[CVE-2026-68138][MODERATE 7.0] net/sched: serialize qdisc_rtab_list against concurrent get/put

[CVE-2026-68138][MODERATE 7.0] net/sched: serialize qdisc_rtab_list against concurrent get/put

[AL-KERNEL]

CVE: CVE-2026-68138
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: serialize qdisc_rtab_list against concurrent get/put
Commit: fb29e1b41052488ee3f2d115d4a870497ebd7f7d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb29e1b41052488ee3f2d115d4a870497ebd7f7d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68138
Analysis date: Mon, 10 Aug 2026 12:52:35 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in net/sched qdisc_get_rtab() and qdisc_put_rtab() can corrupt the global qdisc rate-table list and non-atomic refcount, allowing local CAP_NET_ADMIN or namespaced networking users to trigger a kernel use-after-free or double-free and cause a host-wide DoS, with limited but non-negligible LPE concern due to kernel memory lifetime corruption.

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

The original Linux kernel CVE announcement for CVE-2026-68138 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68138

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-2026-68138	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-362;*CWE-416;CWE-415;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in qdisc_get_rtab() and qdisc_put_rtab() can corrupt the global qdisc_rtab_list and the non atomic refcnt for struct qdisc_rate_table. Two concurrent RTM_NEWTFILTER requests using cls_flower with a police action can reach this path without RTNL and race on the same rate table, causing a use-after-free or double-free of a kmalloc-2k kernel object. For the CVSS the PR:L is selected because reliable triggering needs local ability to issue tc netlink operations, but this may be available to a reduced capability root, container root, or a user with CAP_NET_ADMIN in a network namespace. The issue is not directly network reachable and is triggered through local control plane configuration rather than packet traffic. Impact is at least local denial of service via kernel memory corruption. In the paranoid interpretation, confidentiality and integrity impact are also possible because the primitive is a kernel UAF or double-free rather than a simple NULL dereference or resource leak.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE LOCK RACE LEAK UAF NETWORK HARDWARE LINUS QDISC KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Memory corruption, weak or race-dependent primitive: +1
  Explicit KASAN slab-use-after-free is shown in qdisc_put_rtab(), and the commit also states possible use-after-free / double-free of struct qdisc_rate_table. Because this is race-dependent and no controlled reclaim or overwrite primitive is shown, it is downgraded from strong corruption to weak race-UAF handling.

* Real lifetime corruption: +1
  The bug corrupts lifetime/refcounting of a kmalloc-2k qdisc_rate_table object via unsynchronized get/put operations.

* Reliable kernel crash / strong DoS: +1
  The provided trace shows a concrete KASAN slab-use-after-free in the affected path, so host crash / panic is realistic depending on kernel settings.

* Cross-boundary isolation concern: +1
  qdisc_rtab_list is process-global and not per-netns, so corruption caused from one network namespace can affect global kernel state.

* Hard or timing-dependent race: -1
  Triggering requires concurrent RTM_NEWTFILTER operations on different CPUs racing on the same rate table.

* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1
  tc filter updates normally require CAP_NET_ADMIN, but this is not treated as full -2 because CAP_NET_ADMIN may be delegated to containers or network namespaces, and the corrupted object is global.

Paranoid uplift:

* Paranoid score raises the result to 5 because the bug is an explicit kernel UAF/double-free in a global net/sched object reachable through netlink control-plane operations, with namespace delegation potentially lowering practical privilege requirements.

## 3. Reachability analysis

The bug is triggered locally through rtnetlink / tc control-plane operations, specifically concurrent RTM_NEWTFILTER requests using cls_flower with a police action. It is not directly network-triggerable by packet traffic.

In typical host deployments, the attacker needs CAP_NET_ADMIN. However, in systems with unprivileged user namespaces or delegated container networking, a reduced-privilege container root or namespaced CAP_NET_ADMIN user may be able to reach the path. The global qdisc_rtab_list makes this more concerning because the corrupted object is not isolated per network namespace.

