[CVE-2026-74620][MODERATE 7.0] net/sched: act_gact, act_police: range check the fallback control action

[CVE-2026-74620][MODERATE 7.0] net/sched: act_gact, act_police: range check the fallback control action

[AL-KERNEL]

CVE: CVE-2026-74620
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: act_gact, act_police: range check the fallback control action
Commit: efa58aeb6a99028b1fbc3ab2f31ba3a881211ad4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efa58aeb6a99028b1fbc3ab2f31ba3a881211ad4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74620
Analysis date: Sat, 22 Aug 2026 12:49:29 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
act_gact and act_police accepted invalid fallback control actions such as TC_ACT_CONSUMED, allowing a CAP_NET_ADMIN actor to install a deterministic per-packet skb leak that can exhaust kernel memory and cause denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-74620 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74620

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: NO

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-2026-74620	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-20;**CWE-400;*CWE-401;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'act_gact and act_police allowed user supplied fallback control actions outside the valid UAPI range. A local actor able to configure tc actions could store TC_ACT_CONSUMED as the fallback verdict, causing callers to believe the action took ownership of the skb and therefore skip freeing it. This leaks one sk_buff and its data buffer for each packet that traverses the affected filter, which can lead to unbounded kernel memory consumption and denial of service. For the CVSS the PR:L is used in the paranoid score because CAP_NET_ADMIN may be delegated to containers or service accounts, and such reduced capability control can still affect global kernel memory. The issue is not directly network reachable because the vulnerable state is created through local traffic control configuration, although network traffic can amplify the leak after the rule is installed. Impact is denial of service only, with no concrete UAF, OOB access, information leak, or privilege escalation primitive indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE LEAK LINUS TEST PACKET  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The leak is deterministic once the invalid fallback action is installed. Each matching packet leaks one sk_buff plus its data buffer.

* Availability impact realistic: +1
  The leak is unbounded and per packet, so sustained traffic can realistically exhaust kernel memory.

* Broad/default/common subsystem or broadly deployed exposure: +1
  Traffic control actions are part of a common Linux networking subsystem.

* Common networking core path or packet-processing path: +1
  After configuration, the issue is exercised by packets traversing ingress, egress, or qevent handling.

* Requires CAP_NET_ADMIN in typical deployments: -2
  Creating the affected tc actions normally requires CAP_NET_ADMIN or equivalent administrative control.

Paranoid additional interpretation:

* Cross-boundary isolation/resource concern: +1
  If CAP_NET_ADMIN is delegated to a container, network namespace, or service account, the actor may still consume host kernel memory through skb leaks.

* Privilege penalty reduced to 0 in paranoid scoring
  Delegated CAP_NET_ADMIN is not always equivalent to full host root in practical risk terms.

Not counted:

* No generic memory corruption signal. The patch and commit describe an skb ownership verdict misuse causing leaks, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation signal. There is no concrete LPE primitive.
* No confidentiality or integrity impact. The demonstrated effect is resource exhaustion.

## 3. Reachability analysis

The bug is triggered by a local actor able to configure tc act_gact or act_police fallback actions through netlink traffic control configuration. In normal host deployments this requires CAP_NET_ADMIN, so the conservative interpretation treats it as privileged local. In containerized or delegated network administration setups, CAP_NET_ADMIN may be available to a less trusted actor, and the resulting skb leaks can consume shared host kernel memory. The vulnerable state is not created directly by remote packets, but once installed, ordinary network traffic can amplify the leak.

Call-site confidence: high. The commit explicitly names sch_handle_ingress(), sch_handle_egress(), and tcf_qevent_handle() as callers that skip freeing skb on TC_ACT_CONSUMED.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an Important vulnerability. The impact is deterministic and potentially system-wide memory exhaustion, but it is still a resource leak rather than memory corruption. The realistic exploitation evidence supports DoS, not privilege escalation. The paranoid score is elevated because delegated CAP_NET_ADMIN can make this relevant in container or service-account threat models and because traffic can amplify the leak after setup.

## 5. One-sentence report phrase

act_gact and act_police accepted invalid fallback control actions such as TC_ACT_CONSUMED, allowing a CAP_NET_ADMIN actor to install a deterministic per-packet skb leak that can exhaust kernel memory and cause denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue is a deterministic unbounded per-packet kernel memory leak and may be more relevant in environments where CAP_NET_ADMIN is delegated to containers, network namespaces, or restricted service accounts.

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

Patch: net/sched: act_gact, act_police: range check the fallback control action
Commit: efa58aeb6a99028b1fbc3ab2f31ba3a881211ad4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efa58aeb6a99028b1fbc3ab2f31ba3a881211ad4

Commit description:

tcf_action_check_ctrlact() range checks the primary control action:

	if (!opcode)
		ret = action > TC_ACT_VALUE_MAX ? -EINVAL : 0;

TC_ACT_VALUE_MAX is TC_ACT_TRAP, so kernel-internal verdicts above it
cannot be set that way. But act_gact and act_police each carry a second,
independent control action supplied by user space that never reaches that
helper - TCA_GACT_PROB.paction and TCA_POLICE_RESULT. Both only reject
TC_ACT_GOTO_CHAIN, so any other value is stored verbatim and returned
verbatim from the action.

In particular user space can store TC_ACT_CONSUMED, which is
TC_ACT_VALUE_MAX + 1 and is deliberately not part of the UAPI value
range. That verdict tells every caller the action took ownership of the
skb, so nobody frees it: sch_handle_ingress(), sch_handle_egress() and
tcf_qevent_handle() all deliberately skip the free for it. The result is
one leaked sk_buff plus its data buffer per packet traversing the filter,
unbounded, for all traffic on the chain including kernel-generated
packets.

Both are trivially deterministic. act_gact clamps tcfg_pval to >= 1, so
with pval = 1 gact_determ() returns the fallback for every packet.
act_police has no mandatory rate, so rate = 0 leaves tcfp_mtu = ~0 and
tcf_police_mtu_check() always passes.

TC_ACT_CONSUMED was added by commit 720f22f ("net: sched: refactor
reinsert action"), after both goto-chain guards were written:
commit 9469f37 ("net/sched: act_gact: disallow 'goto chain' on
fallback control action") and
commit c08f5ed ("net/sched: act_police: disallow 'goto chain' on
fallback control action"). Neither guard was widened when the new
verdict appeared.

Factor the existing range test out of tcf_action_check_ctrlact() as
tcf_action_valid() and apply it to both fallbacks. The helper cannot call
tcf_action_check_ctrlact() directly because that also allocates a
goto_chain, which is exactly what these two sites must not do.

Reproduced on v7.2-rc6: kmemleak reports one leaked 232-byte
skbuff_head_cache object plus its 704-byte data buffer per packet. With
this patch both configurations are rejected with -EINVAL and kmemleak
reports none.

Fixes: 720f22f ("net: sched: refactor reinsert action")
Cc: stable@vger.kernel.org # v5.3+
Signed-off-by: Hyunjung Ko <hj351016@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260806101252.809593-1-hj351016@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/act_api.h
  net/sched/act_gact.c
  net/sched/act_police.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=efa58aeb6a99028b1fbc3ab2f31ba3a881211ad4

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

The original Linux kernel CVE announcement for CVE-2026-74620 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74620

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: 2
  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).