[CVE-2026-89481][IMPORTANT] nvme-tcp: fix host memory disclosure on R2T for a read command

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

CVE: CVE-2026-89481
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nvme-tcp: fix host memory disclosure on R2T for a read command
Commit: a4c3c7310156493797c920b10d8648c07aa05403
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4c3c7310156493797c920b10d8648c07aa05403
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89481
Analysis date: Sat, 12 Sep 2026 04:06:25 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89481 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89481

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: IMPORTANT
Manual review required: YES

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

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

CVE-2026-89481	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89481 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L';*CWE-200;CWE-201;*CWE-20;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '7.1';DESCR 'A host memory disclosure in the NVMe TCP host path occurs because nvme_tcp_handle_r2t does not verify that an R2T belongs to a WRITE request. A malicious or compromised NVMe TCP controller can send R2T for a READ command and make the host respond with H2CData from the READ destination buffer. That buffer may contain stale kernel data before the READ completes, and reported testing showed disclosure of a full 4096 byte buffer including struct page pointers. For the CVSS the PR:N because the attacker does not need local privileges on the host once they can act as the controller endpoint or tamper with the controller connection. The issue is network adjacent in the storage network model rather than generally Internet exposed. Impact is primarily high confidentiality loss from host kernel memory disclosure. Integrity impact is not assigned because the analyzed primitive is data disclosure, not memory overwrite.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	REMOTE SIMPLEFIX NVME ERRORPATH NETWORK LINUS TEST KPANIC 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2. Substantial kernel disclosure primitive is established. A malicious NVMe/TCP controller can make the host transmit the READ destination buffer back as H2CData.
* +1. Confidentiality impact is demonstrated. The supplied report says a 4096 byte buffered read leaked stale kernel data including struct page pointers.
* +1. Practical exploit chain is demonstrated. Controller sends R2T for READ, host builds H2CData, host sends the request data buffer, command completes normally.
* +1. Network-adjacent controller reachability. The attacker must act as or control the NVMe/TCP controller endpoint or tamper with that storage connection, so this is meaningful remote protocol influence but not generic Internet exposure.

Paranoid delta:

* +1. Treat the connected malicious controller as directly able to trigger the vulnerable host operation over the established NVMe/TCP protocol session. This raises reachability weight, but still does not assume Internet-wide exposure.

Important non-awarded signals:

* 0. RealUAF not awarded. This is stale buffer content disclosure, not object lifetime reuse after free.
* 0. Arbitrary write not demonstrated. The primitive is host-to-controller data disclosure.
* 0. Integrity not awarded. No overwrite, state corruption, or unauthorized modification primitive is established.
* 0. Reliable kernel crash not awarded. The described vulnerable behavior completes normally and silently.
* 0. Extra availability not awarded. The previous CVSS hint includes A:L, but the patch evidence here primarily supports confidentiality loss.
* 0. Controlled reclaim not demonstrated.
* 0. Local unprivileged trigger not awarded. The attacker model is malicious or compromised controller, not local host user.
* 0. Host-admin penalty not applied. Host-admin-equivalent privilege is not required by the attacker once the host is connected to the malicious or compromised controller.

3. Reachability analysis

The evaluated attacker is a malicious or compromised NVMe/TCP controller, or an actor able to tamper with the controller connection. The host must already be using NVMe/TCP or be configured to connect to the attacker-controlled endpoint, so this is storage-network adjacent rather than a generic public Internet attack.

The attacker does not need local code execution or host-admin privileges on the victim host to send the malformed protocol behavior once the NVMe/TCP relationship exists. Namespace-scoped privilege is not relevant. No race is involved. No special physical hardware is required beyond the NVMe/TCP host functionality and an accessible or compromised controller endpoint.

4. Primitive and severity interpretation

The exact primitive is host kernel memory disclosure through protocol confusion. nvme_tcp_handle_r2t() accepted R2T for a non-WRITE request. For a READ request, the request buffer is the destination buffer, so replying with H2CData can expose stale host memory to the controller.

This is not a UAF and not a repeated release/refcount bug. It is also not merely a crash or validation-only issue, because the disclosure path is explicit and tested. The demonstrated consequence is confidentiality loss of kernel memory contents. The speculative consequence is that leaked kernel pointers may assist later exploitation, but no second-stage arbitrary read, arbitrary write, or privilege escalation is established by this patch.

PrimitiveClass is CLASS_A because substantial kernel disclosure is directly established. ImportantCandidate is YES because host kernel memory disclosure to a malicious storage endpoint is security-significant and should not be automatically downgraded. AutoDowngradeAllowed is NO because the issue has a concrete Class A disclosure primitive.

5. One-sentence report phrase

A malicious NVMe TCP controller can send R2T for a READ command and cause the host to transmit stale READ destination buffer contents back as H2CData, resulting in host kernel memory disclosure without requiring attacker privileges on the host.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue has a demonstrated network-adjacent host kernel memory disclosure primitive, including leaked stale kernel data and struct page pointers.

7. Call-site confidence

Call-site confidence: high

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

Patch: nvme-tcp: fix host memory disclosure on R2T for a read command
Commit: a4c3c7310156493797c920b10d8648c07aa05403
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4c3c7310156493797c920b10d8648c07aa05403

Commit description:

nvme_tcp_handle_r2t() does not check the direction of the request the
R2T refers to. A malicious controller can send an R2T for a READ and
the host will answer it: nvme_tcp_setup_h2c_data_pdu() builds the
H2CData header and nvme_tcp_try_send_data() sends the request's data
buffer. That buffer is the READ destination, so its contents go to the
controller.

The command then completes normally and nothing is logged.

Against a test controller that answers every READ with an R2T, a 4096
byte buffered read returned all 4096 bytes, split over two R2Ts. The
pages contained stale kernel data, including an array of struct page
pointers.

Reject an R2T for a request that is not a write.

Fixes: 3f2304f ("nvme-tcp: add NVMe over TCP host driver")
Cc: stable@vger.kernel.org
Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/nvme/host/tcp.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=a4c3c7310156493797c920b10d8648c07aa05403

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

The original Linux kernel CVE announcement for CVE-2026-89481 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89481

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:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L
  The Best / paranoid CVSS score: 7.1

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: 5
  Paranoid ActionableScore: 6

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).