[CVE-2026-64127][MODERATE REGULAR] Bluetooth: L2CAP: ecred_reconfigure: send packed pdu, not stack pointer

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

CVE: CVE-2026-64127
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Bluetooth: L2CAP: ecred_reconfigure: send packed pdu, not stack pointer
Commit: ed5fcd2a26f0c16fc289c8cd6b03328a0582a687
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed5fcd2a26f0c16fc289c8cd6b03328a0582a687
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64127
Analysis date: Sun, 19 Jul 2026 12:57:17 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A Bluetooth L2CAP ECred reconfigure request can leak a kernel stack pointer to a connected peer because the code sends the address and size of the local `pdu` pointer instead of the packed request payload, resulting in KASLR-relevant information disclosure.

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

The original Linux kernel CVE announcement for CVE-2026-64127 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64127

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
======================================================================

CVE-2026-64127	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N';*CWE-200;CWE-201;CWE-704;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.3';DESCR 'Short SHA only was provided. This is an information disclosure issue rather than DoS or memory corruption. Manual review is recommended because kernel pointer leaks can be security relevant even when direct privilege escalation is not present in the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	REMOTE KASAN OOB LEAK NETWORK PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Adjacent network information disclosure: +1
  The leaked bytes are sent over Bluetooth L2CAP to the paired peer. This is not Internet-reachable, but it is externally observable by an adjacent Bluetooth peer.

* Local unprivileged trigger: +1
  The demonstrated trigger is a local `setsockopt(SOL_BLUETOOTH, BT_RCVMTU, ...)` on a connected L2CAP ECred socket. This likely does not require full root in many Bluetooth-enabled configurations, although exact policy may vary.

* Confidentiality impact plausible: +1
  The packet contains bytes copied from the stack slot holding the `pdu` pointer. The examples show a kernel virtual address, so this is a real kernel pointer leak and KASLR weakening issue.

* Broad or common subsystem exposure: +1 paranoid only
  Bluetooth is a common kernel subsystem on laptops, desktops, embedded devices, and phones. However, the affected ECred reconfigure path is narrower than core Bluetooth pairing or classic L2CAP traffic, so this is only counted in the paranoid score.

* No memory corruption: +0
  The patch fixes a wrong pointer and wrong length passed to `l2cap_send_cmd()`, but the observed effect is copying stack bytes into an outbound skb, not writing out of bounds or corrupting kernel memory.

* No direct privilege escalation primitive: +0
  The leak may assist exploitation of another bug by weakening KASLR, but this patch does not show UAF, OOB write, type confusion, arbitrary write, callback control, or refcount abuse.

## 3. Reachability analysis

The most concrete trigger is local: a process creates or uses a connected Bluetooth L2CAP ECred channel and changes receive MTU, causing `l2cap_ecred_reconfigure()` to send the malformed reconfiguration request. The recipient is the paired or connected Bluetooth peer, which receives the leaked kernel pointer bytes.

This is adjacent-network exposure, not normal IP remote exposure. A remote Internet attacker cannot directly reach this path. A nearby Bluetooth attacker would generally need pairing or a suitable L2CAP connection context, and the local host must initiate the affected reconfigure operation or be driven into that state by local software.

Namespaces and containers may matter only if Bluetooth socket access is delegated into the container. In typical desktop or embedded Bluetooth deployments, local Bluetooth access may be available to non-root users depending on distro policy and device permissions.

## 4. Severity interpretation

This behaves like an actionable information disclosure issue, not an Important memory corruption vulnerability. The demonstrated primitive is a kernel address leak to a Bluetooth peer, which is security-relevant because it can weaken KASLR and help chain exploitation with a separate memory corruption bug.

Realistic impact is confidentiality-only. There is no demonstrated kernel crash, write primitive, UAF, double free, type confusion, or direct LPE path. Therefore it should not be auto-closed as a trivial correctness bug, but it also does not reach Strong Important class on its own.

