[CVE-2026-64283][MODERATE 7.0] KVM: guest_memfd: Treat memslot binding offset+size as unsigned values

[CVE-2026-64283][MODERATE 7.0] KVM: guest_memfd: Treat memslot binding offset+size as unsigned values

[AL-KERNEL]

CVE: CVE-2026-64283
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: guest_memfd: Treat memslot binding offset+size as unsigned values
Commit: f3a98d5881b9bd4807f49156143565f6aabcef1e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3a98d5881b9bd4807f49156143565f6aabcef1e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64283
Analysis date: Sat, 25 Jul 2026 13:10:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A signed offset plus size overflow in KVM guest_memfd memslot binding can bypass the file size range check and allow an out-of-range guest memory backing offset, requiring local KVM ioctl access and warranting manual review for downstream VM memory integrity or availability impact.

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

The original Linux kernel CVE announcement for CVE-2026-64283 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64283

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-2026-64283	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-190;CWE-681;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.6';DESCR 'KVM guest_memfd binding used signed loff_t values for a memslot offset and size even though the values are conceptually unsigned. A very large positive offset with a valid memslot size can overflow into a negative signed sum and bypass the range check against the guest_memfd file size, allowing a memslot to be bound at an out of range file offset. For the CVSS the PR:L is used because an attacker needs local access to KVM ioctls such as a VMM process or a user allowed to open /dev/kvm, not full host administrator privileges. The issue is not directly network reachable and requires local KVM control plane access. Impact is at least local denial of service or VM memory backing inconsistency. For the paranoid score, choose the highest still-defensible interpretation supported by the bug class and patch context, with preference for manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64283 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	VIRT HARDWARE  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Triggering requires local access to KVM ioctls, typically via a VMM process or a user allowed to access `/dev/kvm`, but not full host root.
* Weak/indirect corruption candidate: +1. The bug is a signedness and integer overflow validation bypass that can allow an out-of-range `guest_memfd` binding offset. The patch does not show UAF, OOB write, arbitrary write, or controlled object replacement.
* Integrity impact plausible: +1. Incorrectly accepting an absurd memslot binding can corrupt VM memory backing semantics or cause inconsistent KVM guest private memory state.
* Availability impact realistic: +1. A bad binding in a KVM memory-management path can plausibly cause VM failure or host-side KVM fault handling problems, although the commit does not demonstrate a reliable kernel crash.

Paranoid additional signal:

* Weak LPE concern: +1. The out-of-range binding is later consumed by sensitive KVM memory and page-fault machinery, so manual review is warranted for possible downstream corruption effects. This is weak and not a demonstrated privilege-escalation primitive.

Caps applied:

* This is primarily a validation and integer-overflow fix with no explicit downstream UAF, OOB write, arbitrary write, or page-cache corruption primitive. Conservative score is capped at 4 and paranoid score at 5.

## 3. Reachability analysis

The bug is reachable by a local process that can create and configure KVM guest memory, especially through `KVM_CREATE_GUEST_MEMFD` and memslot binding operations. This generally requires access to `/dev/kvm`, which may be granted to non-root users in the `kvm` group or to a VMM service account. It is not directly network reachable.

Namespaces or containers matter if `/dev/kvm` is passed through to a container or if a reduced-privilege virtualization service can issue the relevant ioctls. In that model, this should not be treated as full PR:H host-root only. The affected feature is KVM `guest_memfd`, mostly relevant to newer private or confidential guest memory use cases, so exposure is narrower than generic KVM but still security-sensitive.

Call-site confidence: medium. The patch shows the vulnerable validation point but not all later consumers of the accepted binding.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate. The demonstrated bug is an integer overflow and range-check bypass in a sensitive KVM guest-memory binding path, not proven host memory corruption.

Theoretical impact includes VM memory backing inconsistency and possible downstream KVM memory-management faults. Realistic exploitation evidence for host privilege escalation is not shown by the patch, so it should not be scored as a strong Important solely from the diff. However, because the affected object is KVM private guest memory backing, it should not be auto-closed without manual review.

## 5. One-sentence report phrase

A signed offset plus size overflow in KVM guest_memfd memslot binding can bypass the file size range check and allow an out-of-range guest memory backing offset, requiring local KVM ioctl access and warranting manual review for downstream VM memory integrity or availability impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The issue affects KVM guest memory binding and can bypass a core range check, but the patch does not prove a concrete UAF, OOB write, arbitrary write, or host privilege-escalation primitive. Manual review should focus on how an accepted out-of-range `gmem.pgoff` is later consumed by fault, invalidation, and private-memory mapping paths.

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

Patch: KVM: guest_memfd: Treat memslot binding offset+size as unsigned values
Commit: f3a98d5881b9bd4807f49156143565f6aabcef1e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3a98d5881b9bd4807f49156143565f6aabcef1e

Commit description:

When binding a memslot to a guest_memfd file, treat the offset and size as
unsigned values to fix a bug where the sum of the two can result in a false
negative when checking for overflow against the size of the file.  Passing
unsigned values also avoids relying on somewhat obscure checks in other
flows for safety, and tracks the offset and size as they are intended to be
tracked, as unsigned values.

On 64-bit kernels, the number of pages a memslot contains and thus the size
(and offset) of its guest_memfd binding are unsigned 64-bit values.  Taking
the offset+size as an loff_t instead of a uoff_t inadvertently converts
the unsigned value to a signed value if the offset and/or size is massive.

Locally storing the offset and size as signed values is benign in and of
itself (though even that is *extremely* difficult to discern), but
operating on their sum is not.

For the offset, KVM explicitly checks against a negative value, which might
seem like a bug as KVM could incorrectly reject a legitimate binding, but
that's not actually the case as KVM_CREATE_GUEST_MEMFD takes a signed value
for its size, i.e. a would-be-negative offset is also greater than the
maximum possible size of any guest_memfd file.

Regarding the size, while KVM lacks an explicit check for a negative value,
i.e. seemingly has a flawed overflow check, KVM restricts the number of
pages in a single memslot to the largest positive signed 32-bit value:

        if (id < KVM_USER_MEM_SLOTS &&
            (mem->memory_size >> PAGE_SHIFT) > KVM_MEM_MAX_NR_PAGES)
                return -EINVAL;

and so that maximum "size" will ever be is 0x7fffffff000.

The sum of the two is, however, problematic.  While the size is restricted
by KVM's memslot logic, the offset is not, i.e. the offset is completely
unchecked until the "offset + size > i_size_read(inode)" check.  If the
offset is the (nearly) largest possible _positive_ value, then adding size
to the offset can result in a signed, negative 64-bit value.  When compared
against the size of the file (guaranteed to be positive), the negative sum
is always smaller, and KVM incorrectly allows the absurd offset.

Opportunistically add missing includes in kvm_mm.h (instead of relying on
its parents).

Fixes: a7800aa ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Cc: stable@vger.kernel.org
Cc: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Michael Roth <michael.roth@amd.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://patch.msgid.link/20260602170921.1304394-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  virt/kvm/guest_memfd.c
  virt/kvm/kvm_mm.h

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=f3a98d5881b9bd4807f49156143565f6aabcef1e

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

The original Linux kernel CVE announcement for CVE-2026-64283 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64283

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: 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).