[CVE-2026-80819][MODERATE 7.0] Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept

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

CVE: CVE-2026-80819
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept
Commit: d8d686dd5662a7c4745e4515f1237a9f3b7df181
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8d686dd5662a7c4745e4515f1237a9f3b7df181
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80819
Analysis date: Fri, 04 Sep 2026 14:01:31 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A Bluetooth RFCOMM deferred setup bug allows an adjacent remote BR/EDR peer to send DISC, clear the session pointer, and cause the next recvmsg() path to dereference NULL, resulting in a reliable kernel crash DoS without local privileges.

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

The original Linux kernel CVE announcement for CVE-2026-80819 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80819

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-2026-80819	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80819 CHECK WITH IMPACT FROM ORIG NN MODERATE	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:L/A:H';**CWE-476;CWE-362;CWE-667;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.6';DESCR 'A NULL pointer dereference in the Bluetooth RFCOMM deferred setup path occurs because rfcomm_sock_recvmsg() can call rfcomm_dlc_accept() without holding rfcomm_mutex while rfcomm_dlc_accept() immediately dereferences d session. A remote BR/EDR peer can send DISC on a deferred DLC so that the session is unlinked while RFCOMM_DEFER_SETUP remains set, and the next recvmsg() on the accepted socket dereferences a NULL session and crashes the kernel. For the CVSS the PR:N because the attacker only needs Bluetooth adjacency to the exposed RFCOMM service using BT_DEFER_SETUP and does not need a local account on the victim. The issue is adjacent network reachable rather than Internet reachable, because it requires Bluetooth radio proximity or an equivalent emulated BR/EDR link. Impact is at least denial of service via kernel crash. In the paranoid score limited confidentiality or integrity impact is kept possible because the bug is a locking and lifetime violation around a session pointer, but the patch evidence primarily supports crash impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 7)	YES	REMOTE KASAN NULLPTR NETWORK TEST KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Bluetooth RFCOMM is adjacent-network reachable. The attacker does not need a local account, but does need Bluetooth BR/EDR adjacency or an equivalent emulated link.

* Real lifetime corruption: +1
  The bug is caused by missing rfcomm_mutex protection around d->session lifetime. A remote DISC can cause rfcomm_dlc_unlink() to set d->session = NULL before the deferred accept path uses it.

* Reliable kernel crash / strong DoS: +1
  The commit describes a reproduced NULL pointer dereference in rfcomm_dlc_accept() through recvmsg(), with no timing window required after DISC is processed.

* Availability impact realistic: +1
  The crash path is direct and high-confidence once the affected deferred setup state is reached.

Paranoid additional signal:

* Memory corruption, weak/indirect corruption candidate: +1
  The patch fixes a locking and object-lifetime invariant around a session pointer. The demonstrated primitive is NULL dereference, not controlled UAF reclaim, but the lifetime nature justifies manual-review sensitivity.

Not awarded:

* Strong LPE plausibility: +0
  No attacker-controlled reclaim, overwrite, callback control, type confusion, or refcount takeover is shown.

* Strong generic memory corruption: +0
  The demonstrated failure is NULL pointer dereference, not UAF with reuse, double free, OOB write, or arbitrary write.

* Hard race penalty: 0
  Although locking is involved, the described trigger does not require winning a narrow timing race. After DISC processing, the next recvmsg() dereference is unconditional.

## 3. Reachability analysis

The bug can be triggered by a remote Bluetooth BR/EDR peer interacting with an RFCOMM service that uses BT_DEFER_SETUP. The attacker does not need local code execution on the victim, so PR is effectively none for the remote peer, but the exposure is adjacent-network rather than Internet-routable. The path depends on Bluetooth being enabled and an affected RFCOMM deferred-setup socket being reachable, so it is not a universal default server exposure. Containers and namespaces do not materially reduce the remote Bluetooth adjacency requirement.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is remotely triggerable over Bluetooth adjacency and has a reliable kernel crash path. It is not a strong Important candidate on the current evidence because the demonstrated primitive is NULL pointer dereference only, with no shown attacker-controlled memory reuse, write primitive, or privilege-escalation chain. The paranoid score keeps it above simple DoS triage because the root cause is an object lifetime and locking violation in a network-reachable protocol path.

