[CVE-2026-89973][MODERATE 7.0] nvme-tcp: check the data direction of a C2HData PDU

[CVE-2026-89973][MODERATE 7.0] nvme-tcp: check the data direction of a C2HData PDU

[AL-KERNEL]

CVE: CVE-2026-89973
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nvme-tcp: check the data direction of a C2HData PDU
Commit: df74950ba6008655d4a977d17df7faf4b6e52b74
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df74950ba6008655d4a977d17df7faf4b6e52b74
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89973
Analysis date: Wed, 16 Sep 2026 12:28:07 -0400
ActionableScore: 4
ActionableScore lower bound: 3
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-2026-89973 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89973

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-2026-89973	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89973 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:N/I:N/A:H';*CWE-20;CWE-754;CWE-755;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'NVMe/TCP host handling accepts a C2HData PDU by command id and payload presence but did not verify that the matching request was a READ. A malicious or compromised NVMe/TCP controller can answer a WRITE request with C2HData and drive the host receive path into _copy_to_iter where a WARN_ON_ONCE is hit and the controller is reset. For the CVSS the PR:N is used because the attacker does not need a local account on the host when they control or can impersonate the controller endpoint. The issue is network reachable in storage fabric terms, but NVMe/TCP is normally deployed on isolated storage or data center networks rather than the public Internet. Impact is at least denial of service through repeated controller resets and may become a host kernel panic when panic_on_warn is enabled. No data copy succeeds and the patch context states that this is not memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	REMOTE DANGER NVME ERRORPATH NETWORK HARDWARE LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Remote or network-adjacent trigger. A malicious or compromised NVMe/TCP controller can send a malformed C2HData PDU for a WRITE request and reach the vulnerable host receive path.
* +1 Availability impact in conservative scoring. The host detects a copy failure, returns -EFAULT, and resets the controller, causing storage-path denial of service.
* +1 Additional paranoid availability impact. On systems booted with panic_on_warn, the reachable WARN_ON_ONCE in _copy_to_iter can become a fatal host kernel panic.

Negative or non-awarded signals:

* RealUAF not awarded. The commit explicitly states that no data is copied and this is not memory corruption.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity not awarded. There is no disclosure, no successful copy, and no unauthorized modification primitive.
* Strong LPE or strong exploitation leverage not awarded. The established behavior is warning, copy failure, and controller reset or panic_on_warn crash.
* Extra availability was not double-counted for the same reset path. The paranoid point is only for the distinct panic_on_warn host-panic deployment.
* No race or timing penalty. The trigger is a protocol-state validation bug, not a timing race.
* No rare-subsystem penalty. NVMe/TCP is specialized storage infrastructure, but the supplied evidence does not justify treating it as rare or unreachable once deployed.

3. Reachability analysis

The evaluated attacker is a malicious, compromised, or impersonated NVMe/TCP controller endpoint. The attacker does not need a local account or host-admin privilege on the victim host if the host connects to the attacker-controlled controller or if an existing controller is compromised.

The issue is network reachable in storage-fabric terms. It is not a generic public Internet remote bug in normal deployments, because NVMe/TCP targets are usually placed on isolated storage or data-center networks. That deployment fact limits practical exposure but does not remove the network-adjacent trigger.

No namespace-scoped privilege is relevant. This is not a local namespace capability issue.

Prior host setup may exist in practice because the host must have an NVMe/TCP connection to a controller. However, the attacker model here is controller-side behavior after the connection exists, not local host reconfiguration. Therefore HostAdminRequired=NO for the victim-side trigger.

No special hardware beyond an NVMe/TCP host deployment is required. No race repeatability issue is present. A malicious controller can repeatedly send the unexpected C2HData response when suitable WRITE requests are present.

4. Primitive and severity interpretation

The exact primitive is missing validation of request data direction in nvme_tcp_handle_c2h_data(). The host accepts a C2HData PDU by command id and payload presence, but does not verify that the matched request is a READ.

This is not active structured UAF and not fixed stale storage. It is a protocol validation and invalid-state bug that reaches a WARN_ON_ONCE in _copy_to_iter.

There is no concrete repeated release or refcount operation. There is no first lifetime operation that may free an object, no surviving dangling reference, and no second lifetime mutation.

