[CVE-2026-64563][MODERATE 7.0] rhashtable: clear stale iter->p on table restart

[CVE-2026-64563][MODERATE 7.0] rhashtable: clear stale iter->p on table restart

[AL-KERNEL]

CVE: CVE-2026-64563
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: rhashtable: clear stale iter->p on table restart
Commit: 3ff7c1dbf722cf3fa538672452ba182318e0fcc3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3ff7c1dbf722cf3fa538672452ba182318e0fcc3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64563
Analysis date: Tue, 04 Aug 2026 02:59:14 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale rhashtable iterator pointer can survive a table restart after resize and cause a local use-after-free read in rhashtable_walk_next, with demonstrated netlink_diag kernel crash impact and enough shared-infrastructure risk to require manual review.

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

The original Linux kernel CVE announcement for CVE-2026-64563 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64563

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-2026-64563	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-825;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free read can occur in rhashtable walk restart handling because a stale iterator pointer is not cleared when the previous table was freed during resize. A local process can trigger affected multi fragment rhashtable dumps across walk_stop/walk_start boundaries, with concrete affected paths including netlink_diag and TIPC socket walks. For the CVSS the PR:L is used because triggering requires local code execution or a local process able to request the affected diagnostic dump, not only remote packet reachability. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, the stale pointer UAF may allow broader confidentiality or integrity impact through object confusion or reuse of freed memory and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	READ DANGER UAF KERNEL_PANIC_PLUS_UAF KPANIC 	NO	NO	checked

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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

Conservative signals:

* Local unprivileged trigger: +1
  A local process can request affected multi-fragment rhashtable dump paths such as netlink_diag. TIPC depends more on configuration.
* Memory corruption, weak/indirect corruption candidate: +1
  This is a real stale pointer use-after-free read, but the patch does not show attacker-controlled reclaim, write-after-free, callback control, or arbitrary write.
* Real lifetime corruption: +1
  iter->p can survive a table restart after the old table was freed during resize.
* Reliable kernel crash / strong DoS: +1
  The report includes a KASAN slab-use-after-free crash in rhashtable_walk_next.
* Broad/shared helper exposure: +1
  rhashtable walking is shared infrastructure, and the commit names multiple concrete users.
* Hard or unreliable race / special timing required: -1
  Triggering requires a restart across walk_stop/walk_start while a table resize/free invalidates the old table.

Paranoid delta:

* Paranoid manual-review uplift: +1
  The UAF is in shared iterator infrastructure and affected callers may interpret returned objects differently. This does not prove LPE, but it is enough to avoid auto-close.

Race-UAF downgrade applied:

* No strong LPE bonus.
* No arbitrary C/I impact in the conservative score.
* Conservative score capped at 4.
* Paranoid score capped at 5.

## 3. Reachability analysis

The likely trigger is local. A local user or process can exercise diagnostic dump interfaces that perform multi-fragment rhashtable walks, with netlink_diag given as a concrete crashing path. This is not directly network-triggerable by packet traffic.

Namespace behavior may matter. Some diagnostic interfaces are reachable by unprivileged users in common configurations, while TIPC and some netlink families may depend on module availability, namespace policy, or capabilities. The bug is not default remote exposure, but it is in common kernel infrastructure and has at least one realistic local diagnostic path.

Call-site confidence: medium. The report provides concrete call stacks for netlink_diag and mentions TIPC, but the full caller source is not included in the patch snippet.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a confirmed UAF in shared rhashtable walk infrastructure with a real crash trace and multiple affected callers. Realistic demonstrated impact is local kernel crash / DoS.

It is not a strong Important-class LPE candidate based on the patch alone. The UAF is read-side and race-dependent, and there is no demonstrated attacker-controlled reclaim, write primitive, callback dispatch, refcount takeover, or type confusion. The paranoid score still reaches Actionable Moderate because shared lifetime bugs of this class are often under-triaged and deserve manual review.

## 5. One-sentence report phrase

A stale rhashtable iterator pointer can survive a table restart after resize and cause a local use-after-free read in rhashtable_walk_next, with demonstrated netlink_diag kernel crash impact and enough shared-infrastructure risk to require manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed UAF plus shared helper exposure plus concrete local crash path. The current evidence supports DoS most strongly, but the stale pointer lifetime pattern should be reviewed manually before closing as ordinary Moderate.

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

Patch: rhashtable: clear stale iter->p on table restart
Commit: 3ff7c1dbf722cf3fa538672452ba182318e0fcc3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3ff7c1dbf722cf3fa538672452ba182318e0fcc3

Commit description:

rhashtable_walk_start_check() has two restart paths when resuming a walk.
When iter->walker.tbl is valid, it re-validates iter->p against the table
and sets iter->p = NULL if the object is gone.  When iter->walker.tbl is
NULL (table was freed during resize), it resets slot and skip but forgets
to clear iter->p.

rhashtable_walk_next() then dereferences the stale iter->p, reading
freed memory.  This is a use-after-free.

Any caller that does multi-fragment rhashtable walks across
walk_stop/walk_start boundaries is affected.  Concrete cases include
netlink_diag (__netlink_diag_dump in net/netlink/diag.c) and TIPC
(tipc_nl_sk_walk in net/tipc/socket.c).

Crash stack (netlink_diag):
  BUG: KASAN: slab-use-after-free in rhashtable_walk_next+0x365/0x3c0
  Read of size 8 at addr ffff88801a9d2438 (freed kmalloc-2k, offset 1080)
  Call Trace:
   rhashtable_walk_next+0x365/0x3c0 (lib/rhashtable.c:1016)
   __netlink_diag_dump+0x160/0x760 (net/netlink/diag.c:122)
   netlink_diag_dump+0xc2/0x240
   netlink_dump+0x5bc/0x1270
   netlink_recvmsg+0x7a3/0x980
   sock_recvmsg+0x1bc/0x200
   __sys_recvfrom+0x1d4/0x2c0

Fixes: 5d240a8 ("rhashtable: improve rhashtable_walk stability when stop/start used.")
Cc: <stable@vger.kernel.org>
Reported-by: AutonomousCodeSecurity@microsoft.com
Reported-by: Yuan Tan <yuantan098@gmail.com>
Closes: https://lore.kernel.org/linux-crypto/CAB8m9Wh559e+=n8z51gB8DrbEyCc2mc0MgGjrRR6_VXBmU=2AQ@mail.gmail.com
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netlink/diag.c
  net/tipc/socket.c
  lib/rhashtable.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=3ff7c1dbf722cf3fa538672452ba182318e0fcc3

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

The original Linux kernel CVE announcement for CVE-2026-64563 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64563

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).