The path is in net/sched, a common kernel networking subsystem, but the vulnerable operation is configuration/control-plane rather than default packet processing.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch fixes a real race leading to UAF/double-free, not just a warning, leak, or validation issue. Realistic demonstrated impact is kernel crash / DoS. Privilege escalation is theoretically plausible because the primitive is kernel lifetime corruption, but the patch does not show controlled reclaim, attacker-controlled replacement, arbitrary write, callback control, or type confusion.

Conservative interpretation: borderline/manual-review Moderate.
Paranoid interpretation: Actionable Moderate at minimum, with Important-candidate attention if namespace reachability is confirmed in the evaluated product.

## 5. One-sentence report phrase

A race in net/sched qdisc_get_rtab() and qdisc_put_rtab() can corrupt the global qdisc rate-table list and non-atomic refcount, allowing local CAP_NET_ADMIN or namespaced networking users to trigger a kernel use-after-free or double-free and cause a host-wide DoS, with limited but non-negligible LPE concern due to kernel memory lifetime corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit UAF/double-free, global object shared across namespaces, and possible reduced-capability CAP_NET_ADMIN reachability make this unsafe to auto-close even though the demonstrated impact is mainly DoS.

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

Patch: net/sched: serialize qdisc_rtab_list against concurrent get/put
Commit: fb29e1b41052488ee3f2d115d4a870497ebd7f7d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb29e1b41052488ee3f2d115d4a870497ebd7f7d

Commit description:

qdisc_get_rtab() and qdisc_put_rtab() mutate the process-global singly
linked list qdisc_rtab_list and a plain non-atomic 'int refcnt' with no
lock. This was only safe because every caller historically held the RTNL
mutex, which serialized all rate-table lookups, inserts and frees.

That invariant no longer holds. cls_flower sets
TCF_PROTO_OPS_DOIT_UNLOCKED, so tc_new_tfilter() keeps rtnl_held == false
for it and sets TCA_ACT_FLAGS_NO_RTNL. That flag propagates through
tcf_exts_validate_ex() -> tcf_action_init() -> tcf_action_init_1() ->
tcf_police_init(), which calls qdisc_get_rtab()/qdisc_put_rtab() with the
RTNL mutex NOT held. Two RTM_NEWTFILTER requests on different CPUs, each
adding a flower filter with a police action carrying the same rate, then
race on qdisc_rtab_list and on the non-atomic refcnt, leading to a
use-after-free / double-free of the kmalloc-2k struct qdisc_rate_table.
qdisc_rtab_list is a single global (not per-netns), so the corrupted
object is shared system-wide.

  BUG: KASAN: slab-use-after-free in qdisc_put_rtab+0x12f/0x160
   qdisc_put_rtab+0x12f/0x160
   tcf_police_init+0xda9/0x1590
   tcf_action_init_1+0x460/0x6b0
   tcf_action_init+0x439/0xa40
   tcf_exts_validate_ex+0x42d/0x550
   fl_change+0xddd/0x7da0
   tc_new_tfilter+0xaa7/0x2420
   rtnetlink_rcv_msg+0x95e/0xe90
  which belongs to the cache kmalloc-2k of size 2048

Protect qdisc_rtab_list and the refcount with a dedicated spinlock. The
(sleeping, GFP_KERNEL) allocation in qdisc_get_rtab() is performed before
taking the lock; if a concurrent inserter added an identical table in the
meantime the freshly allocated one is freed under the lock, so no
duplicate is leaked. qdisc_put_rtab() now decrements the refcount and
unlinks under the same lock.

Fixes: 470502d ("net: sched: unlock rules update API")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
Cc: stable@vger.kernel.org
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260715114114.446841-1-qwe.aldo@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/sch_api.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=fb29e1b41052488ee3f2d115d4a870497ebd7f7d

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

The original Linux kernel CVE announcement for CVE-2026-68138 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68138

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).