[CVE-2026-63989][LOW] bridge: Fix sleep in atomic context in netlink path [ Upstream

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

CVE: CVE-2026-63989
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bridge: Fix sleep in atomic context in netlink path [ Upstream
Commit: c9c2e609e8397bb57b4d73675f33a99183c4a0bd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9c2e609e8397bb57b4d73675f33a99183c4a0bd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63989
Analysis date: Sun, 19 Jul 2026 12:58:56 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A bridge netlink configuration bug allowed `br_setport()` to call a sleepable promiscuity update while holding the bridge spinlock, causing a sleep-in-atomic warning or possible local DoS for actors able to modify bridge port settings.

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

The original Linux kernel CVE announcement for CVE-2026-63989 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63989

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: LOW
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
======================================================================

CVE-2026-63989	LOW	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-667;CWE-821;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A sleep in atomic context can occur in the bridge netlink configuration path because br_setport was called while the bridge spinlock was held, but some non STP port flag updates may call dev_set_promiscuity which can sleep. A local actor who can change bridge port attributes through rtnetlink can trigger a kernel warning or crash on systems configured to panic on such warnings. For the CVSS the PR:L is used in the paranoid score because delegated CAP_NET_ADMIN, container root, or a network management service account may be sufficient in some deployments, even though normal host configuration usually requires administrator privileges. The issue is not directly network reachable and is triggered by local control plane configuration changes rather than packet traffic. Impact is denial of service only, with no evidence of UAF, OOB access, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-63989 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 78cd408356fe3edbac66598772fd347bf3e32c1f YES  NO NO unknown 	MAYBE	LOCK DANGER HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Bridge port configuration through rtnetlink normally requires CAP_NET_ADMIN.
* Availability impact realistic: +1. The bug can trigger a kernel splat from sleeping in atomic context, but the patch does not show UAF, OOB access, arbitrary write, or guaranteed panic.
* Broad subsystem exposure: +1. Linux bridge is common, but this is a control-plane configuration path, not packet-processing.

Conservative total: 0.

Paranoid signals:

* Requires delegated CAP_NET_ADMIN or reduced admin context: -1. In containerized or delegated network-management setups, the actor may not be full host root.
* Reliable kernel crash / strong DoS: +1. On systems with panic_on_warn or strict debug settings, the reported splat can become a host-level DoS.
* Availability impact realistic: +1. The impact is still limited to availability and depends on configuration.
* Broad subsystem exposure: +1. Bridge configuration is common enough to avoid treating this as rare.

Paranoid total: 2.

No memory corruption, UAF, double-free, OOB write, type confusion, information leak, integrity bypass, or privilege escalation primitive is supported by the patch.

## 3. Reachability analysis

The bug is triggered through local bridge netlink configuration, specifically by changing bridge port attributes that can cause `dev_set_promiscuity()` while `br->lock` is held. In normal deployments this requires CAP_NET_ADMIN, so it is not reachable by an ordinary unprivileged local user. Namespaces or containers can reduce the practical privilege requirement if CAP_NET_ADMIN is delegated, but that still requires a configured network-administration context. The path is not remotely reachable through packet traffic.

Call-site confidence: high. The provided stack trace and diff show the relevant call chain from rtnetlink to `br_setlink()`, `br_setport()`, `br_port_flags_change()`, and `dev_set_promiscuity()`.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like DoS/control-plane locking bug. The realistic impact is a warning or crash under debug or panic-on-warning configurations. Theoretical impact does not extend to memory corruption or privilege escalation based on the provided patch. It should not be treated as an Important-class kernel vulnerability.

## 5. One-sentence report phrase

A bridge netlink configuration bug allowed `br_setport()` to call a sleepable promiscuity update while holding the bridge spinlock, causing a sleep-in-atomic warning or possible local DoS for actors able to modify bridge port settings.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED.

Reason: DoS-only, CAP_NET_ADMIN-gated, no memory corruption primitive, no remote packet trigger, and no evidence of confidentiality, integrity, or privilege-escalation impact.

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

Patch: bridge: Fix sleep in atomic context in netlink path [ Upstream
Commit: c9c2e609e8397bb57b4d73675f33a99183c4a0bd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9c2e609e8397bb57b4d73675f33a99183c4a0bd

Changed files:
  net/core/dev_addr_lists.c
  net/core/rtnetlink.c
  include/linux/spinlock.h
  net/bridge/br_netlink.c
  lib/dump_stack.c
  kernel/sched/core.c
  net/core/dev_api.c
  net/bridge/br_if.c
  net/netlink/af_netlink.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=c9c2e609e8397bb57b4d73675f33a99183c4a0bd

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

The original Linux kernel CVE announcement for CVE-2026-63989 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63989

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:H/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: 0
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).