[CVE-2026-74654][LOW] serial: 8250_dma: Clear stale RX state on shutdown

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

CVE: CVE-2026-74654
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: serial: 8250_dma: Clear stale RX state on shutdown
Commit: bf4fb620e02962b2b52500a4b3d8420f351eb46a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf4fb620e02962b2b52500a4b3d8420f351eb46a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74654
Analysis date: Sat, 22 Aug 2026 18:27:15 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74654 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74654

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74654	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-664;CWE-404;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the 8250 DMA serial driver because RX DMA shutdown can leave rx_running set after the RX DMA channel has been released. On the next open, the DesignWare BUSY workaround can call serial8250_rx_dma_flush before a new RX channel is requested, causing dmaengine_pause to receive a NULL rxchan and triggering a kernel Oops. For the CVSS the PR:L is used because triggering may be possible by a local user or service account with access to the affected serial tty device, for example via dialout style permissions. The issue is not network reachable and requires an affected 8250 DMA based serial configuration. Impact is denial of service via kernel crash only, with no supported evidence of information disclosure or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	PRINTF-TTY SIMPLEFIX DMAorINTERRUPT LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown
* Local unprivileged trigger: +1. A local user or service account with access to the affected serial tty device may be able to close and reopen the port while RX DMA state is stale.
* Reliable kernel crash / strong DoS: +1. The commit explicitly describes passing a NULL RX DMA channel to dmaengine_pause(), causing a kernel Oops.
* Hard or configuration-specific condition: -1. Triggering depends on an affected 8250 DMA setup, stale rx_running state after close, and the DesignWare BUSY workaround path during reopen.
* Availability impact realistic: +1. A kernel Oops is a realistic availability impact, but there is no indication of persistence beyond the crash condition.
* No memory corruption bonus. The primitive is a NULL pointer dereference, not UAF, OOB write, double free, stale object reuse, or controlled corruption.
* No confidentiality or integrity impact. The patch does not support an information leak, write primitive, object confusion, or privilege escalation path.
* Call-site confidence: high. The commit explains the direct call path from serial8250_rx_dma_flush() to dmaengine_pause() with rxchan NULL.

Net score: +1 local trigger +1 crash -1 configuration condition +1 availability = 2.

3. Reachability analysis
The bug is local and requires access to the affected serial port. It is not network reachable. In typical systems, serial device access may be restricted to root or users in groups such as dialout, but that is still weaker than full host-root in some deployments. Namespaces do not generally make this broadly reachable unless the serial device is explicitly passed into a container. The affected path also depends on 8250 DMA and the DesignWare BUSY workaround, so exposure is hardware and configuration dependent rather than default across all Linux hosts.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like kernel DoS issue. The realistic impact is a kernel Oops caused by a stale driver state flag and NULL DMA channel use. There is no supported memory corruption primitive, no attacker-controlled reclaim, no arbitrary write, no callback control, and no plausible LPE signal from the patch context.

5. One-sentence report phrase
A stale RX DMA running flag in the 8250 DMA serial driver can cause serial8250_rx_dma_flush() to pass a NULL RX channel to dmaengine_pause() on reopen, leading to a local kernel Oops on affected hardware configurations.

6. Manual review recommendation
NO MANUAL CHECK NEEDED. This can be handled as an auto-closable local DoS in most environments because it is a NULL pointer dereference with hardware-specific reachability and no evidence of memory corruption, privilege escalation, confidentiality impact, or integrity impact.

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

Patch: serial: 8250_dma: Clear stale RX state on shutdown
Commit: bf4fb620e02962b2b52500a4b3d8420f351eb46a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf4fb620e02962b2b52500a4b3d8420f351eb46a

Commit description:

serial8250_release_dma() terminates RX DMA and releases the channel, but
leaves rx_running set.  If the port is closed while an RX transfer is
active, the stale state remains while rxchan is NULL until the channel is
requested again on the next open.

The DesignWare BUSY workaround added by commit a7b9ce3
("serial: 8250_dw: Ensure BUSY is deasserted") calls
serial8250_rx_dma_flush() from the LCR write path during startup.  This
happens before serial8250_request_dma() obtains a new RX channel.  On
reopen, the stale rx_running state therefore makes the flush path pass a
NULL channel to dmaengine_pause(), causing a kernel Oops.

Clear rx_running after terminating RX DMA, matching the TX cleanup.  Also
make the flush helper return if the DMA object or RX channel is not
available so startup and teardown paths cannot pass a NULL channel to the
DMAengine API.

Fixes: 0fcb790 ("tty: serial: 8250_dma: keep own book keeping about RX transfers")
Cc: stable <stable@kernel.org>
Signed-off-by: Cunhao Lu <1579567540@qq.com>
Link: https://patch.msgid.link/tencent_9EE2945F4C933B4D810C73C2D7485E000F06@qq.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/tty/serial/8250/8250_dma.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=bf4fb620e02962b2b52500a4b3d8420f351eb46a

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

The original Linux kernel CVE announcement for CVE-2026-74654 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74654

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:H/PR:L/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: 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).