## 5. One-sentence report phrase

A Bluetooth L2CAP ECred reconfigure request can leak a kernel stack pointer to a connected peer because the code sends the address and size of the local `pdu` pointer instead of the packed request payload, resulting in KASLR-relevant information disclosure.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Kernel pointer leaks are often useful as exploit-chain enablers, especially when they disclose randomized kernel addresses to an external peer. Manual review should confirm Bluetooth access policy, whether pairing is required, and whether the leak is reachable by an unprivileged local user or by a malicious connected peer.

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

Patch: Bluetooth: L2CAP: ecred_reconfigure: send packed pdu, not stack pointer
Commit: ed5fcd2a26f0c16fc289c8cd6b03328a0582a687
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed5fcd2a26f0c16fc289c8cd6b03328a0582a687

Commit description:

Commit 1c08108 ("Bluetooth: L2CAP: Avoid -Wflex-array-member-not-at-end
warnings") converted the on-stack request PDU in l2cap_ecred_reconfigure()
from an explicit packed struct to DEFINE_RAW_FLEX(), but did not adjust the
size and source-pointer arguments to l2cap_send_cmd():

  -    struct {
  -            struct l2cap_ecred_reconf_req req;
  -            __le16 scid;
  -    } pdu;
  +    DEFINE_RAW_FLEX(struct l2cap_ecred_reconf_req, pdu, scid, 1);
       ...
       l2cap_send_cmd(conn, chan->ident, L2CAP_ECRED_RECONF_REQ,
                      sizeof(pdu), &pdu);

After the conversion, DEFINE_RAW_FLEX() expands to declare an anonymous
union pdu_u plus a local pointer "pdu" pointing at it. Therefore:

  - sizeof(pdu) is now sizeof(struct l2cap_ecred_reconf_req *) = 8 on
    64-bit (4 on 32-bit), not the 6 bytes of (mtu, mps, scid[1]).
  - &pdu is the address of the local pointer's stack storage, not the
    address of the request payload.

l2cap_send_cmd() forwards (data, count) to l2cap_build_cmd(), which calls
skb_put_data(skb, data, count). The L2CAP_ECRED_RECONFIGURE_REQ packet
body therefore contains 8 bytes copied from the kernel stack starting at
&pdu -- the 8 bytes overlap the pdu pointer's value, leaking a kernel
stack address to the paired Bluetooth peer. The intended (mtu, mps, scid)
fields are not transmitted at all, so the peer rejects the request as
malformed and the L2CAP_ECRED_RECONFIGURE feature itself has been broken
for the local-side initiator since the introducing commit landed.

The sibling site l2cap_ecred_conn_req() in the same commit was converted
correctly (sizeof(*pdu) + len, pdu); only this site was missed.

Restore the original semantics: pass the full flex-struct size via
struct_size(pdu, scid, 1) and the pdu pointer (the struct address) as
the source.

Validated on a stock 7.0-based host kernel via the real call path:
setsockopt(SOL_BLUETOOTH, BT_RCVMTU, ...) on a BT_CONNECTED
L2CAP_MODE_EXT_FLOWCTL socket emits an L2CAP_ECRED_RECONFIGURE_REQ
whose body is 8 bytes (the on-stack pdu local's value) rather than
the expected 6. Three captures from fresh socket / fresh hciemu peer
on the same host -- low bytes vary per call, high 0xffff confirms a
kernel virtual address (KASLR-randomised stack slot, not a fixed
string):

  RECONF_REQ body (ident=0x02 len=8): 42 fb 54 af 0e ca ff ff
  RECONF_REQ body (ident=0x02 len=8): 52 3d 2e af 0e ca ff ff
  RECONF_REQ body (ident=0x02 len=8): b2 fc 5b af 0e ca ff ff

After this patch the body is 6 bytes carrying the expected
little-endian (mtu, mps, scid).

Cc: stable@vger.kernel.org
Fixes: 1c08108 ("Bluetooth: L2CAP: Avoid -Wflex-array-member-not-at-end warnings")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
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/l2cap_core.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=ed5fcd2a26f0c16fc289c8cd6b03328a0582a687

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

The original Linux kernel CVE announcement for CVE-2026-64127 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64127

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:H/PR:N/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS score: 4.3

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: 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).