[CVE-2026-31766][IMPORTANT] drm/amdgpu: validate doorbell_offset in user queue creation

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

CVE: CVE-2026-31766
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: drm/amdgpu: validate doorbell_offset in user queue creation
Commit: 3543005a42d7e8e12b21897ef6798541bf7cbcd3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3543005a42d7e8e12b21897ef6798541bf7cbcd3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31766
Analysis date: Sat, 01 Aug 2026 13:16:37 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Unchecked user supplied doorbell_offset in AMDGPU user queue creation can produce an out of bounds doorbell index outside the allocated doorbell BO, potentially corrupting kernel doorbell space and causing local DoS or device-state integrity impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-31766 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31766

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: IMPORTANT
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-2026-31766	IMPORTANT	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:H/I:H/A:H';*CWE-20;*CWE-787;CWE-190;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An unchecked user supplied doorbell_offset in AMDGPU user queue creation can be used to compute a doorbell index outside the allocated doorbell BO. This may corrupt kernel doorbell space because amdgpu_userq_get_doorbell_index() passes the offset to amdgpu_doorbell_index_on_bar() before validating that the computed location is inside the BO. For the CVSS the PR:L is used because a local process with access to the AMDGPU user queue interface or DRM render device can provide the offset during queue creation. The issue is not network reachable and requires local GPU device access. Impact is at least local denial of service through GPU or kernel instability. In the paranoid interpretation it may allow confidentiality or integrity impact if the out of bounds doorbell index can influence kernel owned doorbell state in a controlled way.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	OOB SIMPLEFIX HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. A local process with access to the AMDGPU user queue or DRM render interface can provide the user controlled doorbell_offset.
* Memory corruption, Weak/indirect corruption candidate: +1. The bug allows an out of bounds doorbell index relative to the allocated doorbell BO, but the patch does not show an arbitrary kernel memory write primitive.
* Privileged kernel/device-management memory corruption path: +1. The affected object is AMDGPU doorbell space, a trusted GPU/kernel device-management resource.
* Weak LPE concern: +1 paranoid only. Corrupting kernel doorbell space could plausibly affect privileged GPU queue state, but no controlled overwrite, reclaim, callback, or arbitrary write primitive is shown.
* Integrity impact plausible: +1. The bug can corrupt kernel-owned doorbell state or device queue signaling state.
* Availability impact realistic: +1. Incorrect doorbell indexing can plausibly cause GPU instability, queue failure, or kernel/device failure.
* Broad/default/common subsystem exposure: +1 paranoid only. AMDGPU render/device interfaces are common on systems with AMD GPUs, though this specific user queue path may depend on hardware and driver feature availability.

No remote reachable signal is applied. No strong generic memory corruption signal is applied because the patch shows an out of bounds device-doorbell index, not a demonstrated arbitrary RAM write or UAF reclaim primitive.

## 3. Reachability analysis

The likely trigger is local access to the AMDGPU user queue creation path through DRM or render device interfaces. In many deployments render nodes are accessible to non-root users in a graphics or render group, so PR:L is more appropriate than PR:H for practical triage. Containers may matter if GPU devices are passed through or exposed to workloads. The issue is not network reachable. Exploitation depends on AMDGPU hardware, driver support for user queues, and access to the relevant device node.

Call-site confidence: medium. The provided patch shows the vulnerable helper and the unchecked value flow into amdgpu_doorbell_index_on_bar(), but not all later users of the computed doorbell index.

## 4. Severity interpretation

This is stronger than an ordinary Moderate validation bug because the rejected value can move a computed doorbell index outside the allocated doorbell BO and into kernel doorbell space. Realistic evidence supports local device-state corruption and DoS. Theoretical escalation is plausible enough for manual review, but the patch does not demonstrate a strong LPE primitive such as arbitrary write, UAF reclaim, function pointer control, or controlled object confusion.

## 5. One-sentence report phrase

Unchecked user supplied doorbell_offset in AMDGPU user queue creation can produce an out of bounds doorbell index outside the allocated doorbell BO, potentially corrupting kernel doorbell space and causing local DoS or device-state integrity impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly mentions possible corruption of kernel doorbell space from user controlled input, and the downstream security impact depends on how the computed doorbell index is later used by AMDGPU queue and doorbell handling.

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

Patch: drm/amdgpu: validate doorbell_offset in user queue creation
Commit: 3543005a42d7e8e12b21897ef6798541bf7cbcd3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3543005a42d7e8e12b21897ef6798541bf7cbcd3

Commit description:

amdgpu_userq_get_doorbell_index() passes the user-provided
doorbell_offset to amdgpu_doorbell_index_on_bar() without bounds
checking. An arbitrarily large doorbell_offset can cause the
calculated doorbell index to fall outside the allocated doorbell BO,
potentially corrupting kernel doorbell space.

Validate that doorbell_offset falls within the doorbell BO before
computing the BAR index, using u64 arithmetic to prevent overflow.

Fixes: f09c1e6 ("drm/amdgpu: generate doorbell index for userqueue")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit de1ef4f )
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/amdgpu/amdgpu_userq.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=3543005a42d7e8e12b21897ef6798541bf7cbcd3

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

The original Linux kernel CVE announcement for CVE-2026-31766 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31766

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).