[CVE-2026-64581][MODERATE 7.0] xfrm: fix sk_dst_cache double-free in xfrm_user_policy() [ Upstream

[CVE-2026-64581][MODERATE 7.0] xfrm: fix sk_dst_cache double-free in xfrm_user_policy() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64581
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: fix sk_dst_cache double-free in xfrm_user_policy() [ Upstream
Commit: 96b678d08268b5f5c6fc99d4289d9b7e334fc683
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=96b678d08268b5f5c6fc99d4289d9b7e334fc683
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64581
Analysis date: Wed, 05 Aug 2026 05:09:47 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in XFRM per-socket policy handling can double release a socket dst cache entry while the UDP transmit path resets it concurrently, causing a local unprivileged double-free/UAF and at least a kernel crash, with limited but manual-review-worthy privilege escalation concern.

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

The original Linux kernel CVE announcement for CVE-2026-64581 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64581

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-2026-64581	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-362;CWE-415;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in xfrm_user_policy can double release the socket dst cache because __sk_dst_reset clears sk_dst_cache non atomically while the connected UDP transmit path may concurrently reset the same cache with an atomic xchg. Both racing paths can observe the same old dst and call dst_release on the single socket reference, freeing the xfrm_dst bundle while it is still referenced and creating a use-after-free condition. For the CVSS the PR:L is used because triggering is reported to be reachable by an unprivileged local user through user and network namespaces, but it still requires local code execution on the target. The issue is not directly network reachable as a remote packet only attack. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	DANGER UAF KPANIC 	NO	NO	checked

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

* Local unprivileged trigger: +1. The commit explicitly says the bug is reachable by an unprivileged user via user and network namespaces.
* Memory corruption, weak race-dependent primitive: +1. The bug is a double release of `sk_dst_cache` leading to UAF, but the patch does not show attacker-controlled reclaim, type confusion, callback control, or arbitrary write.
* Real lifetime corruption: +1. Both racing paths can release the same `dst` reference, freeing an `xfrm_dst` bundle while it is still referenced.
* Reliable kernel crash / strong DoS: +1. The report includes a KASAN slab-use-after-free in `dst_release()`.
* Broad networking/socket exposure: +1. The affected path is in XFRM/socket dst-cache handling and intersects with the connected UDP transmit path.
* Common networking data path relevance: +1. The race involves `udp_sendmsg -> sk_dst_check -> sk_dst_reset`, not only obscure driver-local code.
* Hard or unreliable race: -1. Triggering requires racing per-socket policy changes with UDP send path cache reset.

Conservative score is capped at 4 because this is a race-dependent UAF without demonstrated reclaim or stronger exploitation primitive. Paranoid score is 5 because the bug is a real local unprivileged double-free/UAF in networking lifetime management and should not be auto-closed.

## 3. Reachability analysis

A local unprivileged user can plausibly trigger the issue if user and network namespaces are available, because the commit explicitly states this reachability. The attack is not directly remote network-triggerable as a packet-only attack. It requires local code execution and a race between `xfrm_user_policy()` setsockopt handling and the connected UDP transmit fast path. Namespace support materially lowers the privilege requirement from host admin to local unprivileged in many deployments.

## 4. Severity interpretation

This behaves above an ordinary Moderate because it is not only a warning, leak, or validation issue. It is a real double-free/UAF in kernel networking lifetime handling with an unprivileged namespace trigger. Realistic demonstrated impact is kernel crash / DoS. Privilege escalation is theoretically concerning due to UAF/double-free, but no concrete reclaim, object replacement, type confusion, or write primitive is shown in the patch context, so the conservative score should not treat LPE as proven.

## 5. One-sentence report phrase

A race in XFRM per-socket policy handling can double release a socket dst cache entry while the UDP transmit path resets it concurrently, causing a local unprivileged double-free/UAF and at least a kernel crash, with limited but manual-review-worthy privilege escalation concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real lifetime corruption bug with double-free and UAF, reachable by an unprivileged local user through namespaces, and therefore should not be automatically closed even though the practical LPE primitive is not demonstrated.

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

Patch: xfrm: fix sk_dst_cache double-free in xfrm_user_policy() [ Upstream
Commit: 96b678d08268b5f5c6fc99d4289d9b7e334fc683
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=96b678d08268b5f5c6fc99d4289d9b7e334fc683

Changed files:
  include/linux/rcuref.h
  include/net/sock.h
  net/xfrm/xfrm_state.c
  net/ipv4/ip_sockglue.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=96b678d08268b5f5c6fc99d4289d9b7e334fc683

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

The original Linux kernel CVE announcement for CVE-2026-64581 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64581

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: 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).