[CVE-2026-63979][MODERATE 7.0] net/handshake: hand off the pinned file reference to accept_doit [ Upstream

[CVE-2026-63979][MODERATE 7.0] net/handshake: hand off the pinned file reference to accept_doit [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63979
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/handshake: hand off the pinned file reference to accept_doit [ Upstream
Commit: c06876d4fac38f35820946ee3b1be7d7da799cd4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c06876d4fac38f35820946ee3b1be7d7da799cd4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63979
Analysis date: Sun, 19 Jul 2026 14:46:44 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum. Strong Important candidate only if manual review confirms exploitable UAF beyond crash
Manual review required: YES

Summary:
A race in net/handshake can let the accept path use a socket file reference after concurrent cancellation releases it, causing NULL dereference or UAF-style access to freed socket memory and at least a local kernel DoS.

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

The original Linux kernel CVE announcement for CVE-2026-63979 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63979

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

CVE-2026-63979	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-416;*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 'A race in net handshake can occur after handshake_req_next removes a request from the pending list and drops the lock while the accept path still plans to read the socket file through the request sock pointer. A concurrent cancel path can release the consumer owned file reference and free or orphan the socket, so the accept side may read NULL or chase freed socket memory. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires a local process or service context able to drive the handshake accept and cancel timing, but it should not be treated as full administrator only access in reduced capability deployments. The issue is not directly network reachable through packet input alone, although network driven TLS or RPC activity may help create handshake requests. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to use-after-free on kernel socket related objects.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum. Strong Important candidate only if manual review confirms exploitable UAF beyond crash (with actual score 5)	YES	UAF KPANIC 	YES	NO	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. Strong Important candidate only if manual review confirms exploitable UAF beyond crash**::

## 2. Signal breakdown

* Local trigger or local service-context trigger: +1
  The accept path is reached through the net/handshake generic netlink service, and reliable triggering requires local accept/cancel timing or a local service context such as tlshd or RPC/NFS-related teardown.

* Memory corruption, weak/indirect primitive: +1
  The patch explicitly describes the accept side either reading NULL through sk_socket or chasing freed memory. This is a UAF candidate, but no attacker-controlled reclaim, overwrite, callback dispatch, or type confusion is shown.

* Real lifetime corruption: +1
  struct sock is pinned, but struct socket and sock->file have independent lifetimes. The bug is a real reference/lifetime mismatch across accept and cancel paths.

* Reliable kernel crash / strong DoS: +1
  NULL dereference or dereference of freed socket/file memory can realistically crash the kernel.

* Hard race / timing window: -1 in conservative interpretation
  Triggering depends on the window between handshake_req_next() dropping hn_lock and handshake_nl_accept_doit() reaching FD_PREPARE().

* Paranoid availability uplift: +1
  In the paranoid score, the host-wide crash impact is treated as sufficiently strong and security-relevant because the patch fixes a concrete freed-object dereference in socket/file lifetime handling.

Race-UAF downgrade applied. No extra LPE bonus is awarded because the patch does not show controlled reclaim, write-after-free, type confusion, callback control, or refcount takeover.

## 3. Reachability analysis

This is not directly packet-triggerable from the network alone. Network-driven TLS or RPC activity may help create or cancel handshake requests, but the vulnerable accept-side dereference is in the local netlink handshake control path.

Typical triggering requires a local process, service, or reduced-privilege context able to participate in the handshake accept/cancel lifecycle. It should not be treated as full host-root-only in all deployments because reduced-capability service accounts or containerized service contexts may drive parts of this flow. Namespaces may matter if the handshake netlink interface and related sockets are exposed inside a network namespace.

The path is not a generic default packet-processing fast path. It is relevant mainly on systems using the kernel handshake infrastructure, especially TLS/RPC/NFS-related users.

Call-site confidence: high. The patch includes the vulnerable accept call site, the request-selection helper, and the cancel-related KUnit adjustments.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is a real lifetime race involving struct socket and struct file references, not just a validation or bookkeeping fix. The realistic demonstrated impact is local kernel crash or Oops. The theoretical memory corruption concern is valid because the accept side can chase freed memory, but the patch does not demonstrate a practical privilege-escalation primitive.

Therefore the conservative score is borderline/manual-review level, while the paranoid score reaches Actionable Moderate. It should not be auto-closed without human review, but it is not a strong Important conclusion unless further analysis shows controlled reuse or another exploitation primitive.

## 5. One-sentence report phrase

A race in net/handshake can let the accept path use a socket file reference after concurrent cancellation releases it, causing NULL dereference or UAF-style access to freed socket memory and at least a local kernel DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes a concrete socket/file lifetime race with possible freed-memory dereference. No strong LPE primitive is shown, but UAF-class lifetime bugs in kernel socket objects require manual review before downgrade or autoclose.

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

Patch: net/handshake: hand off the pinned file reference to accept_doit [ Upstream
Commit: c06876d4fac38f35820946ee3b1be7d7da799cd4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c06876d4fac38f35820946ee3b1be7d7da799cd4

Changed files:
  net/handshake/handshake-test.c
  net/handshake/netlink.c
  net/handshake/request.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=c06876d4fac38f35820946ee3b1be7d7da799cd4

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

The original Linux kernel CVE announcement for CVE-2026-63979 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63979

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).