[CVE-2026-63883][MODERATE 7.0] serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ [ Upstream

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

CVE: CVE-2026-63883
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ [ Upstream
Commit: c91ea13375f70f6271a0183445e34e83b8f4d8f7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c91ea13375f70f6271a0183445e34e83b8f4d8f7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63883
Analysis date: Sun, 19 Jul 2026 20:25:07 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Borderline manual review / Actionable Moderate candidate
Manual review required: NO

Summary:
A race in the qcom_geni serial TX DMA path can corrupt kfifo accounting after a flush before DMA IRQ handling, causing later DMA transmission of stale TX buffer data and possible confidentiality, integrity, or device availability impact.

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

The original Linux kernel CVE announcement for CVE-2026-63883 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63883

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: 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
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CVE-2026-63883	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-665;*CWE-200;CWE-345;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '5.8';DESCR 'A race in the qcom_geni serial TX DMA path can occur when uart_flush_buffer resets the transmit kfifo before the pending DMA completion IRQ is handled. The IRQ handler may then advance the kfifo by tx_remaining even though the fifo is empty, causing the kfifo length accounting to wrap and a later DMA transfer to send stale TX buffer data. For the CVSS the PR:L is used because a local user or process needs access to the affected tty or serial device and must be able to trigger transmit and flush operations. AC:H is used because the issue depends on a narrow timing window between DMA completion, delayed IRQ delivery, and buffer flush. The issue is not network reachable. Impact is at least limited confidentiality and integrity exposure through stale serial data transmission, with possible availability impact for serial communication or attached devices.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review / Actionable Moderate candidate (with actual score 5)	YES	OOB INIT PRINTF-TTY RACE DMAorINTERRUPT HARDWARE LINUS KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Borderline manual review / Actionable Moderate candidate**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process with access to the affected tty or serial device can plausibly transmit data and request a flush.
* Memory corruption, weak/indirect corruption candidate: +1. The bug corrupts kfifo accounting by advancing `out` past `in`, but does not show UAF, OOB write, arbitrary write, reclaim control, or type confusion.
* Privileged kernel/device-management memory corruption path: +1. The corrupted state is in a serial DMA TX path and can cause a later DMA submission based on stale kfifo state.
* Confidentiality impact plausible: +1. The next DMA transfer can send stale TX buffer data to the serial peer.
* Integrity impact plausible: +1. The bug can transmit unintended stale bytes to an attached serial device.
* Hard or unreliable race / special timing required: -1. Triggering requires a timing window between DMA completion, delayed IRQ handling, and `uart_flush_buffer()`.

Conservative total: 4

Paranoid additional signal:

* Availability impact realistic: +1. In device-dependent deployments, spurious stale DMA transfers may disrupt serial communication or attached-device behavior beyond simple data exposure.

Paranoid total: 5

Not counted:

* Remote reachable: +0. No network path is shown.
* Privilege escalation plausible: +0. No reclaim, write primitive, object replacement, callback control, or type confusion is shown.
* Strong generic memory corruption: +0. The primitive is kfifo bookkeeping corruption and stale DMA transmission, not a demonstrated kernel memory overwrite.
* Reliable kernel crash / strong DoS: +0. The patch describes stale data transfer, not a reliable host crash.

## 3. Reachability analysis

The likely trigger is a local process or service that has access to the affected Qualcomm GENI serial tty and can cause TX DMA activity followed by a flush. This is not normally reachable from an arbitrary network attacker. Namespaces or containers do not automatically make the device reachable, but container or service configurations that pass through the serial device may expose the path to reduced-privilege workloads.

The path depends on Qualcomm GENI serial hardware and DMA-enabled serial operation, so exposure is platform-specific. However, on affected embedded or mobile platforms the serial driver path may be ordinary rather than a special debug-only configuration.

Call-site confidence: high. The provided patch includes the affected handler and explains the later `start_tx_dma()` consequence from the corrupted kfifo length.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an Important vulnerability. The bug has a real race and a concrete stale-data transmission effect, which supports confidentiality and integrity concern for serial traffic. However, the realistic evidence does not support privilege escalation or arbitrary kernel memory corruption.

The theoretical risk is limited by timing requirements and by needing access to the affected serial device. The practical impact is mainly stale data disclosure or unintended serial output, with possible device-specific availability impact.

## 5. One-sentence report phrase

A race in the qcom_geni serial TX DMA path can corrupt kfifo accounting after a flush before DMA IRQ handling, causing later DMA transmission of stale TX buffer data and possible confidentiality, integrity, or device availability impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue is not just a crash or warning. It involves a race-induced kfifo state corruption and stale DMA data transmission, but the available evidence does not justify treating it as strong memory corruption or likely privilege escalation.

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

Patch: serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ [ Upstream
Commit: c91ea13375f70f6271a0183445e34e83b8f4d8f7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c91ea13375f70f6271a0183445e34e83b8f4d8f7

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=c91ea13375f70f6271a0183445e34e83b8f4d8f7

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

The original Linux kernel CVE announcement for CVE-2026-63883 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63883

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:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4
  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: 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).