[CVE-2026-80854][MODERATE 7.0] usb: gadget: f_tcm: keep port count until LUN teardown completes

[CVE-2026-80854][MODERATE 7.0] usb: gadget: f_tcm: keep port count until LUN teardown completes

[AL-KERNEL]

CVE: CVE-2026-80854
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: usb: gadget: f_tcm: keep port count until LUN teardown completes
Commit: c494c5562ca69b61a82f566e3b87a445d2c28929
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c494c5562ca69b61a82f566e3b87a445d2c28929
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80854
Analysis date: Fri, 04 Sep 2026 16:33:30 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in usb gadget f_tcm can free session command state during last LUN unlink while an in flight usbg_cmd work item is still active, causing a kernel UAF candidate and observed target core BUG/Oops, with typical exploitation requiring local gadget or target configuration privileges.

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

The original Linux kernel CVE announcement for CVE-2026-80854 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80854

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80854	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80854 CHECK WITH IMPACT FROM ORIG NN LOW	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-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A lifetime race in usb gadget f_tcm can occur when the last LUN is unlinked while nexus removal is running at the same time. The port count may reach zero before core_dev_del_lun has drained active LUN references, allowing target_remove_session to free sess_cmd_map while an in flight usbg_cmd and its work item can still be accessed. This is a kernel use after free candidate and has been observed as a DEBUG_OBJECTS free active warning followed by a target core BUG or Oops. For the CVSS the PR:L is used in the paranoid score because reliable triggering may be possible for a local actor with delegated gadget or target configfs control, such as a reduced capability service account or container root, even though typical systems require administrator privileges. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the worst defensible case may allow confidentiality or integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	WARNONLY RACE LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1. The patch describes session state being freed while an in-flight `usbg_cmd` and work item can still be accessed, but no controlled reclaim or overwrite primitive is shown.
* Real lifetime corruption: +1. This is a real lifetime bug involving premature `target_remove_session()` and freeing of `sess_cmd_map`.
* Reliable kernel crash / strong DoS: +1. The commit reports DEBUG_OBJECTS "free active" followed by target-core BUG/Oops.
* Hard or unreliable race / special timing required: -1. Trigger requires overlapping last-LUN unlink with nexus removal.
* Requires admin/root/CAP_* in typical deployments: -2. LUN unlink and nexus management through target/configfs normally require administrative privileges.
* Rare AND difficult-to-reach subsystem/configuration: -1. USB gadget f_tcm / target UASP or BOT gadget use is not a broad default deployment.

Paranoid additional interpretation:

* Local unprivileged or reduced-privilege trigger concern: +1. In some products, gadget or target configfs operations may be delegated to a service account, container root, or reduced-capability management component rather than full host root.
* Weak LPE concern: +1. The bug is a UAF/lifetime violation involving an in-flight command and active work item, which is more concerning than a simple NULL dereference, although no practical reclaim or overwrite primitive is demonstrated.
* Privileged kernel/device-management memory corruption path: +1. The corruption is in USB gadget target-core session and command lifetime management, a privileged kernel subsystem with complex asynchronous state.

Paranoid score is capped at 5 under the race-UAF downgrade rule because there is no demonstrated attacker-controlled reclaim, arbitrary write, type confusion, callback control, or refcount takeover.

## 3. Reachability analysis

The realistic trigger is local control-plane manipulation of the USB gadget target configuration: unlinking the last LUN while nexus removal runs concurrently. Typical systems require root or equivalent privileges to manage target/configfs and USB gadget setup, so the conservative privilege model is admin-only.

The paranoid model uses a lower privilege assumption because embedded or appliance-style deployments may delegate gadget management to a reduced-privilege service, container root, or management agent. A connected USB host may help keep commands in flight, but the issue is not network reachable and is not a pure external USB-host-only trigger based on the supplied patch.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup bug. It is a race-driven kernel lifetime violation with a concrete UAF candidate and observed BUG/Oops. Realistic impact is local DoS under specialized configuration and timing conditions.

Theoretical impact could extend beyond DoS because freed session command mapping state may still be accessed by active command/workqueue paths. However, the patch does not show controlled object replacement, writable stale object access, arbitrary write, or a demonstrated LPE chain, so Important classification is plausible only as a paranoid/manual-review candidate rather than as a confirmed LPE.

## 5. One-sentence report phrase

A race in usb gadget f_tcm can free session command state during last LUN unlink while an in flight usbg_cmd work item is still active, causing a kernel UAF candidate and observed target core BUG/Oops, with typical exploitation requiring local gadget or target configuration privileges.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a race plus real lifetime corruption/UAF candidate in target-core USB gadget code, with observed kernel BUG/Oops and possible reduced-privilege deployment models. It should not be auto-closed even though the common case requires privileged local configuration access.

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

Patch: usb: gadget: f_tcm: keep port count until LUN teardown completes
Commit: c494c5562ca69b61a82f566e3b87a445d2c28929
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c494c5562ca69b61a82f566e3b87a445d2c28929

Commit description:

tcm_usbg_drop_nexus() permits session removal once tpg_port_count
reaches zero. However, usbg_port_unlink() currently decrements that
count from the fabric_pre_unlink() callback, before core_dev_del_lun()
waits for active se_lun references to drain.

If removal of the last LUN races a nexus removal, the latter can observe
a zero port count and call target_remove_session(). This frees
sess_cmd_map while an in-flight struct usbg_cmd, including its work item,
can still be accessed.

Overlapping the last-LUN unlink with nexus removal reproduces this
lifetime violation as a DEBUG_OBJECTS "free active" warning for
usbg_cmd_work, followed by a target-core BUG/Oops.

The generic target-core unlink path has no callback after
core_dev_del_lun() completes. Add an optional fabric_post_unlink()
callback and use it for the f_tcm port count. The count now remains
nonzero until core_dev_del_lun() has finished draining active LUN
references, preventing nexus removal from freeing the session during
command completion.

Fixes: c52661d ("usb-gadget: Initial merge of target module for UASP + BOT")
Cc: stable@vger.kernel.org
Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Link: https://patch.msgid.link/20260807060733.3186624-1-shuangpeng.kernel@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/target/target_core_fabric_configfs.c
  drivers/usb/gadget/function/f_tcm.c
  include/target/target_core_fabric.h

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=c494c5562ca69b61a82f566e3b87a445d2c28929

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

The original Linux kernel CVE announcement for CVE-2026-80854 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80854

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:N/I:N/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: 3
  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).