* [CVE-2026-98821][MODERATE REGULAR] block: reject polled dio with user integrity metadata [ Upstream
@ 2026-10-19 2:33 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 2:33 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98821
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: block: reject polled dio with user integrity metadata [ Upstream
Commit: c97ef13ad96be0e08af38dfa2ed40cded0b53e1b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c97ef13ad96be0e08af38dfa2ed40cded0b53e1b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98821
Analysis date: Sun, 18 Oct 2026 22:33:27 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A local process with access to an integrity capable block device can combine io_uring IOPOLL with user metadata so that a uio_meta pointer is interpreted as a bio, causing an invalid dereference and kernel denial of service.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98821 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98821
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-2026-98821 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98821 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-843;**CWE-476;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'Polled direct I O with user integrity metadata can cause a kernel pointer type confusion because IOCB_HAS_METADATA leaves iocb private pointing to uio_meta while iocb_bio_iopoll interprets it as a bio. A local process can trigger the invalid dereference by combining io_uring IOPOLL with a PI attribute on an integrity capable block device, which can cause a general protection fault and kernel crash. For the CVSS the PR:L reflects that local code execution and access to a suitable block device are required, even if no administrative capability is needed after the device has been delegated. The issue is not network reachable. The demonstrated impact is denial of service, while the wrong type interpretation creates enough uncertainty around downstream pointer and polling state use to justify limited confidentiality and integrity impact only in the paranoid score. Manual review should determine whether attacker influenced uio_meta fields can reach a stronger memory access primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.6 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE KASAN OOB NULLPTR HARDWARE IO_URING DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
Positive signals:
* +1 Weak or indirect corruption candidate. `iocb->private` contains a `uio_meta`, but `iocb_bio_iopoll()` reinterprets it as a `bio`. This is a concrete wrong-type structured-object access.
* +1 Reliable kernel crash or strong DoS. The supplied reproducer reaches `bio_poll()` and produces a general protection fault with a KASAN invalid dereference.
Negative signals:
* None. Host-admin access, rare-hardware restrictions, and difficult reachability were not sufficiently established to justify penalties.
Important non-awarded signals:
* No UAF, OOB write, repeated release, refcount corruption, or controlled reclaim is established.
* Strong type-confusion leverage was not awarded because the demonstrated path faults while interpreting `uio_meta` as `bio`, without evidence of controlled target selection, callback influence, or a useful read or write primitive.
* No extra availability point was awarded because it would duplicate the same kernel crash.
* Confidentiality and integrity points were not awarded. The supplied evidence does not show disclosure or unauthorized modification.
* B5-R does not apply because there is no retry-accumulated heap OOB write.
* B3-PENDING does not apply because there is no shared-backing write.
* V33 and V34 do not apply because no attacker-influenced write into active networking state exists.
* B4-PENDING does not apply because no authorization decision is bypassed.
3. Reachability analysis
The demonstrated trigger combines io_uring IOPOLL, user integrity metadata, and an integrity-capable block device. A local process with access to such a device can submit the incompatible operation, but the supplied evidence does not establish whether an ordinary user receives that access by default or whether host administration is required to provision it. Therefore `HostAdminRequired` remains `UNKNOWN`.
No namespace-scoped capability chain is shown. A delegated block-device file descriptor could permit a below-host-admin process to reach the path, but delegation is not established by the supplied patch. The reproducer uses `scsi_debug` with DIF and DIX, while applicability to other PI-capable devices remains plausible but unresolved. This is not a race and does not require a narrow timing window. It is not network reachable.
4. Primitive and severity interpretation
The exact primitive is a wrong-type pointer interpretation. `IOCB_HAS_METADATA` leaves `iocb->private` pointing to `uio_meta`, while the polling path expects a published `bio` and passes the pointer to `bio_poll()`.
This is active structured-state consumption rather than a lifetime violation, but the demonstrated operation is an invalid dereference ending in a kernel fault. There is no evidence that the object was freed, that attacker-controlled replacement data is consumed, or that the mismatch yields a directed kernel read or write. It is therefore `CLASS_C`, not `CLASS_A` or `CLASS_B`.
The demonstrated consequence is local kernel DoS. A stronger consequence would require evidence that attacker-influenced `uio_meta` fields can survive the initial accesses and meaningfully control `bio_poll()` queue, cookie, or downstream polling state. That mechanism is not established here.
`ImportantCandidate=NO` because no independently useful corruption or boundary-crossing primitive is demonstrated. `AutoDowngradeAllowed=YES` because the current evidence supports a constrained invalid-state crash rather than unresolved Important-grade corruption.
5. One-sentence report phrase
A local process with access to an integrity capable block device can combine io_uring IOPOLL with user metadata so that a uio_meta pointer is interpreted as a bio, causing an invalid dereference and kernel denial of service.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Review whether supported production PI devices expose this operation below host-admin privilege and whether any architecture or layout lets attacker-influenced `uio_meta` fields reach deeper `bio_poll()` processing before the fault.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: block: reject polled dio with user integrity metadata [ Upstream
Commit: c97ef13ad96be0e08af38dfa2ed40cded0b53e1b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c97ef13ad96be0e08af38dfa2ed40cded0b53e1b
Changed files:
block/fops.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=c97ef13ad96be0e08af38dfa2ed40cded0b53e1b
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98821 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98821
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:L/I:L/A:H
The Best / paranoid CVSS score: 5.8
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: 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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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