PrimitiveClass=CLASS_C because the demonstrated result is warning, failed receive, controller reset, and optionally panic_on_warn host crash. No independently useful corruption primitive is established.

Demonstrated consequence: denial of service through controller reset. Paranoid consequence: host kernel panic when panic_on_warn is enabled.

Speculative consequence: stronger memory corruption or privilege escalation is not supported by the patch or commit context.

ImportantCandidate=NO because the issue lacks memory corruption, authorization bypass, ownership violation, controlled write, UAF, or a demonstrated stronger exploit chain.

AutoDowngradeAllowed=YES because this is a Class C validation bug with DoS-only behavior and no unresolved kernel corruption primitive. Manual review is still recommended because the trigger is controller-side and network-adjacent in storage-fabric deployments.

Likely operational severity: Moderate. It is actionable for exposed or security-sensitive NVMe/TCP environments, especially when panic_on_warn is enabled, but it is not an Important-class kernel corruption candidate based on supplied evidence.

5. One-sentence report phrase

A malicious NVMe/TCP controller can send C2HData for a WRITE request and trigger a host warning followed by controller reset or panic_on_warn kernel panic, causing denial of service without evidence of memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network-adjacent from a malicious NVMe/TCP controller and can become a host panic with panic_on_warn, but the patch explicitly indicates no data copy and no memory corruption, so Important-class treatment is not established.

7. Call-site confidence

Call-site confidence: high

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

Patch: nvme-tcp: check the data direction of a C2HData PDU
Commit: df74950ba6008655d4a977d17df7faf4b6e52b74
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df74950ba6008655d4a977d17df7faf4b6e52b74

Commit description:

nvme_tcp_handle_c2h_data() finds the request by command id and checks
that it has a payload, but it does not check that the command asked for
data to be read.  A controller that answers a write command with C2HData
therefore reaches nvme_tcp_recv_data(), where _copy_to_iter() hits
WARN_ON_ONCE(i->data_source) and returns 0.  The receive path turns that
into -EFAULT and resets the controller.

No data is copied, so this is not memory corruption.  What a controller
gets is a kernel warning it can raise at will, which is fatal on a host
booted with panic_on_warn.

The send path already knows the direction - it consults rq_data_dir()
when it builds a command - and nvme_tcp_handle_r2t() checks the length
and the offset of the request it names.  The C2HData path does not check
the direction at all.

Reject a C2HData PDU whose command is not a read.  Rejecting it fails
the command and resets the controller, as the neighbouring check in this
function does; what goes away is the warning.

  [    6.885580] ------------[ cut here ]------------
  [    6.886457] WARNING: lib/iov_iter.c:193 at _copy_to_iter+0x289/0x1330, CPU#0: kworker/0:1H/71
  [    6.888137] CPU: 0 UID: 0 PID: 71 Comm: kworker/0:1H Not tainted 7.2.0-rc5-NVMETCP-gf5098b6bae76 #1 PREEMPT(lazy)
  [    6.891165] Workqueue: nvme_tcp_wq nvme_tcp_io_work
  [    6.891875] RIP: 0010:_copy_to_iter+0x289/0x1330
  [    6.903739] Call Trace:
  [    6.904085]  <TASK>
  [    6.909254]  __skb_datagram_iter+0x433/0x820
  [    6.911026]  skb_copy_datagram_iter+0x37/0x120
  [    6.911622]  nvme_tcp_recv_skb+0xa07/0x4320
  [    6.913378]  __tcp_read_sock+0x1ab/0x810
  [    6.915788]  nvme_tcp_try_recv+0x152/0x1e0
  [    6.918222]  nvme_tcp_io_work+0x1e4/0x6c0
  [    6.926906]  </TASK>
  [    6.927226] ---[ end trace 0000000000000000 ]---
  [    6.927878] nvme nvme0: queue 1 failed to copy request 0x71 data
  [    6.928709] nvme nvme0: receive failed:  -14

Fixes: 3f2304f ("nvme-tcp: add NVMe over TCP host driver")
Cc: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
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:
  lib/iov_iter.c
  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=df74950ba6008655d4a977d17df7faf4b6e52b74

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

The original Linux kernel CVE announcement for CVE-2026-89973 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89973

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 3
  Paranoid ActionableScore: 4

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).