[CVE-2026-64328][MODERATE REGULAR] usb: gadget: f_fs: Fix DMA fence leak

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

CVE: CVE-2026-64328
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: usb: gadget: f_fs: Fix DMA fence leak
Commit: b7475b2dce5e121e687280ba5732ccefe77ffd2f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7475b2dce5e121e687280ba5732ccefe77ffd2f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64328
Analysis date: Sat, 25 Jul 2026 06:50:43 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like, not Important
Manual review required: NO

Summary:
A missing dma_fence_put in the USB gadget FunctionFS DMABUF transfer path leaks ffs_dma_fence objects across repeated transfers, allowing a local process with access to the configured gadget interface to cause gradual kernel memory exhaustion and denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64328 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64328

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64328	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'The USB/gadget FunctionFS DMABUF transfer path can leak ffs_dma_fence objects because the initial dma_fence reference is not released after dma_resv_add_fence takes its own reference. Repeated transfers can slowly consume kernel memory and may lead to denial of service through resource exhaustion. For the CVSS the PR:L is used because a local process or gadget service with access to the FunctionFS DMABUF transfer interface is needed to trigger the leak. The issue is not directly network reachable and there is no evidence of a memory corruption primitive beyond resource leakage. Impact is denial of service only in the realistic case.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, not Important (with actual score 3)  SKIP CVE-2026-64328 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB USB SIMPLEFIX LEAK DMAorINTERRUPT IMPROVEONLY LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like, not Important**::

## 2. Signal breakdown

Conservative signals:

* Local trigger: +1. Triggering requires a local process or gadget service with access to the FunctionFS DMABUF transfer path.
* Real lifetime or refcount bug: +1. The initial dma_fence reference is not released after dma_resv_add_fence takes its own reference.
* Availability impact realistic: +1. Repeated transfers can slowly leak kernel memory and may eventually cause resource exhaustion.
* Rare or configuration-dependent subsystem: -1. USB gadget FunctionFS with DMABUF import is not a broad default server or desktop exposure.

Paranoid additional signal:

* Reliable resource-exhaustion DoS under sustained abuse: +1. If an attacker can repeatedly drive the transfer path, the leak can accumulate until memory pressure or failure.

Not applied:

* No remote reachable signal.
* No generic memory corruption signal. This is a reference leak, not UAF, OOB write, double free, or type confusion.
* No privilege escalation signal.
* No confidentiality or integrity impact.
* No Dirty-Pipe-like or page-cache corruption primitive.

## 3. Reachability analysis

The bug is reachable only when USB gadget FunctionFS with the DMABUF import interface is configured and usable. In typical deployments, gadget setup is controlled by root or a privileged service, but the actual transfer path may be reachable by a local gadget process with delegated access to FunctionFS endpoint files. Namespaces do not by themselves make this broadly unprivileged unless the device and FunctionFS access are explicitly delegated. The path is not network reachable and is not enabled on most general-purpose systems.

Call-site confidence: high. The patch and commit message directly describe the reference ownership sequence around dma_fence_init, dma_resv_add_fence, ffs_dmabuf_cleanup, and dma_fence_put.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel resource leak. The realistic impact is local denial of service through gradual kernel memory exhaustion. The paranoid score recognizes that sustained abuse could make availability impact high, but the patch does not support memory corruption, privilege escalation, confidentiality loss, or integrity impact. This should not be treated as an Important-class vulnerability.

## 5. One-sentence report phrase

A missing dma_fence_put in the USB gadget FunctionFS DMABUF transfer path leaks ffs_dma_fence objects across repeated transfers, allowing a local process with access to the configured gadget interface to cause gradual kernel memory exhaustion and denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a clear resource leak with no concrete memory corruption primitive, no network exposure, and no plausible privilege escalation path shown by the patch.

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

Patch: usb: gadget: f_fs: Fix DMA fence leak
Commit: b7475b2dce5e121e687280ba5732ccefe77ffd2f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7475b2dce5e121e687280ba5732ccefe77ffd2f

Commit description:

In ffs_dmabuf_transfer(), a ffs_dma_fence object is kmalloc'd, with the
underlying dma_fence later initialized by dma_fence_init(), which sets
its kref counter to 1. Then, dma_resv_add_fence() gets a second
reference, and a pointer to the ffs_dma_fence is passed as the
usb_request's "context" field.

The dma-resv mechanism will manage the second reference, but the first
reference is never properly released; the ffs_dmabuf_cleanup() function
decreases the reference count, but only to balance with the reference
grab in ffs_dmabuf_signal_done().

The code will then slowly leak memory as more ffs_dma_fence objects are
created without being ever freed.

Address this issue by transferring ownership of the fence to the DMA
reservation object, by calling dma_fence_put() right after
dma_resv_add_fence(). The ffs_dma_fence then gets properly discarded
after being signalled.

Fixes: 7b07a2a ("usb: gadget: functionfs: Add DMABUF import interface")
Cc: stable <stable@kernel.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20260609152905.729328-1-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/gadget/function/f_fs.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=b7475b2dce5e121e687280ba5732ccefe77ffd2f

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

The original Linux kernel CVE announcement for CVE-2026-64328 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64328

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).