[CVE-2026-53283][MODERATE 7.0] iommu/amd: Bounds-check devid in __rlookup_amd_iommu()

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

CVE: CVE-2026-53283
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: iommu/amd: Bounds-check devid in __rlookup_amd_iommu()
Commit: f0a0f01787ecece814414b0665df879b69849d09
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0a0f01787ecece814414b0665df879b69849d09
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53283
Analysis date: Fri, 26 Jun 2026 15:59:08 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An unchecked `devid` index in the AMD IOMMU reverse lookup table can read past the allocated IVRS-derived table and interpret adjacent slab data as an IOMMU pointer, causing a boot-time kernel GPF and denial of service on affected firmware or virtualized PCI topologies.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53283 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53283

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-2026-53283	MODERATE	CHECK	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-125;CWE-129;**CWE-476;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An out of bounds read in the AMD IOMMU lookup path can occur when a PCI device has a BDF that is not covered by the IVRS described rlookup table. __rlookup_amd_iommu indexes rlookup_table with devid before the caller can reject an out of range device, so the lookup may read adjacent slab contents and treat the result as an amd_iommu pointer. This can cause an invalid pointer dereference and a boot time general protection fault, resulting in denial of service. For the CVSS the PR:L is used for the paranoid score because some virtualized or delegated device environments may allow a less privileged actor to influence device exposure or hotplug behavior, while the normal physical platform case is closer to PR:H. The issue is not network reachable and does not show a concrete write primitive or user visible information leak. Impact is denial of service via boot failure or kernel crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	OOB DANGER HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	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:

* +1 Firmware-mediated external influence: the triggering condition comes from PCI or IVRS topology supplied by firmware or a hypervisor, not from ordinary packet or syscall input.
* +1 Memory corruption, weak/indirect corruption candidate: `devid` is used as an unchecked index into `rlookup_table`, producing an out-of-bounds read and possible bogus `struct amd_iommu *` interpretation.
* +1 Reliable kernel crash / strong DoS: the commit shows a boot-time GPF in `amd_iommu_probe_device`.
* +1 Availability impact realistic: the failure can prevent the system or VM from booting, which is persistent system-wide availability impact.
* -1 Rare/configuration-dependent condition: requires AMD IOMMU plus IVRS not describing a PCI device that is still enumerated.
* -1 Constrained primitive: there is no attacker-controlled write, no UAF reclaim, no user-visible leak, and the adjacent slab value is not shown to be attacker-controlled.

Paranoid additions/interpretation:

* +1 Privileged kernel/device-management memory corruption path: the bug is in IOMMU device discovery and interprets firmware-fed PCI metadata inside a trusted boot-time kernel path.
* Paranoid scoring does not subtract the rarity penalty because the issue was observed on a real cloud VM configuration and affects boot-critical initialization.

## 3. Reachability analysis

The bug is triggered during AMD IOMMU initialization while the kernel walks PCI devices and probes IOMMU ownership. A normal local unprivileged user cannot directly create the faulty IVRS or PCI topology. In physical deployments, triggering typically requires platform firmware or privileged hardware configuration control. In virtualized environments, the hypervisor or cloud platform controls the exposed PCI and IVRS state, so this is best modeled as firmware-mediated or platform-mediated influence rather than remote network reachability.

Namespaces and containers do not normally lower the privilege requirement because this path runs during host or VM boot and is not reachable through ordinary container interfaces. Realistic exploitation is mainly a denial-of-service or boot-failure scenario for affected AMD IOMMU systems or VMs with incomplete IVRS coverage.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an Important-class vulnerability. The patch shows a concrete out-of-bounds read and invalid pointer dereference in a privileged IOMMU path, so it should not be treated as a trivial compatibility bug. However, the evidence supports boot-time crash and availability impact, not privilege escalation, confidentiality loss, or integrity compromise. The theoretical memory-safety concern is higher than an ordinary NULL dereference, but the realistic exploitation evidence is limited to GPF and boot failure.

## 5. One-sentence report phrase

An unchecked `devid` index in the AMD IOMMU reverse lookup table can read past the allocated IVRS-derived table and interpret adjacent slab data as an IOMMU pointer, causing a boot-time kernel GPF and denial of service on affected firmware or virtualized PCI topologies.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a firmware-mediated IOMMU memory-safety bug with an out-of-bounds read and invalid object interpretation in a boot-critical path. No LPE primitive is evident, but the combination of IOMMU context, real cloud VM trigger, and reliable boot failure warrants manual triage rather than auto-close.

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

Patch: iommu/amd: Bounds-check devid in __rlookup_amd_iommu()
Commit: f0a0f01787ecece814414b0665df879b69849d09
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0a0f01787ecece814414b0665df879b69849d09

Commit description:

iommu_device_register() walks every device on the PCI bus via
bus_for_each_dev() and calls amd_iommu_probe_device() for each. The
inlined check_device() path computes the device's sbdf, calls
rlookup_amd_iommu() to find the owning IOMMU, and only afterwards
verifies devid <= pci_seg->last_bdf. __rlookup_amd_iommu() indexes
rlookup_table[devid] with no bounds check of its own, so for a PCI
device whose BDF is not described by the IVRS, the lookup reads past
the end of the allocation before the caller's bounds check can run.

This was harmless before commit e874c66 ("iommu/amd: Change
rlookup, irq_lookup, and alias to use kvalloc()"): the table was a
zeroed page-order allocation, so the over-read returned NULL and the
caller's NULL check skipped the device. After that commit the table is
a tight kvcalloc() and the over-read returns adjacent slab contents,
which check_device() then dereferences as a struct amd_iommu *,
causing a boot-time GPF.

Seen on Google Compute Engine ct6e VMs, where the virtualized IVRS
describes only the four TPU endpoints 00:04.0-07.0; the gVNIC at
00:08.0 (devid 0x40) indexes 56 bytes past the 456-byte allocation,
into the adjacent kmalloc-512 slab object:

  pci 0000:00:04.0: Adding to iommu group 0
  pci 0000:00:05.0: Adding to iommu group 1
  pci 0000:00:06.0: Adding to iommu group 2
  pci 0000:00:07.0: Adding to iommu group 3
  Oops: general protection fault, probably for non-canonical address 0x3a64695f78746382: 0000 [#1] SMP NOPTI
  CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.18.22 #1
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/06/2025
  RIP: 0010:amd_iommu_probe_device+0x54/0x3a0
  Call Trace:
   __iommu_probe_device+0x107/0x520
   probe_iommu_group+0x29/0x50
   bus_for_each_dev+0x7e/0xe0
   iommu_device_register+0xc9/0x240
   iommu_go_to_state+0x9c0/0x1c60
   amd_iommu_init+0x14/0x40
   pci_iommu_init+0x16/0x60
   do_one_initcall+0x47/0x2f0

Guard the array access in __rlookup_amd_iommu(). With the fix applied
on 6.18.22, the gVNIC at 00:08.0 is skipped cleanly and the VM boots.

Fixes: e874c66 ("iommu/amd: Change rlookup, irq_lookup, and alias to use kvalloc()")
Cc: stable@vger.kernel.org
Reported-by: Ziyuan Chen <zc@anthropic.com>
Tested-by: Ziyuan Chen <zc@anthropic.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Assisted-by: Claude:unspecified
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/iommu/amd/iommu.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=f0a0f01787ecece814414b0665df879b69849d09

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

The original Linux kernel CVE announcement for CVE-2026-53283 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53283

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: 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).