[CVE-2026-74563][MODERATE 7.0] rds: tcp: hold the RCU lock across ipv6_chk_addr() in rds_tcp_laddr_check() [ Upstream

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

CVE: CVE-2026-74563
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: rds: tcp: hold the RCU lock across ipv6_chk_addr() in rds_tcp_laddr_check() [ Upstream
Commit: 76dd48886eeeb5fcf2b837d2f4c3d17eebeac9ef
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76dd48886eeeb5fcf2b837d2f4c3d17eebeac9ef
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74563
Analysis date: Sat, 15 Aug 2026 13:32:23 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local race in RDS TCP IPv6 bind handling can use a `net_device` pointer after its RCU lifetime ends, allowing a local attacker with suitable namespace or netdevice control to trigger a kernel use-after-free and crash the system.

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

The original Linux kernel CVE announcement for CVE-2026-74563 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74563

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74563	MODERATE	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-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'rds_tcp_laddr_check obtains a scoped IPv6 net_device pointer with dev_get_by_index_rcu but previously dropped the RCU read lock before calling ipv6_chk_addr. A concurrent RTM_DELLINK can free the device before ipv6_chk_addr and __ipv6_chk_addr_and_flags dereference it, resulting in a slab use after free on a stale net_device pointer. For the CVSS the PR:L is used because reliable triggering requires a local process able to call bind on an RDS socket and to arrange or race interface removal in the relevant network namespace. AC:H is used because the issue depends on a timing window between RDS bind processing and netdevice unregister. The issue is not directly network reachable. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to use after free memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74563 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE READ OOB LOCK DANGER UAF NETWORK HARDWARE LINUS IPV6 KPANIC 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Applied signals:

* **Local unprivileged trigger: +1**
  The vulnerable path is reached from `bind()` on an RDS socket. Reliable triggering also needs racing netdevice removal, which may require `CAP_NET_ADMIN` unless user or network namespaces make this available to a reduced-privilege local attacker.

* **Memory corruption, weak primitive: +1**
  This is a KASAN-confirmed slab use-after-free, but the demonstrated access is a stale `net_device` read in `ipv6_chk_addr()` rather than attacker-controlled reclaim, overwrite, callback control, or type confusion.

* **Real lifetime corruption: +1**
  The bug is an RCU lifetime violation. `dev_get_by_index_rcu()` returns a pointer valid only inside the same RCU read-side critical section, but the old code used it after `rcu_read_unlock()`.

* **Reliable kernel crash / strong DoS: +1**
  The report shows a concrete KASAN slab-use-after-free in the syscall path, with realistic kernel crash potential.

Paranoid-only signal:

* **Availability impact realistic: +1**
  In the paranoid score, repeated local triggering could cause host-wide kernel crashes or repeated service disruption. This raises the practical triage score but does not imply proven privilege escalation.

Not applied:

* **Remote reachable / network-triggerable: +0**
  The issue is triggered through local RDS `bind()` plus netdevice unregister, not by receiving network packets.

* **Privilege escalation plausible: +0**
  No attacker-controlled reclaim, write-after-free, callback dispatch, refcount takeover, or type confusion is shown.

* **Confidentiality / Integrity impact: +0**
  The demonstrated primitive is a stale read causing UAF detection or crash. No leak or write primitive is demonstrated.

* **Hard race subtraction: 0**
  Although this is a race, the commit includes an `exploit` task and a concrete KASAN trace, so it appears reproducible enough for manual triage rather than ordinary theoretical race handling.

## 3. Reachability analysis

A local process can reach the vulnerable call path through `rds_bind()` with an IPv6 scoped address. The race requires concurrent netdevice removal via `RTM_DELLINK`, which normally requires `CAP_NET_ADMIN`, but this may be available inside user or network namespaces depending on product configuration. The path is not directly reachable from the network. RDS is less common than core TCP or UDP, but it remains an ordinary kernel socket subsystem when enabled, so this should not be auto-closed solely as a rare configuration issue.

Call-site confidence: **high**, because the supplied trace shows the full path from `bind()` to `rds_tcp_laddr_check()` to `ipv6_chk_addr()` to the UAF.

## 4. Severity interpretation

This behaves more like an **actionable Moderate** than an ordinary Moderate. The realistic demonstrated impact is local DoS via a race-triggered UAF read of a freed `net_device`. The theoretical memory-corruption concern exists because this is a real stale-pointer lifetime bug in kernel networking code, but the patch does not show a strong LPE primitive such as controlled reuse, writable stale access, or callback hijacking. Therefore it is not a strong Important candidate on the provided evidence, but it should receive manual review.

## 5. One-sentence report phrase

A local race in RDS TCP IPv6 bind handling can use a `net_device` pointer after its RCU lifetime ends, allowing a local attacker with suitable namespace or netdevice control to trigger a kernel use-after-free and crash the system.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is a confirmed UAF in a networking path with namespace-dependent reachability. Even though the demonstrated primitive is crash-oriented and not a proven LPE, memory lifetime bugs of this type should not be auto-closed.

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

Patch: rds: tcp: hold the RCU lock across ipv6_chk_addr() in rds_tcp_laddr_check() [ Upstream
Commit: 76dd48886eeeb5fcf2b837d2f4c3d17eebeac9ef
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=76dd48886eeeb5fcf2b837d2f4c3d17eebeac9ef

Changed files:
  net/ipv6/addrconf.c
  mm/kasan/report.c
  net/rds/tcp.c
  net/rds/bind.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=76dd48886eeeb5fcf2b837d2f4c3d17eebeac9ef

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

The original Linux kernel CVE announcement for CVE-2026-74563 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74563

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:N/I:N/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).