[CVE-2026-74630][IMPORTANT] ipv6: prevent in6_dev_get() from resurrecting inet6_dev

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-74630
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: prevent in6_dev_get() from resurrecting inet6_dev
Commit: 785d908f8d21c8bc78b6fb2c2932ab662bf6918a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=785d908f8d21c8bc78b6fb2c2932ab662bf6918a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74630
Analysis date: Sat, 22 Aug 2026 13:57:03 -0400
ActionableScore: 9
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local unprivileged user can race IPv6 device teardown so that `in6_dev_get()` resurrects an `inet6_dev` after its refcount reached zero, causing an RCU use-after-free that can crash the kernel and may warrant privilege-escalation review.

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

The original Linux kernel CVE announcement for CVE-2026-74630 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74630

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: IMPORTANT
Manual review required: YES

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
======================================================================

CVE-2026-74630	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-416;CWE-362;CWE-911;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'in6_dev_get could read dev ip6_ptr under RCU and then increment the inet6_dev reference count even after teardown had dropped the last reference. This can resurrect an object whose RCU free is already queued and allows callers to use the inet6_dev after it has been freed. The provided traces show refcount addition on 0, reference underflow, and KASAN slab use after free in mutex_lock with reproduction as UID 1000. For the CVSS the PR:L is used for the paranoid score because a local unprivileged user can reach the affected IPv6 path while teardown or namespace cleanup creates the race. The issue is not directly network reachable as AV:N because local execution or a local user context is needed to drive the race. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to use after free memory corruption, so privilege escalation cannot be ruled out without manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)  SKIP CVE-2026-74630 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE OOB DANGER UAF HARDWARE IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **9**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* **Local unprivileged trigger: +1**
  The reproducer is reported as running as UID 1000, so PR:L is appropriate.

* **Memory corruption, strong corruption primitive: +2**
  The bug is a real UAF/refcount lifetime issue. `refcount_inc()` can resurrect an `inet6_dev` after its refcount reached zero and after RCU freeing has been queued.

* **Real lifetime corruption: +1**
  This is explicitly an RCU/refcount lifetime violation with stale object use after teardown.

* **Reliable kernel crash / strong DoS: +1**
  The supplied traces show `refcount_t: addition on 0`, reference underflow, and KASAN `slab-use-after-free` in `mutex_lock`.

* **Privilege escalation plausible: +1 conservative / +2 paranoid**
  Conservative score treats this as a UAF with LPE concern but without demonstrated reclaim or controlled replacement. Paranoid score gives stronger LPE weight because the primitive is refcount resurrection of a freed networking object, with a post-RCU use-after-free write path shown.

* **Broad/default/common subsystem: +1**
  IPv6 device state and addrconf-related paths are common kernel networking functionality.

* **Common networking core path or packet-processing-adjacent path: +1**
  `inet6_dev` lifetime is central to IPv6 per-device state, multicast, and addrconf users.

* **Confidentiality impact plausible: +1 paranoid only**
  Not demonstrated directly, but UAF exploitation can plausibly expose stale kernel object state in worst-case review.

* **Integrity impact plausible: +1 paranoid only**
  The KASAN trace shows a write after free through `mutex_lock`, so worst-case integrity impact is plausible even though no controlled overwrite is demonstrated.

* **Hard or unreliable race / special timing required: -1**
  The issue depends on a teardown race between RCU lookup, last reference drop, and later use.

## 3. Reachability analysis

A local unprivileged user appears able to trigger the affected path, based on the UID 1000 reproduction. The bug is not directly remote-network-triggerable as AV:N, because the attacker needs local execution or local namespace/process control to drive the race around IPv6 device teardown or related cleanup. Network namespaces may make triggering more realistic if unprivileged users can create or manipulate namespaces in the evaluated environment. IPv6 is broadly deployed, so this is not a rare subsystem. Call-site confidence: **medium-high**, because the changed helper is a refcounting/lifetime helper and the report includes concrete downstream traces in `ip6_mc_source()` and KASAN UAF.

## 4. Severity interpretation

This behaves more like an **Important-class local kernel vulnerability candidate** than an ordinary Moderate. The realistic demonstrated impact is local kernel crash and refcount/UAF corruption. The theoretical worst case includes privilege escalation because this is not a NULL dereference or pure warning: it is refcount resurrection of an object whose RCU free has already been queued, followed by actual use-after-free. However, practical LPE is not proven because there is no demonstrated attacker-controlled reclaim, object replacement, or arbitrary write primitive in the supplied material.

## 5. One-sentence report phrase

A local unprivileged user can race IPv6 device teardown so that `in6_dev_get()` resurrects an `inet6_dev` after its refcount reached zero, causing an RCU use-after-free that can crash the kernel and may warrant privilege-escalation review.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a real local UAF/refcount resurrection issue in a common IPv6 lifetime path, with UID 1000 reproduction and KASAN evidence. Even though reliable privilege escalation is not demonstrated, the bug class and affected subsystem are strong enough to require manual security review.

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

Patch: ipv6: prevent in6_dev_get() from resurrecting inet6_dev
Commit: 785d908f8d21c8bc78b6fb2c2932ab662bf6918a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=785d908f8d21c8bc78b6fb2c2932ab662bf6918a

Commit description:

in6_dev_get() reads dev->ip6_ptr under RCU and then unconditionally
increments its refcount. Device teardown can clear the pointer and drop
the last reference between these operations. The increment then
resurrects an object whose RCU free has already been queued, so callers
can use it after it is freed.

Use refcount_inc_not_zero() and return NULL when the object has already
reached zero. RCU keeps the memory accessible through the attempted
reference acquisition, and a successful increment pins the object for
the caller.

An independent run on the exact unpatched 6f5156d (v7.2-rc3)
kernel reproduced the invalid reference acquisition as UID 1000:

  refcount_t: addition on 0; use-after-free.
  ip6_mc_source+0xef4/0x17e0

It was followed by the corresponding reference underflow in
ip6_mc_source(). The supplied trace from the same unpatched revision
additionally shows the access after the RCU read-side section ends:

  BUG: KASAN: slab-use-after-free in mutex_lock+0x76/0xe0
  Write of size 8 at addr ffff888015b50240 by task poc/1219

Bug found and triaged by OpenAI Security Research and
validated by Trail of Bits.

Fixes: 8814c4b ("[IPV6] ADDRCONF: Convert addrconf_lock to RCU.")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Co-developed-by: David Lee <david.lee@trailofbits.com>
Signed-off-by: David Lee <david.lee@trailofbits.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260803122758.666112-1-david.lee@trailofbits.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/addrconf.h

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=785d908f8d21c8bc78b6fb2c2932ab662bf6918a

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

The original Linux kernel CVE announcement for CVE-2026-74630 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74630

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: 7
  Paranoid ActionableScore: 9

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).