[CVE-2026-63981][MODERATE 7.0] net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow

[CVE-2026-63981][MODERATE 7.0] net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow

[AL-KERNEL]

CVE: CVE-2026-63981
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow
Commit: 25fc9352590f5ef21ebf290432bd768b336693bc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=25fc9352590f5ef21ebf290432bd768b336693bc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63981
Analysis date: Sun, 19 Jul 2026 15:46:12 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local unprivileged user with access to user and network namespaces can configure TC mirred blockcast recursion so that `tcf_mirred_act()` bypasses its nesting limit and overflows the kernel stack, causing a reliable kernel panic.

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

The original Linux kernel CVE announcement for CVE-2026-63981 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63981

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-2026-63981	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:L/I:L/A:H';CWE-674;**CWE-400;CWE-770;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'act_mirred blockcast handling can bypass the sched_mirred_nest recursion counter because the blockcast path returns before normal mirred nesting accounting is applied. A local user can build a TC configuration in a network namespace where two devices share an egress block and mirror packets back and forth, causing repeated re entry into tcf_mirred_act until the kernel stack overflows and the system panics. For the CVSS the PR:L is selected because reliable triggering requires local code execution, but unprivileged user namespaces can grant CAP_NET_ADMIN inside a new network namespace on many systems. The issue is not directly network reachable from a remote host and is triggered through local TC control plane configuration plus packet transmission. Impact is at least local denial of service via kernel panic. The paranoid score keeps limited confidentiality and integrity impact because kernel stack overflow is a memory safety adjacent condition, but the patch context does not show a reliable arbitrary write or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-63981 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: fe946a751d9b52b7c45ca34899723b314b79b249 YES  NO NO unknown 	MAYBE	REMOTE KASAN OOB NETWORK SKB HARDWARE QDISC KPANIC PACKET 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  The commit states the bug is reachable by an unprivileged user via `unshare(CLONE_NEWUSER | CLONE_NEWNET)`, because the user obtains `CAP_NET_ADMIN` inside the new network namespace.

* Reliable kernel crash / strong DoS: +1
  The recursion bypass leads to kernel stack overflow, stack guard page hit, Oops, and kernel panic.

* Availability impact realistic: +1
  This is not only a warning. The provided trace shows a reliable host-wide panic from a constructed TC configuration and packet transmission.

* Common networking / packet-processing path: +1
  The affected code is in `net/sched` and executes in the TC egress packet-processing path once configured.

Paranoid-only signal:

* Weak / indirect corruption candidate: +1
  Kernel stack overflow is a memory-safety-adjacent failure mode, but the patch does not show controlled overwrite, UAF reclaim, type confusion, callback control, or an arbitrary write primitive. This supports manual review but not Important-class scoring by itself.

Not triggered:

* Remote reachable: +0
  The issue is not triggered by ordinary remote packets alone. It requires local TC setup.

* Strong LPE plausibility: +0
  No practical privilege escalation primitive is shown.

* Generic strong memory corruption: +0
  The bug causes stack exhaustion and panic, not a demonstrated attacker-controlled memory corruption primitive.

* Requires admin/root penalty: +0 in the main score
  Although TC normally needs `CAP_NET_ADMIN`, the provided advisory explicitly describes unprivileged reachability through user and network namespaces. If unprivileged user namespaces are disabled, the practical score should be lower.

## 3. Reachability analysis

A local unprivileged user can trigger the issue on systems where unprivileged user namespaces are enabled. The attacker can create a new user and network namespace, obtain `CAP_NET_ADMIN` in that namespace, create dummy devices, attach `clsact` qdiscs with shared blocks, install `mirred blockcast` filters, and transmit a packet that recurses between the devices.

This is not a remote network vulnerability in the usual sense. Remote traffic alone is insufficient unless a local actor has already configured the vulnerable TC topology. The path is realistic on distributions that allow unprivileged user namespaces, but less reachable on hardened systems where user namespaces are disabled or restricted.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate because it is a reliable local unprivileged kernel panic when namespaces are available. It does not reach a Strong Important candidate level because the demonstrated impact is DoS, and the patch does not show a controlled corruption primitive or a plausible LPE chain.

The conservative interpretation is local DoS. The paranoid interpretation keeps it as Actionable Moderate because kernel stack overflow is worth manual review, but there is no evidence for arbitrary write, UAF reclaim, type confusion, or privilege escalation.

## 5. One-sentence report phrase

A local unprivileged user with access to user and network namespaces can configure TC mirred blockcast recursion so that `tcf_mirred_act()` bypasses its nesting limit and overflows the kernel stack, causing a reliable kernel panic.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is locally reachable without host-root on systems with unprivileged user namespaces enabled and causes kernel stack overflow. The current evidence supports DoS, but the memory-safety-adjacent failure mode and namespace-based reachability make it unsuitable for auto-close.

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

Patch: net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow
Commit: 25fc9352590f5ef21ebf290432bd768b336693bc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=25fc9352590f5ef21ebf290432bd768b336693bc

Commit description:

tcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4)
to prevent deep recursion.  However, when the action uses blockcast
(tcfm_blockid != 0), the function returns at the tcf_blockcast() call
BEFORE reaching the counter increment.  As a result, the recursion
counter never advances and the limit check is entirely bypassed.

When two devices share a TC egress block with a mirred blockcast rule,
a packet egressing on device A is mirrored to device B via blockcast;
device B's egress TC re-enters tcf_mirred_act() via blockcast and
mirrors back to A, creating an unbounded recursion loop:

  tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit
  -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat)

This recursion continues until the kernel stack overflows.

The bug is reachable from an unprivileged user via
unshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant
CAP_NET_ADMIN in the new network namespace, which is sufficient to
create dummy devices, attach clsact qdiscs with shared blocks, and
install mirred blockcast filters.

 BUG: TASK stack guard page was hit at ffffc90000b7fff8
 Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI
 CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410
 RIP: 0010:xas_find+0x17/0x480
 Call Trace:
  xa_find+0x17b/0x1d0
  tcf_mirred_act+0x640/0x1060
  tcf_action_exec+0x400/0x530
  basic_classify+0x128/0x1d0
  tcf_classify+0xd83/0x1150
  tc_run+0x328/0x620
  __dev_queue_xmit+0x797/0x3100
  tcf_mirred_to_dev+0x7b1/0xf70
  tcf_mirred_act+0x68a/0x1060
  [repeating ~30+ times until stack overflow]
 Kernel panic - not syncing: Fatal exception in interrupt

Fix this by incrementing sched_mirred_nest before calling
tcf_blockcast() and decrementing it on return, mirroring the
non-blockcast path.  This ensures subsequent recursive entries see the
updated counter and are correctly limited by MIRRED_NEST_LIMIT.

Fixes: fe946a7 ("net/sched: act_mirred: add loop detection")
Signed-off-by: Kito Xu (veritas501) <hxzene@gmail.com>
Link: https://patch.msgid.link/20260525122556.973584-7-jhs@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/act_mirred.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=25fc9352590f5ef21ebf290432bd768b336693bc

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

The original Linux kernel CVE announcement for CVE-2026-63981 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63981

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:L/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:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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).