[CVE-2026-64253][MODERATE REGULAR] kernel/fork: clear PF_BLOCK_TS in copy_process()

[CVE-2026-64253][MODERATE REGULAR] kernel/fork: clear PF_BLOCK_TS in copy_process()

[AL-KERNEL]

CVE: CVE-2026-64253
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: kernel/fork: clear PF_BLOCK_TS in copy_process()
Commit: ee0801aceabdf583392477baf69a290b09448b8f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ee0801aceabdf583392477baf69a290b09448b8f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64253
Analysis date: Fri, 24 Jul 2026 12:57:38 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate, not Important
Manual review required: NO

Summary:
A child task can inherit PF_BLOCK_TS while copy_process resets its block plug pointer to NULL, allowing later block-layer code that trusts the flag to dereference a NULL plug pointer and crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-64253 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64253

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 REGULAR
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64253	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A child task can inherit PF_BLOCK_TS from the parent while copy_process sets the child block plug pointer to NULL. Later block layer code may treat PF_BLOCK_TS as proof that current plug is valid and may dereference a NULL pointer, causing a kernel crash (DoS). For the CVSS the PR:L value is used because a local unprivileged process may be enough if it can reach the affected fork and block I/O paths. The issue is not network reachable and requires local code execution. Impact is denial of service only based on the shown invariant break and NULL pointer dereference.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Moderate, not Important (with actual score 4)	YES	SIMPLEFIX 	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate, not Important**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process may be able to influence fork and block I/O paths, although the exact reliable trigger depends on reaching copy_process while PF_BLOCK_TS is set.
* Reliable kernel crash / strong DoS: +1. The commit explicitly says callers may dereference current->plug unguarded when PF_BLOCK_TS is set, so inheriting the flag with plug NULL can lead to NULL pointer dereference.
* Important filesystem/storage path: +1. The invariant belongs to the block layer timestamp and plug handling path.
* Hard or special state required: -1. The child must be forked while the parent has PF_BLOCK_TS set, which likely requires a narrower block-plug timing/state window than ordinary fork alone.

Paranoid extra interpretation:

* Broad/default/common subsystem exposure: +1. fork and CONFIG_BLOCK are common on normal Linux systems, so the affected invariant is not tied to rare hardware or a niche optional feature.

No memory corruption, UAF, OOB write, arbitrary write, information leak, page-cache corruption, or LPE primitive is shown by the patch.

## 3. Reachability analysis

The bug is local. It is not network reachable. A realistic trigger requires a task to enter copy_process while PF_BLOCK_TS is inherited from a parent whose plug state is not inherited. In normal deployments CONFIG_BLOCK is broadly enabled, but reliable exploitation depends on whether an unprivileged process can naturally create the needed fork plus block-plug state. Namespaces do not obviously change the privilege requirement, because this is not a CAP_NET_ADMIN or CAP_SYS_ADMIN control-plane bug. The most defensible PR assumption is local unprivileged user access, but with some uncertainty around reliable triggering.

Call-site confidence: medium. The changed code is directly in copy_process, and the commit explains the invariant and the downstream unguarded dereference expectation, but the exact crashing caller is not included in the patch context.

## 4. Severity interpretation

This behaves like a Moderate local DoS issue, not an Important-class vulnerability. The theoretical concern is a broken kernel invariant leading to an invalid pointer dereference. The realistic evidence supports kernel crash only. There is no demonstrated stale object reuse, write-after-free, type confusion, attacker-controlled reclaim, arbitrary write, or security-boundary bypass, so privilege escalation is not supported by the patch.

## 5. One-sentence report phrase

A child task can inherit PF_BLOCK_TS while copy_process resets its block plug pointer to NULL, allowing later block-layer code that trusts the flag to dereference a NULL plug pointer and crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This is not an Important candidate based on the patch alone, but manual review is reasonable because the affected code is in fork plus block-layer state inheritance and the practical triggerability by an unprivileged local process is not fully demonstrated in the provided context.

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

Patch: kernel/fork: clear PF_BLOCK_TS in copy_process()
Commit: ee0801aceabdf583392477baf69a290b09448b8f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ee0801aceabdf583392477baf69a290b09448b8f

Commit description:

PF_BLOCK_TS is only set in blk_time_get_ns() when current->plug is
non-NULL, and blk_finish_plug() clears it via __blk_flush_plug()
before NULLing the plug pointer.  copy_process() breaks the
invariant by inheriting PF_BLOCK_TS from the parent while resetting
the child's plug to NULL.

Clear PF_BLOCK_TS alongside that assignment so callers can rely on
"PF_BLOCK_TS set implies current->plug != NULL" and dereference
current->plug unguarded.

Fixes: 06b23f9 ("block: update cached timestamp post schedule/preemption")
Cc: stable@vger.kernel.org
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Link: https://patch.msgid.link/20260616141604.328820-2-usama.arif@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/fork.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=ee0801aceabdf583392477baf69a290b09448b8f

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

The original Linux kernel CVE announcement for CVE-2026-64253 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64253

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).