[CVE-2026-53327][LOW] debugobjects: Do not fill_pool() if pi_blocked_on

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

CVE: CVE-2026-53327
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: debugobjects: Do not fill_pool() if pi_blocked_on
Commit: 3a408cae608d9c075dd3a9e5cfc03b3cb0726863
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a408cae608d9c075dd3a9e5cfc03b3cb0726863
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53327
Analysis date: Wed, 01 Jul 2026 09:56:21 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
`debugobjects on PREEMPT_RT can attempt pool refill while the current task is already PI-blocked, causing an RT lock assertion and possible local denial of service without evidence of memory corruption or privilege escalation.`

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

The original Linux kernel CVE announcement for CVE-2026-53327 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53327

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: LOW
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-2026-53327	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-667;CWE-755;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'debugobjects on PREEMPT_RT can call debug_objects_fill_pool while the current task already has pi_blocked_on set. On RT kernels this can make fill_pool take an rt_mutex through rtlock_lock while the task is already blocked on another PI lock, triggering an assertion or warning and potentially a kernel panic depending on configuration. For the CVSS the PR:L is used for the paranoid score because a local user or local process is needed to drive kernel paths that allocate debug objects, but full administrator privileges are not necessarily required in a fuzzing or delegated workload environment. The issue is not network reachable and requires an RT enabled kernel with debugobjects enabled. Impact is denial of service only via warning, assertion, or possible kernel crash, with no supported evidence of confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	WARNONLY SIMPLEFIX LINUS SYZBOT 	YES	NO	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local task may be able to drive kernel paths that allocate debug objects, especially in fuzzing-like workloads. No full admin capability is clearly required for every possible trigger path.

* Reliable kernel crash / strong DoS: +1 in paranoid scoring
  The commit describes an RT lock assertion when `current->pi_blocked_on` is already set. This may become a warning, assertion failure, or panic depending on kernel configuration.

Subtracted signals:

* Hard or special timing / state required: -1
  The task must already be blocked on an RT mutex PI chain while `debug_objects_fill_pool()` attempts a pool refill.

* Rare / configuration-dependent path: -1
  The issue requires PREEMPT_RT behavior, RT mutexes, and debugobjects pool refill behavior. This is not a broad default production path on all kernels.

Not applied:

* No generic memory corruption bonus.
  The phrase about PI chain corruption refers to an invalid RT locking state, not a concrete UAF, OOB write, double free, type confusion, or arbitrary write primitive.

* No privilege escalation bonus.
  The patch does not show attacker-controlled reclaim, stale object reuse, writable stale object access, callback dispatch, refcount takeover, or another LPE-relevant primitive.

* No confidentiality or integrity impact.
  The demonstrated effect is assertion or crash behavior only.

## 3. Reachability analysis

The bug is locally reachable only if a workload can cause debug object allocation or pool refill while the current task is already PI-blocked on an RT mutex. It is not network reachable. Namespaces or containers do not obviously create a stronger boundary bypass here, although a container workload on an RT/debug kernel might contribute to triggering kernel paths. The affected configuration is specialized because it depends on PREEMPT_RT and debugobjects behavior.

Call-site confidence: high, because the changed function and relevant conditional are present in the patch.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel DoS issue, not an Important-class vulnerability. The realistic impact is a local availability failure through RT lock assertion or panic. Theoretical state corruption is limited to locking invariants and does not provide evidence of memory corruption or privilege escalation.

## 5. One-sentence report phrase

`debugobjects on PREEMPT_RT can attempt pool refill while the current task is already PI-blocked, causing an RT lock assertion and possible local denial of service without evidence of memory corruption or privilege escalation.`

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the issue is configuration-dependent, local-only, and DoS-only, with no concrete memory corruption, security-boundary bypass, information disclosure, or privilege escalation primitive shown by the patch.

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

Patch: debugobjects: Do not fill_pool() if pi_blocked_on
Commit: 3a408cae608d9c075dd3a9e5cfc03b3cb0726863
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a408cae608d9c075dd3a9e5cfc03b3cb0726863

Commit description:

On RT enabled kernels, fill_pool() ends up calling rtlock_lock(), which
asserts if current::pi_blocked_on is set, because a task can obviously only
block on one lock as otherwise the priority inheritenace chain gets
corrupted.

Prevent this by expanding the conditional to take current::pi_blocked_on
into account.

Fixes: 4bedcc2 ("debugobjects: Make them PREEMPT_RT aware")
Reported-by: syzbot+b8ca586b9fc235f0c0df@syzkaller.appspotmail.com
Signed-off-by: Helen Koike <koike@igalia.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260511215359.3351259-1-koike@igalia.com
Closes: https://syzkaller.appspot.com/bug?extid=b8ca586b9fc235f0c0df
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  lib/debugobjects.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=3a408cae608d9c075dd3a9e5cfc03b3cb0726863

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

The original Linux kernel CVE announcement for CVE-2026-53327 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53327

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).