[CVE-2026-52969][IMPORTANT] KVM: Reject wrapped offset in kvm_reset_dirty_gfn()

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

CVE: CVE-2026-52969
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: Reject wrapped offset in kvm_reset_dirty_gfn()
Commit: 74f1a22f7a80f03d28ad8551a2d25d563433addf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74f1a22f7a80f03d28ad8551a2d25d563433addf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52969
Analysis date: Thu, 25 Jun 2026 16:59:34 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local process with access to `/dev/kvm` can craft dirty-ring entries that exploit a wrapped offset in `kvm_reset_dirty_gfn()`, potentially causing out-of-bounds KVM rmap access, host kernel crash, and possible privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-52969 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52969

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
======================================================================

CVE-2026-52969	IMPORTANT	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:H/I:H/A:H';CWE-190;CWE-125;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'KVM dirty ring reset trusts slot and offset values stored in a MAP_SHARED dirty ring entry and can accept a wrapped u64 offset during kvm_reset_dirty_gfn. A VMM process with access to /dev/kvm can craft offsets that pass the coalescing and bounds checks, causing the legacy MMU rmap path to index outside slot arch rmap and then operate on a bogus kvm_rmap_head. This is a local KVM memory corruption candidate rather than a simple crash because the described path includes an out of bounds load followed by a conditional write through the loaded pointer. For the CVSS the PR:L because triggering requires local code execution with access to /dev/kvm, which is often available to users in a kvm group but is still a local privilege requirement. The issue is not network reachable. Impact is at least local denial of service and in the paranoid case may include host privilege escalation or broader confidentiality and integrity impact. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-52969  SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	WRITE OOB VIRT HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1
  Triggering requires a local process with access to `/dev/kvm`. This is often available to users in the `kvm` group, so PR:L is more appropriate than PR:H.

* Weak LPE concern: +1
  The path reaches an out-of-bounds `gfn_to_rmap()` lookup and may operate on a bogus `kvm_rmap_head`. This is more than a pure NULL dereference, but a reliable privilege-escalation primitive is not demonstrated.

* Memory corruption, weak or indirect corruption candidate: +1
  The bug is caused by integer wraparound leading to out-of-bounds rmap indexing. The described sink includes a conditional write through the loaded rmap state, but target and payload control appear constrained.

* Reliable kernel crash / strong DoS: +1
  Invalid rmap access in KVM MMU paths can realistically crash the host kernel.

* Confidentiality impact plausible: +1
  OOB access in trusted KVM MMU metadata makes limited confidentiality impact plausible, even if no direct leak is shown.

* Integrity impact plausible: +1
  Conditional clearing of page-table writable state through a bogus rmap-derived pointer creates plausible integrity impact.

* Broad/common subsystem exposure: +1
  KVM is widely deployed, and `/dev/kvm` access is commonly delegated to non-root users.

* Hard or special conditions required: -1
  The most concerning path depends on dirty-ring use plus legacy MMU/rmap conditions such as shadow paging, allocated shadow roots, or write-tracked slots.

Paranoid additions:

* Stronger memory-corruption interpretation: +1 additional
  In the paranoid view, the OOB rmap access plus conditional write through attacker-influenced dirty-ring state is treated closer to a strong KVM MMU corruption primitive.

* Stronger LPE plausibility: +1 additional
  KVM MMU metadata corruption is historically sensitive, and host privilege escalation is plausible enough to require manual analysis even if not proven.

## 3. Reachability analysis

The bug is locally reachable by a process that can create or operate a KVM VM and access the dirty-ring interface through `/dev/kvm`. It is not network reachable by itself. Containers or sandboxes matter because `/dev/kvm` is sometimes passed through to reduced-privilege workloads, CI jobs, nested-virtualization environments, or users in the `kvm` group. This should not be treated as full host-root access.

The affected path is configuration-dependent but realistic for KVM deployments using dirty-ring based dirty logging. The strongest corruption path depends on KVM MMU rmap usage, especially legacy MMU or write-tracked slots. That makes exploitation less universal, but not rare enough to downgrade below actionable handling.

