[CVE-2026-43496][LOW] net/sched: sch_red: Replace direct dequeue call with peek and qdisc_dequeue_peeked

[CVE-2026-43496][LOW] net/sched: sch_red: Replace direct dequeue call with peek and qdisc_dequeue_peeked

[AL-KERNEL]

CVE: CVE-2026-43496
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: sch_red: Replace direct dequeue call with peek and qdisc_dequeue_peeked
Commit: 36aa34f42cb6842cf371f3a2d3e855d24fd57a50
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=36aa34f42cb6842cf371f3a2d3e855d24fd57a50
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43496
Analysis date: Thu, 21 May 2026 08:57:28 -0400
ActionableScore: 3
ActionableScore lower bound: 0
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A sch_red dequeue logic bug can desynchronize peeked qdisc state with child qdiscs such as QFQ, causing a NULL pointer dereference and kernel panic when a specially configured traffic-control hierarchy is exercised.

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

The original Linux kernel CVE announcement for CVE-2026-43496 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43496

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: LOW
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-2026-43496	LOW	CHECK	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-703;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the Linux traffic control scheduler when sch_red calls a child qdisc dequeue callback directly after a prior peek operation. With a child such as qfq using qdisc_peek_dequeued, the peeked skb can be stored in the gso_skb path and the later direct dequeue can observe an inconsistent queue state, leading to a crash in qfq_dequeue. For the CVSS the PR:L is used for the paranoid score because delegated CAP_NET_ADMIN inside a network namespace can let a reduced privilege local user configure qdisc trees in some deployments. In the more typical host setup PR:H applies because creating the red qfq tbf style hierarchy requires administrative traffic control privileges. The issue is not directly network reachable because packet traffic alone does not create the vulnerable qdisc configuration. Impact is denial of service via kernel panic only.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	REMOTE KASAN NULLPTR NETWORK SKB KERNEL_PANIC_PLUS_UAF HARDWARE LINUS QDISC PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The report shows a KASAN NULL pointer dereference in qfq_dequeue leading to kernel panic.
* Availability impact realistic: +1. A configured vulnerable qdisc hierarchy can crash the kernel during packet dequeue.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Creating the RED/QFQ/TBF qdisc hierarchy normally requires traffic-control privileges.

Paranoid additional signals:

* Local unprivileged trigger: +1. If unprivileged user namespaces allow a user to obtain CAP_NET_ADMIN inside a network namespace, the qdisc setup may become reachable by a reduced-privilege local user.
* Reduced capability namespace interpretation: 0 penalty instead of -2. Delegated CAP_NET_ADMIN in a netns is not equivalent to full host root.
* No memory corruption signal. The demonstrated primitive is a NULL pointer dereference, not UAF, OOB write, double free, or controlled overwrite.

## 3. Reachability analysis

The bug is triggered through Linux traffic control configuration involving RED with a child qdisc such as QFQ using qdisc_peek_dequeued, and then packet transmission causes the dequeue path to run. In normal host deployments, configuring this requires CAP_NET_ADMIN, so the conservative score is low. In systems with unprivileged user namespaces and delegated network administration inside a netns, a local reduced-privilege user may be able to create a similar qdisc hierarchy and trigger a host kernel crash. The issue is not directly network reachable because external packets alone do not create the vulnerable qdisc configuration.

## 4. Severity interpretation

This behaves like an ordinary Moderate or borderline actionable local DoS, not an Important memory-corruption vulnerability. The realistic demonstrated impact is kernel panic from a NULL pointer dereference caused by inconsistent qdisc peek/dequeue semantics. There is no evidence of a reclaimable UAF, OOB write, arbitrary write, information leak, or privilege escalation primitive.

## 5. One-sentence report phrase

A sch_red dequeue logic bug can desynchronize peeked qdisc state with child qdiscs such as QFQ, causing a NULL pointer dereference and kernel panic when a specially configured traffic-control hierarchy is exercised.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the bug is DoS-only and normally requires CAP_NET_ADMIN, but namespace delegation may lower the practical privilege requirement on some systems, so it should not be treated as Important but may deserve a quick policy check before autoclosing.

