[CVE-2026-74655][MODERATE 7.0] serial: qcom-geni: fix TX DMA buffer flush

[CVE-2026-74655][MODERATE 7.0] serial: qcom-geni: fix TX DMA buffer flush

[AL-KERNEL]

CVE: CVE-2026-74655
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: serial: qcom-geni: fix TX DMA buffer flush
Commit: 313ae287442e5e8d3f7b53b73f37d384bda072d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=313ae287442e5e8d3f7b53b73f37d384bda072d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74655
Analysis date: Sat, 22 Aug 2026 17:18:05 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local user with access to an affected qcom-geni UART can trigger a TX DMA flush race using a large write followed by TCOFLUSH, causing repeated corrupted DMA transmissions and a persistent tcdrain hang, with a paranoid concern for limited stale data exposure over the serial line.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74655 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74655

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-2026-74655	MODERATE	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:L/I:L/A:H';CWE-362;*CWE-672;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.6';DESCR 'qcom-geni UART TX DMA flushing can leave an in flight DMA transfer active while the serial core resets the transmit kfifo. A local user can trigger the failure with a large write followed by TCOFLUSH, after which corrupted TX DMA frames may be repeated indefinitely and a later tcdrain can hang. For the CVSS the PR:L is used because a local process must have access to the affected UART device and issue normal tty operations, but full root privileges are not necessarily required if device permissions or a service account allow this access. The issue is not network reachable and requires local access to a Qualcomm GENI UART using DMA mode. Impact is mainly denial of service through a stuck UART TX path and hung waiters. Worst case may include limited confidentiality or integrity impact if DMA reads stale or unintended TX buffer contents and transmits them over the serial line.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	OOB INIT PRINTF-TTY ERRORPATH DMAorINTERRUPT LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local trigger: +1. The commit states that userspace can trigger it with a large write followed by TCOFLUSH on the affected tty device.
* Real lifetime corruption: +1. The old stop path could unmap the TX DMA buffer while the serial engine could still read it.
* Reliable DoS / strong device hang: +1. The UART can get stuck indefinitely repeating corrupted TX DMA frames and tcdrain can hang forever.
* Privileged kernel/device/DMA-adjacent state corruption path: +1. The affected path is a DMA-backed serial driver state machine with tx_remaining, tx_queued, and DMA mapping lifetime involved.
* Hardware/device-specific exposure: -1. The issue requires Qualcomm GENI UART DMA mode on affected hardware, such as the reproduced QRB2210 setup.

Conservative total: 3.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Because the serial engine may continue reading after unmap or after TX state corruption, stale or unintended buffer contents could plausibly be transmitted over the UART.
* Integrity impact plausible: +1. The bug can produce corrupted repeated TX frames, affecting data integrity on the serial link.

Paranoid total: 5.

Not awarded:

* Remote reachable: +0. No network path is shown.
* Privilege escalation plausible: +0. The patch does not show attacker-controlled reclaim, overwrite, callback control, type confusion, or a practical LPE chain.
* Strong generic memory corruption: +0. This is DMA lifetime and device-state corruption, but not a demonstrated arbitrary write or kernel object replacement primitive.

## 3. Reachability analysis

The bug is locally triggerable by a process that can open and operate the affected qcom-geni UART device, for example `/dev/ttyHS1`. Full root privileges are not inherently required if device permissions, group membership, or a service account allow access to the tty node. Containers or namespaces only matter if the affected device node is passed through or delegated into that environment.

The path is not network reachable. It is also not a broad default Linux path because it depends on Qualcomm GENI UART hardware and DMA-mode operation. However, on affected embedded or mobile platforms where the device node is available, the trigger is simple and reproducible according to the commit.

## 4. Severity interpretation

This behaves more like an actionable Moderate issue than an ordinary low-risk driver bug. The demonstrated impact is primarily local DoS against the UART TX path, including indefinite corrupted transmission and a hanging tcdrain. The more serious paranoid concern is limited stale-data exposure or serial-output integrity corruption due to DMA read-after-unmap behavior.

There is no strong evidence of practical privilege escalation. The theoretical memory/lifetime issue is real, but the patch does not support arbitrary write, controlled reclaim, object replacement, or kernel code execution.

## 5. One-sentence report phrase

A local user with access to an affected qcom-geni UART can trigger a TX DMA flush race using a large write followed by TCOFLUSH, causing repeated corrupted DMA transmissions and a persistent tcdrain hang, with a paranoid concern for limited stale data exposure over the serial line.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: the main impact is DoS, but the patch explicitly fixes a DMA unmap-before-engine-stop condition, which is a real lifetime issue and may have limited confidentiality or integrity consequences beyond a simple hang.

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

Patch: serial: qcom-geni: fix TX DMA buffer flush
Commit: 313ae287442e5e8d3f7b53b73f37d384bda072d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=313ae287442e5e8d3f7b53b73f37d384bda072d0

Commit description:

When transmit flushing a qcom-geni UART during an ongoing TX DMA, the
UART gets stuck infinitely repeating corrupted TX DMA frames.

The DMA-mode uart_ops does not provide a flush_buffer callback, so an
in-flight transfer can complete after serial core has reset the transmit
kfifo, underflowing its length and resubmitting page-sized transfers
indefinitely. Add one that stops the transfer and clears tx_remaining
and tx_queued.

The stop path was also broken: it unmapped the buffer while the serial
engine could still read it, and never reset the TX DMA state machine.
Cancel the main sequencer command first, then reset the state machine
and wait for it before unmapping. Drop the early return so a pending
mapping is also cleaned up when the main command is inactive.

The bug can be triggered from userspace with a large write immediately
followed by TCOFLUSH. A following tcdrain will hang forever. The bug was
reproduced and this fix was validated on Arduino Uno Q (QRB2210)
using /dev/ttyHS1.

Assisted-by: Claude:claude-5-opus Codex:gpt-5
Signed-off-by: Jan Sebastian Götte <linux@jaseg.de>
Fixes: 2aaa43c ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Cc: stable <stable@kernel.org>
Reviewed-by: Praveen Talari <praveen.talari@oss.qualcomm.com>
Link: https://patch.msgid.link/20260729174105.21838-2-git@jaseg.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/tty/serial/qcom_geni_serial.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=313ae287442e5e8d3f7b53b73f37d384bda072d0

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

The original Linux kernel CVE announcement for CVE-2026-74655 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74655

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:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 3
  Paranoid ActionableScore: 5

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 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).