## 4. Severity interpretation

This behaves like an Important candidate rather than an ordinary Moderate. The conservative view is Actionable Moderate because the bug is local, requires KVM access, and has special MMU-path conditions. The paranoid view is Strong Important candidate because the patch context supports an integer-overflow bounds bypass leading to out-of-bounds KVM rmap access and a possible kernel-memory corruption sink.

Theoretical memory-corruption potential is strong enough for manual review. Realistic exploitation as LPE is not proven from the patch alone, but the KVM MMU/rmap context makes auto-closing unsafe.

## 5. One-sentence report phrase

A local process with access to `/dev/kvm` can craft dirty-ring entries that exploit a wrapped offset in `kvm_reset_dirty_gfn()`, potentially causing out-of-bounds KVM rmap access, host kernel crash, and possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a KVM MMU/rmap bounds-bypass issue with plausible memory corruption and possible host privilege-escalation impact, not a simple DoS-only bug.

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

Patch: KVM: Reject wrapped offset in kvm_reset_dirty_gfn()
Commit: 74f1a22f7a80f03d28ad8551a2d25d563433addf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=74f1a22f7a80f03d28ad8551a2d25d563433addf

Commit description:

kvm_reset_dirty_gfn() guards the gfn range with

	if (!memslot || (offset + __fls(mask)) >= memslot->npages)
		return;

but offset is u64 and the addition is unchecked.  The check can be
silently bypassed by a u64 wrap.

The dirty ring backing those entries is MAP_SHARED at
KVM_DIRTY_LOG_PAGE_OFFSET of the vcpu fd, so the VMM can rewrite the
slot and offset fields of any entry between when the kernel pushes
them and when KVM_RESET_DIRTY_RINGS consumes them.  On reset,
kvm_dirty_ring_reset() re-reads the values via READ_ONCE() and feeds
them straight back into this check; only the flags handshake is
treated as the handover, the slot/offset payload is taken on trust.

Crafting two entries

	entry[i].offset   = 0xffffffffffffffc1
	entry[i+1].offset = 0

makes the coalescing loop in kvm_dirty_ring_reset() compute

	delta = (s64)(0 - 0xffffffffffffffc1) = 63

which falls in [0, BITS_PER_LONG), so it folds entry[i+1] into the
existing mask by setting bit 63.  The trailing kvm_reset_dirty_gfn()
call then sees offset = 0xffffffffffffffc1 and __fls(mask) = 63;
the sum is 0 in u64 and the bounds check passes.

That offset propagates into kvm_arch_mmu_enable_log_dirty_pt_masked()
unchanged.  On the legacy MMU path -- kvm_memslots_have_rmaps() ==
true, i.e. shadow paging, any VM that has allocated shadow roots, or
a write-tracked slot -- it reaches gfn_to_rmap(), which indexes
slot->arch.rmap[0][] with a near-U64_MAX gfn.  That is an
out-of-bounds load of a kvm_rmap_head, followed by a conditional
clear of PT_WRITABLE_MASK in whatever the loaded pointer points at.
The path is reachable from any process holding /dev/kvm.

Range-check offset on its own first, so the addition cannot wrap.
memslot->npages is bounded well below U64_MAX, so once offset <
npages holds, offset + __fls(mask) (with __fls(mask) < BITS_PER_LONG)
stays in range.

Fixes: fb04a1e ("KVM: X86: Implement ring-based dirty memory tracking")
Cc: stable@vger.kernel.org
Signed-off-by: Aaron Sacks <contact@xchglabs.com>
Link: https://patch.msgid.link/20260512060742.1628959-1-contact@xchglabs.com/
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  virt/kvm/dirty_ring.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=74f1a22f7a80f03d28ad8551a2d25d563433addf

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

The original Linux kernel CVE announcement for CVE-2026-52969 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52969

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 6
  Paranoid ActionableScore: 8

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).