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

Patch: net/sched: sch_red: Replace direct dequeue call with peek and qdisc_dequeue_peeked
Commit: 36aa34f42cb6842cf371f3a2d3e855d24fd57a50
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=36aa34f42cb6842cf371f3a2d3e855d24fd57a50

Commit description:

When red qdisc has children (eg qfq qdisc) whose peek() callback is
qdisc_peek_dequeued(), we could get a kernel panic. When the parent of such
qdiscs (eg illustrated in patch #3 as tbf) wants to retrieve an skb from
its child (red in this case), it will do the following:
 1a. do a peek() - and when sensing there's an skb the child can offer, then
     - the child in this case(red) calls its child's (qfq) peek.
        qfq does the right thing and will return the gso_skb queue packet.
        Note: if there wasnt a gso_skb entry then qfq will store it there.
 1b. invoke a dequeue() on the child (red). And herein lies the problem.
     - red will call the child's dequeue() which will essentially just
       try to grab something of qfq's queue.

[   78.667668][  T363] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[   78.667927][  T363] CPU: 1 UID: 0 PID: 363 Comm: ping Not tainted 7.1.0-rc1-00033-g46f74a3f7d57-dirty #790 PREEMPT(full)
[   78.668263][  T363] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   78.668486][  T363] RIP: 0010:qfq_dequeue+0x446/0xc90 [sch_qfq]
[   78.668718][  T363] Code: 54 c0 e8 dd 90 00 f1 48 c7 c7 e0 03 54 c0 48 89 de e8 ce 90 00 f1 48 8d 7b 48 b8 ff ff 37 00 48 89 fa 48 c1 e0 2a 48 c1 ea 03 <80> 3c 02 00 74 05 e8 ef a1 e1 f1 48 8b 7b 48 48 8d 54 24 58 48 8d
[   78.669312][  T363] RSP: 0018:ffff88810de573e0 EFLAGS: 00010216
[   78.669533][  T363] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   78.669790][  T363] RDX: 0000000000000009 RSI: 0000000000000004 RDI: 0000000000000048
[   78.670044][  T363] RBP: ffff888110dc4000 R08: ffffffffb1b0885a R09: fffffbfff6ba9078
[   78.670297][  T363] R10: 0000000000000003 R11: ffff888110e31c80 R12: 0000001880000000
[   78.670560][  T363] R13: ffff888110dc4150 R14: ffff888110dc42b8 R15: 0000000000000200
[   78.670814][  T363] FS:  00007f66a8f09c40(0000) GS:ffff888163428000(0000) knlGS:0000000000000000
[   78.671110][  T363] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   78.671324][  T363] CR2: 000055db4c6a30a8 CR3: 000000010da67000 CR4: 0000000000750ef0
[   78.671585][  T363] PKRU: 55555554
[   78.671713][  T363] Call Trace:
[   78.671843][  T363]  <TASK>
[   78.671936][  T363]  ? __pfx_qfq_dequeue+0x10/0x10 [sch_qfq]
[   78.672148][  T363]  ? __pfx__printk+0x10/0x10
[   78.672322][  T363]  ? srso_alias_return_thunk+0x5/0xfbef5
[   78.672496][  T363]  ? lockdep_hardirqs_on_prepare+0xa8/0x1a0
[   78.672706][  T363]  ? srso_alias_return_thunk+0x5/0xfbef5
[   78.672875][  T363]  ? trace_hardirqs_on+0x19/0x1a0
[   78.673047][  T363]  red_dequeue+0x65/0x270 [sch_red]
[   78.673217][  T363]  ? srso_alias_return_thunk+0x5/0xfbef5
[   78.673385][  T363]  tbf_dequeue.cold+0xb0/0x70c [sch_tbf]
[   78.673566][  T363]  __qdisc_run+0x169/0x1900

The right thing to do in #1b is to grab the skb off gso_skb queue.
This patchset fixes that issue by changing #1b to use qdisc_dequeue_peeked()
method instead.

Fixes: 77be155 ("pkt_sched: Add peek emulation for non-work-conserving qdiscs.")
Reported-by: Manas <ghandatmanas@gmail.com>
Reported-by: Rakshit Awasthi <rakshitawasthi17@gmail.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260430152957.194015-2-jhs@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/sch_red.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=36aa34f42cb6842cf371f3a2d3e855d24fd57a50

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

The original Linux kernel CVE announcement for CVE-2026-43496 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43496

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:L/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: 0
  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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).