## 5. One-sentence report phrase

A Bluetooth RFCOMM deferred setup bug allows an adjacent remote BR/EDR peer to send DISC, clear the session pointer, and cause the next recvmsg() path to dereference NULL, resulting in a reliable kernel crash DoS without local privileges.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: adjacent-network reachable kernel crash plus locking/lifetime violation. The proven impact is DoS, but the protocol exposure and lifetime semantics make this unsuitable for auto-closure.

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

Patch: Bluetooth: RFCOMM: take rfcomm_mutex for the deferred setup accept
Commit: d8d686dd5662a7c4745e4515f1237a9f3b7df181
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d8d686dd5662a7c4745e4515f1237a9f3b7df181

Commit description:

rfcomm_sock_recvmsg() completes a deferred setup by calling
rfcomm_dlc_accept() without holding any RFCOMM lock:

	if (test_and_clear_bit(RFCOMM_DEFER_SETUP, &d->flags)) {
		rfcomm_dlc_accept(d);
		return 0;
	}

and rfcomm_dlc_accept() dereferences the session on its first line:

	struct sock *sk = d->session->sock->sk;

Every other path that touches d->session runs under rfcomm_mutex:
rfcomm_dlc_open(), rfcomm_dlc_close(), rfcomm_dlc_exists(),
rfcomm_dlc_send_rpn(), and the RFCOMM thread through
rfcomm_process_sessions(). rfcomm_connect_ind() is even documented as
"called under rfcomm_lock()". This call site is the only one that skips
it.

The RFCOMM_DEFER_SETUP bit looks like it serialises the accept against
teardown, since __rfcomm_dlc_close() returns early when it wins the
test_and_clear. But rfcomm_recv_disc() forces the state first:

	d->state = BT_CLOSED;
	__rfcomm_dlc_close(d, err);

and the early return only covers BT_CONNECT, BT_CONFIG, BT_OPEN and
BT_CONNECT2. With the state already BT_CLOSED that switch does not
match, the bit is never consulted, and __rfcomm_dlc_close() falls
through to rfcomm_dlc_unlink(), which sets d->session = NULL.

So a remote DISC on a deferred dlc clears the session while leaving
RFCOMM_DEFER_SETUP set. The next recvmsg() then passes the
test_and_clear and dereferences a NULL session. No timing window is
needed: once the DISC has been processed, the dereference is
unconditional.

Give rfcomm_dlc_accept() the same shape as rfcomm_dlc_open() and
rfcomm_dlc_close(): an exported wrapper that takes rfcomm_mutex and
re-checks the session, around a __rfcomm_dlc_accept() that the two
in-core callers, which already hold the mutex, keep using.

Reproduced on a KASAN + PROVE_LOCKING kernel with a BR/EDR peer emulated
over /dev/vhci: the peer brings up an ACL link, opens L2CAP on the
RFCOMM PSM, starts a session, opens a dlc on a channel bound with
BT_DEFER_SETUP, and sends DISC after the socket is accepted. recv() on
the accepted socket then hits:

  Oops: general protection fault
  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:rfcomm_dlc_accept+0x54/0x350
  Call Trace:
    rfcomm_sock_recvmsg+0x1cd/0x230
    sock_recvmsg+0x166/0x1c0
    __sys_recvfrom+0x20d/0x300

0x10 is the offset of sock in struct rfcomm_session. With this patch the
same run completes with recv() returning 0 and no report, and lockdep
stays quiet, confirming rfcomm_mutex is still taken before lock_sock on
this path as it is on the thread side.

Fixes: bb23c0a ("Bluetooth: Add support for deferring RFCOMM connection setup")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.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/rfcomm/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=d8d686dd5662a7c4745e4515f1237a9f3b7df181

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

The original Linux kernel CVE announcement for CVE-2026-80819 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80819

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7.6

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: 5
  Paranoid ActionableScore: 6

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).