-conserving child qdisc, the peeked skb can be stashed in gso_skb while the direct dequeue returns a different skb, leaving qlen and backlog state out of sync. Affected configurations can panic the kernel from softirq during ordinary egress traffic when a peeking parent and qfq style child are combined. For the CVSS the PR:L is used in the paranoid score because some deployments delegate CAP_NET_ADMIN to containers or service accounts, or an already configured vulnerable qdisc hierarchy may let a local user trigger traffic through the broken path. The issue is not directly network reachable as a standalone remote attack because the risky qdisc hierarchy normally has to be configured locally. Impact is denial of service via kernel crash, with no concrete evidence of confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline. Manual review recommended (with actual score 2)	YES	REMOTE DANGER SIMPLEFIX NETWORK SKB LINUS QDISC PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline. Manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The commit explicitly describes NULL dereference and kernel panic from softirq.

* Common networking packet-processing path: +1
  The bug is in net/sched egress qdisc dequeue handling.

* Requires CAP_NET_ADMIN in typical deployments: -2
  Creating the risky qdisc hierarchy normally requires traffic-control privileges.

* Availability impact realistic: +1
  The described outcome is a host kernel panic, not only a local process failure.

Total conservative: 1

Paranoid signals:

* Local unprivileged trigger: +1
  If the vulnerable qdisc hierarchy is already configured, ordinary local egress traffic may trigger the crash.

* Reliable kernel crash / strong DoS: +1
  The failure mode is a NULL pointer dereference leading to panic.

* Common networking packet-processing path: +1
  The affected logic is in the packet scheduling transmit path.

* Availability impact realistic: +1
  Panic from softirq is system-wide availability impact.

* Rare or special configuration: -1
  The issue needs a specific qdisc combination involving multiq, a non-work-conserving child, and a peeking parent such as tbf/qfq.

Total paranoid: 3

No memory-corruption or LPE signals are awarded. The supported primitive is qdisc state desynchronization followed by NULL pointer dereference, not UAF, OOB write, type confusion, arbitrary write, or controlled object reuse.

## 3. Reachability analysis

Typical triggering requires CAP_NET_ADMIN to configure the affected qdisc hierarchy. In containerized or delegated networking environments, CAP_NET_ADMIN may be available to less trusted service accounts or container root, so the paranoid interpretation treats this as more reachable than full host-root only.

The issue is not directly remote-network-triggerable as a standalone attack. Remote packets alone do not normally configure the required qdisc stack, but traffic may exercise the broken dequeue path after local configuration exists.

The path is not default in the sense of ordinary qdisc setups. It depends on a specific composition of peeking and non-work-conserving qdiscs. Call-site confidence: high, because the patch directly changes the affected dequeue call and the commit explains the qdisc interaction.

## 4. Severity interpretation

This behaves like a configuration-dependent local DoS, not an Important-class memory corruption issue. The realistic impact is kernel crash through NULL dereference. Theoretical escalation is not supported by the patch because there is no demonstrated stale object reuse, write-after-free, callback control, refcount takeover, or attacker-controlled corruption primitive.

This should not be treated as a high-priority LPE candidate, but it is worth manual review where CAP_NET_ADMIN is delegated or complex traffic shaping is used.

## 5. One-sentence report phrase

sch_multiq can desynchronize child qdisc state by directly dequeuing after peek, allowing specific traffic-control configurations to trigger a NULL pointer dereference and kernel panic during ordinary egress traffic.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended for products that delegate CAP_NET_ADMIN, use containers with traffic-control permissions, or deploy complex qdisc hierarchies. For default systems without such delegation or qdisc configuration, this is closer to ordinary Moderate / local privileged DoS.

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

Patch: net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked
Commit: 3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a

Commit description:

multiq_dequeue() takes a packet from a band's child with a direct
->dequeue() call after multiq_peek() peeked it. When the child is
non-work-conserving the peek stashes the skb in the child's gso_skb, so
the direct dequeue returns a different skb and orphans the stash,
desyncing the child's qlen/backlog. With a qfq child reached through a
peeking parent (e.g. tbf) this re-enters the child on an emptied list and
dereferences NULL, panicking the kernel from softirq on ordinary egress.

Take the packet through qdisc_dequeue_peeked(), as sch_prio already does
and as sch_red and sch_sfb were just fixed to do. The helper is a no-op
when the child has no stash, so a work-conserving child is unaffected.

Fixes: 77be155 ("pkt_sched: Add peek emulation for non-work-conserving qdiscs.")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260625-b4-disp-31bcb279-v1-2-85c40b83c529@proton.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/sch_multiq.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=3e5fd9d14f2d228e7260251f2e4a1d41ba8f705a

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

The original Linux kernel CVE announcement for CVE-2026-72036 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72036

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 3

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 REGULAR

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).