[CVE-2026-31772][IMPORTANT] Bluetooth: hci_sync: fix stack buffer overflow in hci_le_big_create_sync

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

CVE: CVE-2026-31772
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: Bluetooth: hci_sync: fix stack buffer overflow in hci_le_big_create_sync
Commit: f5d446624345d309e7a4a1b27ea9f028d6a8c5d9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5d446624345d309e7a4a1b27ea9f028d6a8c5d9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31772
Analysis date: Thu, 21 May 2026 16:33:39 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local Bluetooth ISO socket can set a valid BIS count larger than the stack allocation in `hci_le_big_create_sync()`, causing a small but real kernel stack out-of-bounds write and potential kernel crash or corruption.

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

The original Linux kernel CVE announcement for CVE-2026-31772 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31772

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
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CVE-2026-31772	IMPORTANT	CHECK	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:H/A:H';CWE-121;*CWE-787;CWE-119;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A stack out-of-bounds write can occur in hci_le_big_create_sync because the stack allocated flexible array has room for only 17 BIS entries while conn num_bis can reach 31. A local user can trigger this through an ISO socket by setting bc_num_bis to ISO_MAX_NUM_BIS and calling listen(), causing the HCI command sync worker to copy past the stack buffer. For the CVSS the PR:L is used because triggering requires local access to Bluetooth ISO socket operations but not full host root in the typical risk model. The issue is not directly network reachable because the attacker must create and configure the local ISO socket. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact because the primitive is a kernel stack overflow.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE WRITE KASAN OOB NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local trigger: +1. The described reproducer uses a local Bluetooth ISO socket with `bc_num_bis = ISO_MAX_NUM_BIS` followed by `listen()`.
* Memory corruption, strong corruption primitive: +2. The commit explicitly describes a KASAN-confirmed stack out-of-bounds write in `hci_le_big_create_sync()`.
* Reliable kernel crash / strong DoS: +1. The overflow is trivially reproducible and reaches a KASAN-detectable kernel stack write.
* Weak LPE concern: +1. This is a real kernel stack overwrite, but the overflow is small and tied to BIS entry copying rather than a demonstrated arbitrary write primitive.
* Availability impact realistic: +1. The bug can crash the kernel from a local trigger path.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Kernel stack corruption can theoretically corrupt control or data state in ways that later expose memory, although no direct leak is shown.
* Integrity impact plausible: +1. Stack OOB write may corrupt adjacent stack variables or control-relevant state.
* No remote/network signal. The patch supports local socket triggering, not over-the-air Bluetooth packet triggering.
* No full admin penalty. Triggering appears to require local Bluetooth ISO socket access and possibly capability-gated Bluetooth/socket privileges, but not necessarily full host root in all deployments.

## 3. Reachability analysis

The realistic trigger is local: a process creates or configures an ISO socket with an excessive but accepted BIS count and calls `listen()`, causing the HCI command sync worker to hit the undersized stack object. This is not directly remote over Bluetooth radio traffic based on the patch. Practical reachability depends on kernel Bluetooth ISO support, Bluetooth being enabled, and local permissions for ISO socket operations. Namespaces do not make this a generic remote issue, but reduced-privilege local services or users with access to Bluetooth socket APIs may be sufficient depending on distribution policy.

## 4. Severity interpretation

This is more than an ordinary Moderate DoS because the primitive is a confirmed kernel stack out-of-bounds write, not a NULL dereference or resource leak. The demonstrated impact is reliable crash or KASAN-detected corruption. Practical privilege escalation is not demonstrated, and the overwrite is limited, so conservative handling is Actionable Moderate. The paranoid score treats the stack memory corruption as a Strong Important candidate because confidentiality and integrity impact remain technically plausible.

## 5. One-sentence report phrase

A local Bluetooth ISO socket can set a valid BIS count larger than the stack allocation in `hci_le_big_create_sync()`, causing a small but real kernel stack out-of-bounds write and potential kernel crash or corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: confirmed kernel stack OOB write with local triggerability and plausible corruption impact beyond DoS, even though no reliable LPE primitive is demonstrated.

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

Patch: Bluetooth: hci_sync: fix stack buffer overflow in hci_le_big_create_sync
Commit: f5d446624345d309e7a4a1b27ea9f028d6a8c5d9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5d446624345d309e7a4a1b27ea9f028d6a8c5d9

Commit description:

hci_le_big_create_sync() uses DEFINE_FLEX to allocate a
struct hci_cp_le_big_create_sync on the stack with room for 0x11 (17)
BIS entries.  However, conn->num_bis can hold up to HCI_MAX_ISO_BIS (31)
entries — validated against ISO_MAX_NUM_BIS (0x1f) in the caller
hci_conn_big_create_sync().  When conn->num_bis is between 18 and 31,
the memcpy that copies conn->bis into cp->bis writes up to 14 bytes
past the stack buffer, corrupting adjacent stack memory.

This is trivially reproducible: binding an ISO socket with
bc_num_bis = ISO_MAX_NUM_BIS (31) and calling listen() will
eventually trigger hci_le_big_create_sync() from the HCI command
sync worker, causing a KASAN-detectable stack-out-of-bounds write:

  BUG: KASAN: stack-out-of-bounds in hci_le_big_create_sync+0x256/0x3b0
  Write of size 31 at addr ffffc90000487b48 by task kworker/u9:0/71

Fix this by changing the DEFINE_FLEX count from the incorrect 0x11 to
HCI_MAX_ISO_BIS, which matches the maximum number of BIS entries that
conn->bis can actually carry.

Fixes: 42ecf19 ("Bluetooth: ISO: Do not emit LE BIG Create Sync if previous is pending")
Cc: stable@vger.kernel.org
Signed-off-by: hkbinbin <hkbinbinbin@gmail.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bluetooth/hci_sync.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=f5d446624345d309e7a4a1b27ea9f028d6a8c5d9

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

The original Linux kernel CVE announcement for CVE-2026-31772 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31772

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 6
  Paranoid ActionableScore: 8

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).