[CVE-2026-64339][MODERATE REGULAR] usb: misc: usbio: bound bulk IN response length to the received transfer

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

CVE: CVE-2026-64339
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: usb: misc: usbio: bound bulk IN response length to the received transfer
Commit: 48394f94211cf8fe0ea8604fc441633abf90fc94
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=48394f94211cf8fe0ea8604fc441633abf90fc94
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64339
Analysis date: Sat, 25 Jul 2026 12:22:05 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A malicious USBIO-compatible USB device can cause `usbio_bulk_msg()` to read past the bulk IN receive buffer and disclose adjacent kernel slab memory through `i2c-dev`, requiring local or physical device-mediated access and providing confidentiality impact without a write primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64339 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64339

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: 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-2026-64339	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:H/I:N/A:N';CWE-126;CWE-125;*CWE-200;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N';BEST CVSS score: '5.5';DESCR 'A kernel out-of-bounds read in usbio_bulk_msg can occur because a USB device controls bpkt_len and the old checks compare the requested input length against txbuf_len rather than the actual bulk IN receive size. A malicious or malfunctioning device can advertise a large bulk OUT endpoint and a small bulk IN endpoint, then return a short transfer with a larger claimed response length. This makes memcpy read past the rxbuf slab object and return adjacent kernel memory through the i2c layer and i2c-dev. For the CVSS the PR:L is used for the paranoid score because a local user or process that can reach i2c-dev can receive the leaked bytes while the device supplies the crafted response. The issue is not network reachable. Impact is primarily confidentiality loss via kernel memory disclosure, with no direct write primitive or UAF shown by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 3)  SKIP CVE-2026-64339 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 121a0f839dbb397af5fabb701cea3e9983223e50 YES  NO NO unknown 	MAYBE	READ KASAN OOB USB HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	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: **Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: +1
  The device controls endpoint sizes and the response length field `bpkt_len`.

* Limited read-only OOB primitive: +1
  The bug is a confirmed slab out-of-bounds read, not an OOB write or UAF.

* Confidentiality impact plausible: +1
  Adjacent slab bytes can be copied into the caller buffer and exposed through the i2c layer and `i2c-dev`.

* Physical-only or rare hardware-only condition: -2
  Realistic exploitation requires a malicious or malfunctioning USB device, or a USB gadget style setup. It is not network reachable.

* Practical exploit chain demonstrated or strongly implied: +1
  The commit describes confirmation with an AddressSanitizer model and a USB raw-gadget plus dummy_hcd reproducer.

Paranoid additional signal:

* Local unprivileged trigger: +1
  If a local user can access the relevant `i2c-dev` node after the malicious device is attached, that user can receive the leaked bytes.

No added signals:

* No generic strong memory corruption: no OOB write, UAF, double-free, type confusion, or arbitrary write is shown.
* No privilege escalation plausible signal: the primitive is read-only disclosure.
* No network trigger: USB device interaction is local or physical.
* No strong DoS: the primary impact is information disclosure, not reliable crash.

## 3. Reachability analysis

The bug is triggered by a malicious or faulty USBIO-compatible device that advertises mismatched bulk OUT and bulk IN endpoint sizes and then returns a crafted bulk IN response. A local process using the i2c path can receive data copied from beyond `usbio->rxbuf`.

Namespaces and containers do not make this broadly reachable by themselves. Practical access depends on device node permissions, USB device attachment policy, and whether the attacker can attach or emulate the USB device. The path is not remotely reachable over the network.

The affected driver is a USB misc driver, not a broad default kernel data path. Exposure is therefore deployment-specific, but the leak primitive is concrete and confirmed.

## 4. Severity interpretation

This behaves like a confidentiality-impacting kernel memory disclosure rather than an Important-class memory corruption bug. The theoretical impact is meaningful because adjacent slab memory can include kernel pointers or sensitive data, but the patch does not show a write primitive, UAF reclaim, callback control, or privilege escalation path.

Realistic exploitation requires physical or device-mediated influence plus access to the resulting userspace-visible path. This is stronger than an ordinary low-risk correctness fix, but it is not a strong Important candidate.

## 5. One-sentence report phrase

A malicious USBIO-compatible USB device can cause `usbio_bulk_msg()` to read past the bulk IN receive buffer and disclose adjacent kernel slab memory through `i2c-dev`, requiring local or physical device-mediated access and providing confidentiality impact without a write primitive.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a confirmed kernel slab out-of-bounds read with user-visible information disclosure. Even though it is physical or device-mediated and not network reachable, kernel memory disclosure should not be auto-closed without manual review.

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

Patch: usb: misc: usbio: bound bulk IN response length to the received transfer
Commit: 48394f94211cf8fe0ea8604fc441633abf90fc94
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=48394f94211cf8fe0ea8604fc441633abf90fc94

Commit description:

usbio_bulk_msg() copies bpkt_len = le16_to_cpu(bpkt->len) bytes out of
the bulk IN buffer (usbio->rxbuf, allocated with size usbio->rxbuf_len)
into the caller's buffer.  bpkt_len is fully controlled by the device
and is only checked against ibuf_len; ibuf_len in turn is checked
against usbio->txbuf_len, not against rxbuf_len:

	if ((obuf_len > (usbio->txbuf_len - sizeof(*bpkt))) ||
	    (ibuf_len > (usbio->txbuf_len - sizeof(*bpkt))))
		return -EMSGSIZE;

txbuf_len and rxbuf_len are taken independently from the bulk OUT and
bulk IN endpoint wMaxPacketSize in usbio_probe().  A malicious or
malfunctioning device that advertises a large bulk OUT endpoint and a
small bulk IN endpoint (e.g. by claiming one of the quirk-free IDs such
as the Lattice NX33U, 0x2ac1:0x20cb) therefore makes ibuf_len, and
hence the device-supplied bpkt_len, exceed rxbuf_len.  memcpy() then
reads up to txbuf_len - rxbuf_len bytes past the end of the rxbuf slab
object.  The over-read bytes are handed back to the i2c layer and on to
user space through i2c-dev, disclosing adjacent slab memory; with KASAN
this is reported as a slab-out-of-bounds read.

The number of bytes actually received is already known: act equals the
URB actual_length and is bounded by rxbuf_len.  Reject any response
that claims more payload than was received, mirroring the existing
"act < sizeof(*bpkt)" check just above.

The control path (usbio_ctrl_msg()) is not affected: it uses a single
buffer (ctrlbuf) for both directions, so its analogous copy can never
leave the allocation.

Found by code review.  The out-of-bounds read was confirmed under
AddressSanitizer with a faithful userspace model of usbio_bulk_msg()'s
receive path (an rxbuf_len-sized buffer, the same act/ibuf_len/bpkt_len
checks and the memcpy).  A USB raw-gadget + dummy_hcd reproducer is
also available.

Fixes: 121a0f8 ("usb: misc: Add Intel USBIO bridge driver")
Cc: stable <stable@kernel.org>
Signed-off-by: HE WEI (ギカク) <skyexpoc@gmail.com>
Link: https://patch.msgid.link/20260624090952.86439-1-skyexpoc@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/misc/usbio.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=48394f94211cf8fe0ea8604fc441633abf90fc94

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

The original Linux kernel CVE announcement for CVE-2026-64339 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64339

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:P/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  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).