[CVE-2026-63877][LOW] serial: dz: Convert to use a platform device

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

CVE: CVE-2026-63877
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: serial: dz: Convert to use a platform device
Commit: c9e78361fe92fb64662fc3c8f34e2cdbb8c25bc6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9e78361fe92fb64662fc3c8f34e2cdbb8c25bc6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63877
Analysis date: Sun, 19 Jul 2026 23:12:08 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A NULL pointer dereference in the DECstation DZ serial driver can crash the kernel during early boot because serial ports were registered without a valid parent device, but exploitation requires affected legacy hardware and privileged control of boot or driver initialization.

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

The original Linux kernel CVE announcement for CVE-2026-63877 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63877

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-2026-63877	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-476;CWE-665;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the DECstation DZ serial driver during early port registration because UART ports are created without a valid parent device. The serial core may later access the device fwnode through this missing parent and crash during boot or driver initialization. For the CVSS the PR:L is used in the paranoid score because a reduced capability administrator or service account might be able to trigger boot or driver binding of an affected system in some lab or appliance environments. In typical deployments the required privileges are higher because controlling the kernel image, platform device setup, or legacy serial driver initialization is an administrator level action. The issue is not network reachable and is limited to affected MIPS DECstation DZ configurations. Impact is denial of service through a kernel Oops or failed boot, with no supported evidence of information disclosure or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 0)  SKIP CVE-2026-63877 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB DANGER INIT DMAorINTERRUPT KERNEL_PANIC_PLUS_UAF HARDWARE LINUS  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=0

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Reliable kernel crash / strong DoS: +1
  The commit includes a concrete boot-time Oops caused by dereferencing a pointer derived from a NULL parent device during DZ serial port registration.

* Availability impact realistic: +1
  The failure can prevent successful boot or serial driver initialization on affected systems, so the availability impact is system-visible.

Negative signals:

* Requires admin/root/platform control in typical deployments: -2
  Reliable triggering normally requires booting an affected kernel on the affected platform, enabling the relevant driver, or controlling platform/driver initialization. This is not reachable from an ordinary local user interface.

* Physical-only or rare hardware-only condition: -2
  The affected path is specific to MIPS DECstation DZ serial hardware and legacy platform initialization.

Not awarded:

* Remote reachable: +0
  No network path exists.

* Local unprivileged trigger: +0
  No syscall, device node, netlink, ioctl, or other unprivileged runtime trigger is shown.

* Generic memory corruption: +0
  This is a NULL pointer dereference, not UAF, double free, OOB write, type confusion, refcount takeover, or arbitrary write.

* Privilege escalation plausible: +0
  No attacker-controlled reclaim, overwrite, callback dispatch, object replacement, or sensitive boundary bypass is visible.

Call-site confidence: high. The crash path is explicitly shown through `serial_core_register_port()` to `serial_base_ctrl_add()` and `__dev_fwnode()`.

## 3. Reachability analysis

The bug is triggered during early serial driver initialization on affected DECstation DZ configurations. In practical terms, the actor must control the kernel image, platform configuration, boot environment, or driver binding path. Namespace and container delegation do not materially lower the trigger requirement because this is not a normal runtime interface exposed to containers or unprivileged local users. The path is not network reachable and is limited to rare legacy MIPS hardware.

## 4. Severity interpretation

This behaves like a Low-like or ordinary Moderate availability issue, not an Important-class vulnerability. The realistic impact is a boot-time kernel Oops or failed driver initialization. The patch fixes invalid device-model setup by ensuring UART ports have a valid platform device parent. There is no evidence of a practical LPE path, memory lifetime corruption, arbitrary write, page-cache corruption, information disclosure, or cross-boundary policy bypass.

## 5. One-sentence report phrase

A NULL pointer dereference in the DECstation DZ serial driver can crash the kernel during early boot because serial ports were registered without a valid parent device, but exploitation requires affected legacy hardware and privileged control of boot or driver initialization.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a rare hardware-specific boot-time NULL dereference with DoS-only impact, no network reachability, no unprivileged runtime trigger, and no supported memory-corruption or privilege-escalation primitive.

