[CVE-2026-45851][LOW] efi: Fix reservation of unaccepted memory table [ Upstream

[CVE-2026-45851][LOW] efi: Fix reservation of unaccepted memory table [ Upstream

[AL-KERNEL]

CVE: CVE-2026-45851
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: efi: Fix reservation of unaccepted memory table [ Upstream
Commit: b7bc182ec1846be437351e44164089d988f9d0dd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7bc182ec1846be437351e44164089d988f9d0dd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45851
Analysis date: Thu, 28 May 2026 00:54:26 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
An EFI unaccepted memory table reservation size miscalculation can leave part of an unaligned table unreserved on affected TDX guests, causing early boot memory handling to lose or overwrite the table and panic in `accept_memory()`.

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

The original Linux kernel CVE announcement for CVE-2026-45851 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45851

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.

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

CVE-2026-45851	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-682;CWE-131;CWE-665;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'EFI unaccepted memory table reservation used an aligned table size but did not account for an unaligned physical start address. On affected TDX guests this can leave the tail of the table outside the reserved memblock range, so early boot memory use can overwrite or lose access to the table and accept_memory can panic. For the CVSS the PR:L is used for the paranoid score only in environments where a tenant or reduced privilege operator can choose TDX VM memory size and repeatedly boot the guest. The base case is closer to administrator or firmware controlled configuration. The issue is not network reachable and the supported impact is availability only.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) 	MAYBE	OOB DANGER SIMPLEFIX  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The failure can cause a panic in `accept_memory()` during early boot.
* Requires admin/root or VM configuration control in typical deployments: -2. Triggering generally requires control over TDX VM launch parameters, firmware-provided layout, or memory sizing.
* Rare / configuration-dependent environment: -1. The issue is tied to EFI unaccepted memory handling in Intel TDX guests with specific memory layouts.

Paranoid additional signals:

* Availability impact realistic: +1. Affected guests may fail to boot.
* Firmware or platform-mediated condition: +1. The problematic physical address alignment and unaccepted memory table layout come from early firmware/platform-provided boot state.

Not counted:

* No remote/network trigger.
* No local unprivileged runtime trigger inside an already booted system.
* No UAF, double free, OOB write, arbitrary write, info leak, or privilege escalation primitive.
* No meaningful confidentiality or integrity impact is supported.

## 3. Reachability analysis

The issue is reachable during early boot of affected TDX guests when the EFI unaccepted memory table starts at an unaligned physical address and the guest memory layout causes the table tail to fall outside the reserved memblock range. A normal local user inside the guest cannot trigger this after boot.

In typical deployments, the relevant control belongs to the hypervisor, cloud platform, firmware environment, or an operator selecting VM memory size. A tenant may be able to reproduce the failure by choosing certain TDX VM sizes, but the impact is primarily denial of service of that guest instance.

This path is not default-exposed to network attackers and does not affect ordinary non-TDX boot paths in the same way.

## 4. Severity interpretation

This behaves like an ordinary Moderate / Low-like availability bug. It is important for affected TDX deployments because it can prevent guest boot, but it is not an Important-class kernel security issue.

Theoretical table overwrite exists only as an early boot reservation bug and does not provide an attacker-controlled memory corruption primitive. Realistic evidence supports guest boot DoS only.

## 5. One-sentence report phrase

An EFI unaccepted memory table reservation size miscalculation can leave part of an unaligned table unreserved on affected TDX guests, causing early boot memory handling to lose or overwrite the table and panic in `accept_memory()`.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be handled as availability-only TDX boot failure unless the deployment specifically exposes guest memory sizing as a security boundary. No memory corruption primitive, information disclosure, privilege escalation, or network reachability is supported.

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

Patch: efi: Fix reservation of unaccepted memory table [ Upstream
Commit: b7bc182ec1846be437351e44164089d988f9d0dd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b7bc182ec1846be437351e44164089d988f9d0dd

Changed files:
  drivers/firmware/efi/efi.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=b7bc182ec1846be437351e44164089d988f9d0dd

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

The original Linux kernel CVE announcement for CVE-2026-45851 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45851

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  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).