[CVE-2026-89718][MODERATE REGULAR] zram: fix out-of-bounds access in writeback_store()

[CVE-2026-89718][MODERATE REGULAR] zram: fix out-of-bounds access in writeback_store()

[AL-KERNEL]

CVE: CVE-2026-89718
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: zram: fix out-of-bounds access in writeback_store()
Commit: 07a82a66f8c1894d92e51d97542c32ebf2c51b09
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07a82a66f8c1894d92e51d97542c32ebf2c51b09
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89718
Analysis date: Sat, 12 Sep 2026 01:51:51 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89718 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89718

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: YES

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

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

CVE-2026-89718	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89718 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-119;*CWE-787;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in zram writeback_store can use stale scan bounds after a reset and reconfiguration to a smaller device size. The function calculates the number of pages before taking dev_lock, so the zram table can be replaced while the caller waits for the lock and the later scan can operate on the new smaller table with the old larger upper bound. This can cause out-of-bounds slot access in kernel memory and can lead to at least a local denial of service via crash. For the CVSS the PR:L value is used for the paranoid score because some deployments may delegate zram control to a container root, storage service, or local management process rather than a full host administrator. The issue is not network reachable and requires local access to zram control interfaces. Impact should be manually reviewed because the bug class is a race with out-of-bounds kernel table access and may plausibly affect confidentiality or integrity beyond a simple crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 2)	YES	WRITE OOB DANGER HARDWARE MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=1
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied. The difference is from the conservative standard deployment view where zram sysfs reset, disksize, and writeback control are host-admin-controlled, versus a paranoid product-triage view where zram control might be delegated to a storage service or constrained local administrator.

2. Signal breakdown

Conservative signals:

* +1 Weak / indirect kernel corruption candidate. The stale upper bound can cause out-of-bounds accesses to zram table slots after reset and reinitialization with a smaller device size.
* +1 Reliable local DoS concern. Out-of-bounds zram table access can plausibly crash the kernel or trigger fatal memory-safety diagnostics.
* +1 Important storage / block memory-management path. zram writeback operates in a kernel block-memory subsystem where table-slot integrity matters.
* -2 Host-admin-equivalent control is normally required. The complete sequence requires zram sysfs control such as writeback, reset, and disksize reconfiguration on the host.

Paranoid signals:

* +1 Weak / indirect kernel corruption candidate. Same OOB slot-access primitive.
* +1 Reliable local DoS concern. Same crash/DoS consequence.
* +1 Important storage / block memory-management path.
* 0 privilege penalty in the paranoid score because product deployments may delegate zram management to a less-than-full-host-admin service or constrained local management role, but ordinary unprivileged reachability is not established.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Concrete attacker-controlled payload or target selection not demonstrated.
* Real UAF not established.
* Concrete repeated release/refcount mutation not present.
* Strong LPE leverage not awarded because the patch supports OOB table access, not controlled object replacement, callback control, credential corruption, or directed kernel write.
* Confidentiality and integrity bonuses are not awarded because no disclosure path or unauthorized modification of protected data is established.
* Extra Availability is not awarded because the same crash/DoS is already counted once.
* Remote reachability is not established.
* Namespace-scoped privilege is not established.

Repeated release/refcount checklist:

* First release can end lifetime: not applicable.
* Exact same reference survives: not applicable.
* Second operation is a real lifetime/refcount mutation: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The vulnerable path is `writeback_store()` in zram. Triggering the issue requires one operation to enter `writeback_store()` after calculating scan bounds from the old `zram->disksize`, while another operation resets and reconfigures the same zram device with a smaller size before `writeback_store()` obtains `dev_lock`.

In normal Linux deployments, zram reset, disksize configuration, backing-device setup, and writeback sysfs operations are host-controlled administrative operations. This supports `HostAdminRequired=YES` for the standard conservative path.

