[CVE-2026-98843][MODERATE 7.0] netfs: zero gaps in read-gaps folio to avoid writing back stale data

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

CVE: CVE-2026-98843
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfs: zero gaps in read-gaps folio to avoid writing back stale data
Commit: 512eaf01ffabf4258e945016b50df5aed940b705
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=512eaf01ffabf4258e945016b50df5aed940b705
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98843
Analysis date: Sat, 24 Oct 2026 12:10:03 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
A short netfs gap read can leave stale folio bytes that later leak into and corrupt the affected remote file during writeback.

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

The original Linux kernel CVE announcement for CVE-2026-98843 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98843

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-2026-98843	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98843 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-226;*CWE-200;CWE-665;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 'A short or EOF response while netfs_read_gaps reads around a streaming write can leave the unread part of a folio containing stale data. A later writeback may copy those bytes into the remote file and can cause limited information disclosure and persistent file integrity corruption. For the CVSS the PR:L value reflects that reliable local triggering normally requires a user or process permitted to write the affected netfs file. This is not a kernel memory corruption primitive and there is no supported path to code execution or privilege escalation. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 5 with recommendation Actionable Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5. The independent weaker Aegis shadow evaluated CVSS 5.3 and ActionableScore 3 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	MAYBE	REMOTE DISK SIMPLEFIX LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; stronger corruption or boundary effects are unsupported.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger: the supplied evidence grounds triggering in a user or process permitted to write the affected netfs file; host administration or UID 0 is unnecessary.
* +1 Confidentiality plausible: unread folio contents may be written into the remote file and subsequently disclosed to an authorized reader.
* +1 Integrity plausible: stale bytes can persistently replace the intended contents of the affected remote file.
* +1 Practical consequence chain: short/EOF gap read → unread stale folio region → later writeback into the file.
* +1 Important filesystem/storage path: the flaw affects netfs buffered I/O and writeback data correctness.
* No OOB write, UAF, arbitrary kernel write, COW ownership breach, cross-file overwrite, authorization bypass, LPE, or kernel crash is shown.
* No extra availability point: no host-wide or filesystem-wide failure is established.

3. Reachability analysis

A below-host-admin process with write permission to the affected netfs file can exercise the streaming-write/read-gap path. A short or EOF server response is also required. No special hardware, namespace capability, host setup, or timing race is identified. The evidence does not establish that namespace-scoped privilege is relevant.

4. Primitive and severity reading

The primitive is stale folio data surviving an incomplete gap read and later being consumed by writeback. This is a constrained CLASS_B data-confidentiality and file-integrity defect, not kernel memory corruption. The patch zeros only the unread iterator tail while preserving sink pages for the dirty region.

The shown impact is limited stale-data disclosure and persistent corruption of the file already writable by the triggering actor. Cross-user protected-file corruption, arbitrary payload selection, privilege escalation, and code execution remain unproven. ImportantCandidate is therefore NO. Auto-downgrade is allowed after required human validation because no unresolved Important-grade boundary primitive is shown.

5. One-sentence report phrase

A short netfs gap read can leave stale folio bytes that later leak into and corrupt the affected remote file during writeback.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm filesystem-specific short-read behavior, stale-byte provenance, and whether any deployment permits writes to storage shared with a separately authorized reader.

7. Call-site confidence

Call-site confidence: high

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

Patch: netfs: zero gaps in read-gaps folio to avoid writing back stale data
Commit: 512eaf01ffabf4258e945016b50df5aed940b705
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=512eaf01ffabf4258e945016b50df5aed940b705

Commit description:

netfs_read_gaps() leaves gaps around a streaming write's dirty region
unread on the server side, so a short/EOF response there left stale
folio content that later got written back.

Zero it after the read completes, not before: netfs_wait_for_read()
returns rreq->transferred once ret >= 0, so build an iterator over the
same bvec array the read used, advance past ret, and zero the rest.
The dirty region already maps to sink pages in that array rather than
the real folio, so this can't touch it, and only the actual shortfall
gets zeroed instead of the whole gap upfront.

Fixes: ee4cdf7 ("netfs: Speed up buffered reading")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/netfs/buffered_read.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=512eaf01ffabf4258e945016b50df5aed940b705

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

The original Linux kernel CVE announcement for CVE-2026-98843 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98843

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:L/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.3

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: unknown
  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).