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

Patch: serial: dz: Convert to use a platform device
Commit: c9e78361fe92fb64662fc3c8f34e2cdbb8c25bc6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9e78361fe92fb64662fc3c8f34e2cdbb8c25bc6

Commit description:

Prevent a crash from happening as the first serial port is initialised:

  Console: switching to colour frame buffer device 160x64
  tgafb: SFB+ detected, rev=0x02
  fb0: Digital ZLX-E1 frame buffer device at 0x1e000000
  DECstation DZ serial driver version 1.04
  CPU 0 Unable to handle kernel paging request at virtual address 000000bc, epc == 8048b3a4, ra == 80470a78
  Oops[#1]:
  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.19.0-dirty #35 NONE
  $ 0   : 00000000 1000ac00 00000004 804707ac
  $ 4   : 00000000 80e20850 80e20858 81000030
  $ 8   : 00000000 8072c81c 00000008 fefefeff
  $12   : 6c616972 00000006 80c5917f 69726420
  $16   : 80e20800 00000000 808f8968 80e20800
  $20   : 00000000 807f5a9 808b0094 808d3bc8
  $24   : 00000018 80479030
  $28   : 80c2e000 80c2fd70 00000069 80470a78
  Hi    : 00000004
  Lo    : 00000000
  epc   : 8048b3a4 __dev_fwnode+0x0/0xc
  ra    : 80470a78 serial_base_ctrl_add+0xa0/0x168
  Status: 1000ac04	IEp
  Cause : 30000008 (ExcCode 02)
  BadVA : 000000bc
  PrId  : 00000220 (R3000)
  Modules linked in:
  Process swapper/0 (pid: 1, threadinfo=(ptrval), task=(ptrval), tls=00000000)
  Stack : 00400044 00400040 8046f4cc 00000000 808a6148 808a0000 808f8968 8086983c
          808e0000 8046fc84 1000ac01 00000028 80e20700 802ba3f8 80e20700 80d34a94
          80c1b900 80e20700 80e20700 80e20700 80e20700 80444650 00000000 00000000
          00000000 807f5a9 808b0094 80447080 00400040 808e0000 80d34a94 808a6148
          80d34a94 00000004 80e20700 00000000 8076974c 80469810 80c2fe3c 1000ac01
          ...
  Call Trace:
  [<8048b3a4>] __dev_fwnode+0x0/0xc
  [<80470a78>] serial_base_ctrl_add+0xa0/0x168
  [<8046fc84>] serial_core_register_port+0x1c8/0x974
  [<808c6af0>] dz_init+0x74/0xc8
  [<800470e0>] do_one_initcall+0x44/0x2d4
  [<808b111c>] kernel_init_freeable+0x258/0x308
  [<8072e434>] kernel_init+0x20/0x114
  [<80049cd0>] ret_from_kernel_thread+0x14/0x1c

  Code: 27bd0018  03e00008  2402ffea <8c8200bc> 03e00008  00000000  27bdffc0  afbe0038  afb30024

  ---[ end trace 0000000000000000 ]---

-- where a pointer is dereferenced that has been derived from a null
pointer to the port's parent device.

Since no device is available with legacy probing and it's not anymore a
preferable way to discover devices anyway, switch the driver to using a
platform device and use it as the port's parent device.  Update resource
handling accordingly and only request the actual span of addresses used
within the slot, which will have had its resource already requested by
generic platform device code.

Use platform_driver_probe() not just because the DZ device is fixed with
solder on board and not straightforward to remove, but foremost because
the associated TTY's major device number is the same as used by the zs
driver and the first driver to claim it will prevent the other one from
using it.  Either one DZ device or some SCC devices will be present in a
given system but never both at a time, and therefore we want the major
device number to be claimed by the first driver to actually successfully
bind to its device and platform_driver_probe() is a way to fulfil that.

An unfortunate consequence of the switch to a platform device is we now
hand the console over from the bootconsole much later in the bootstrap.
The firmware console handler appears good enough though to work so late
and in particular with interrupts enabled.

Conversely only starting the console port so late lets the reset code
fully utilise our delay handlers, so switch from udelay() to fsleep()
for transmitter draining so as to avoid busy-waiting for an excessive
amount of time.

Fixes: 84a9582 ("serial: core: Start managing serial controllers to enable runtime PM")
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Cc: stable@vger.kernel.org # needs to use .remove_new for <= 6.10
Link: https://patch.msgid.link/alpine.DEB.2.21.2605062326540.46195@angie.orcam.me.uk

[Commit description truncated; see the upstream URL below]

Changed files:
  arch/mips/dec/platform.c
  drivers/tty/serial/dz.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=c9e78361fe92fb64662fc3c8f34e2cdbb8c25bc6

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

The original Linux kernel CVE announcement for CVE-2026-63877 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63877

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:H
  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: unknown
  Paranoid ActionableScore: 0

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).