Ordinary local user reachability is not established. Namespace-scoped reachability is not established because zram devices and their sysfs control plane are not shown to be fully creatable and controllable inside an attacker-created namespace. A product-specific delegation model could reduce the practical privilege requirement, for example if a storage-management service exposes zram controls to a less privileged actor, but that is not proven by the patch.

No direct network trigger exists. No special rare hardware is required beyond zram being configured. The race is real, but the same privileged actor can likely attempt the reset/reconfiguration and writeback sequence repeatedly, so no separate hard-race penalty is applied.

4. Primitive and severity interpretation

The exact primitive is stale scan-bound use in `writeback_store()`. The function computed `nr_pages` and `hi` before acquiring `dev_lock`. If the zram device was reset and reinitialized with a smaller `disksize` while the caller waited for the lock, the scan could operate on the new smaller table using the old larger upper bound.

This is a kernel out-of-bounds table-slot access. It is not a demonstrated UAF. It is not a concrete double release or refcount mutation. It is not a demonstrated arbitrary read/write primitive. The affected storage is an active structured kernel table, but the patch does not establish controlled target selection, controlled replacement contents, callback influence, or credential/security-object corruption.

PrimitiveClass is CLASS_B because a real kernel memory-safety primitive is established, but exploitation leverage beyond crash is unresolved and constrained. The demonstrated practical consequence is local DoS via kernel crash or memory-safety fault. The speculative consequence is broader kernel memory corruption, but the available evidence does not justify Strong Important classification.

ImportantCandidate is NO because the complete trigger is normally host-admin-controlled and the OOB primitive is not shown to be controlled enough for Important-grade exploitation. ManualReview is RECOMMENDED because OOB kernel table access in a storage/memory subsystem should not be treated as a harmless warning. AutoDowngradeAllowed is YES because no strong unresolved boundary bypass, controlled corruption, UAF, repeated lifetime mutation, or low-privilege exploitation path is established.

5. One-sentence report phrase

A race in zram writeback_store can leave stale scan bounds after reset and smaller reconfiguration, causing out-of-bounds zram table slot access and a likely local kernel crash when zram control is available.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the patch establishes an out-of-bounds kernel table access, but the available evidence points mainly to local administrative or delegated-control reachability and does not demonstrate controlled corruption, UAF, arbitrary write, disclosure, or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: zram: fix out-of-bounds access in writeback_store()
Commit: 07a82a66f8c1894d92e51d97542c32ebf2c51b09
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07a82a66f8c1894d92e51d97542c32ebf2c51b09

Commit description:

Patch series "zram: fix stale scan bounds after reinitialization".

Both writeback_store() and read_block_state() derive their table scan
bounds from zram->disksize before acquiring dev_lock.  If the device is
reset and reinitialized with a smaller disksize between that read and lock
acquisition, the bound can describe the old table while the scan operates
on the new one.  This can lead to out-of-bounds slot accesses.

Move both bound calculations under dev_lock so each bound remains
consistent with the table throughout its scan.  Keep the fixes separate
because the affected interfaces originate from different commits and can
be backported independently.

This patch (of 2):

writeback_store() calculates the table scan bounds before taking dev_lock.
A reset followed by reconfiguration with a smaller disksize can therefore
replace zram->table while writeback_store() is waiting for the lock.  Once
it acquires the lock, it sees an initialized device but scans the new
table using the old upper bound, resulting in an out-of-bounds access.

Calculate the number of pages while holding dev_lock so the scan bound
matches the table protected by the lock.

Link: https://lore.kernel.org/20260804065919.3970386-1-xialonglong2025@163.com
Link: https://lore.kernel.org/20260804065919.3970386-2-xialonglong2025@163.com
Fixes: a939888 ("zram: support idle/huge page writeback")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Longlong Xia <xialonglong@kylinos.cn>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/block/zram/zram_drv.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=07a82a66f8c1894d92e51d97542c32ebf2c51b09

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

The original Linux kernel CVE announcement for CVE-2026-89718 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89718

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:H/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: 1
  Paranoid ActionableScore: 3

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).