[CVE-2026-64457][LOW] virtio_pci: fix vq info pointer lookup via wrong index

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

CVE: CVE-2026-64457
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: virtio_pci: fix vq info pointer lookup via wrong index
Commit: 41e6dc1a10036c9f47057033f19af7e52ec464b6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=41e6dc1a10036c9f47057033f19af7e52ec464b6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64457
Analysis date: Sat, 25 Jul 2026 08:01:49 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A wrong virtqueue info index in virtio_pci can cause a NULL pointer dereference during virtio balloon device unbind, allowing a local privileged or delegated device-management user to crash the guest kernel.

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

The original Linux kernel CVE announcement for CVE-2026-64457 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64457

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.

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

CVE-2026-64457	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-476;CWE-682;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in virtio_pci can occur during virtio device unbind because vq info pointers are stored with the sparse caller index while later cleanup looks them up with the dense virtqueue index. This mismatch is reachable when a virtio device such as virtio_balloon has optional queues disabled by feature bits, leaving holes in the caller array. For the CVSS the PR:L is used in the paranoid score because some deployments may delegate device management to a reduced capability root, container root, or service account, although the typical base case requires administrator access to write the sysfs unbind file. The issue is not network reachable and is triggered through local device management or removal paths. Impact is denial of service via guest kernel crash only, with no concrete UAF, OOB write, information leak, or privilege escalation primitive indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-64457 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 89a1c435aec269d109ed46ca7bbb10fa8edf7ace YES  NO NO unknown 	MAYBE	DANGER NULLPTR VIRT HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit shows a reproducible NULL pointer dereference during virtio balloon unbind in `vp_del_vqs()`.
* Requires admin/root/CAP in typical deployments: -2. Writing to `/sys/bus/virtio/drivers/virtio_balloon/unbind` normally requires root or equivalent device-management privileges.
* Weak corruption / bookkeeping mismatch: +0. This is a wrong-index lookup causing NULL dereference, not UAF, OOB write, double free, refcount abuse, or type confusion.

Paranoid additional signals:

* Broad/default/common subsystem: +1. `virtio_pci` and `virtio_balloon` are common in virtualized Linux deployments.
* Privilege penalty refinement: 0 instead of -2. In some environments, device unbind may be delegated to reduced-capability root, container root, or service automation, so the paranoid interpretation should not treat it as full host-root only.

Not applied:

* No remote/network trigger.
* No firmware-mediated external influence counted. Device feature bits are a precondition, but the trigger is local device unbind.
* No privilege escalation plausible.
* No generic memory corruption.
* No confidentiality or integrity impact.

Call-site confidence: high. The commit message includes the failing call trace and the patch directly aligns the store index with the later cleanup lookup index.

## 3. Reachability analysis

The realistic trigger is local device-management access to unbind a virtio balloon device. In normal deployments this requires root or equivalent permissions in the affected guest. Namespace or container delegation could lower the practical privilege level if a reduced-privilege service or container root is allowed to manage the device, but this is not a default unprivileged local-user path. The issue is not network reachable and does not appear triggerable by ordinary packet or storage traffic.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel DoS, not an Important-class issue. The crash is real and reproducible, but the primitive is a NULL pointer dereference from an index mismatch in virtqueue bookkeeping. There is no evidence of UAF, attacker-controlled reclaim, stale callback, OOB write, object replacement, information disclosure, or privilege escalation.

## 5. One-sentence report phrase

A wrong virtqueue info index in virtio_pci can cause a NULL pointer dereference during virtio balloon device unbind, allowing a local privileged or delegated device-management user to crash the guest kernel.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a local privileged or delegated-admin NULL dereference DoS with no demonstrated memory corruption or privilege escalation primitive, and the practical triage score remains below the actionable threshold.

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

Patch: virtio_pci: fix vq info pointer lookup via wrong index
Commit: 41e6dc1a10036c9f47057033f19af7e52ec464b6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=41e6dc1a10036c9f47057033f19af7e52ec464b6

Commit description:

Unbinding a virtio balloon device:

    echo virtio0 > /sys/bus/virtio/drivers/virtio_balloon/unbind

triggers a NULL pointer dereference. The dmesg says:

    BUG: kernel NULL pointer dereference, address: 0000000000000008
    [...]
    RIP: 0010:__list_del_entry_valid_or_report+0x5/0xf0
    Call Trace:
    <TASK>
    vp_del_vqs+0x121/0x230
    remove_common+0x135/0x150
    virtballoon_remove+0xee/0x100
    virtio_dev_remove+0x3b/0x80
    device_release_driver_internal+0x187/0x2c0
    unbind_store+0xb9/0xe0
    kernfs_fop_write_iter.llvm.11660790530567441834+0xf6/0x180
    vfs_write+0x2a9/0x3b0
    ksys_write+0x5c/0xd0
    do_syscall_64+0x54/0x230
    entry_SYSCALL_64_after_hwframe+0x29/0x31
    [...]
    </TASK>

The virtio_balloon device registers 5 queues (inflate, deflate, stats,
free_page, reporting) but only the first two are unconditional. The
stats, free_page and reporting queues are each conditional on their
respective feature bits. When any of these features are absent, the
corresponding vqs_info entry has name == NULL, creating holes in the
array.

The root cause is an indexing mismatch introduced when vq info storage
was changed to be passed as an argument. vp_find_vqs_msix() and
vp_find_vqs_intx() store the info pointer at vp_dev->vqs[i], where 'i'
is the caller's sparse array index. However, the virtqueue itself gets
vq->index assigned from queue_idx, a dense index that skips NULL
entries. When holes exist, 'i' and queue_idx diverge. Later,
vp_del_vqs() looks up info via vp_dev->vqs[vq->index] using the dense
index into the sparsely-populated array, and hits NULL.

Fix this by storing info at vp_dev->vqs[queue_idx] instead of
vp_dev->vqs[i], so the store index matches the lookup index
(vq->index). Apply the fix to both the MSIX and INTX paths.

Cc: Yichun Zhang <yichun@openresty.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Cc: stable@vger.kernel.org # v6.11+
Tested-by: Yuka <yuka@umeyashiki.org>
Fixes: 89a1c43 ("virtio_pci: pass vq info as an argument to vp_setup_vq()")
Signed-off-by: Ammar Faizi <ammarfaizi2@openresty.com>
Message-Id: <20260315141808.547081-1-ammarfaizi2@openresty.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/virtio/virtio_pci_common.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=41e6dc1a10036c9f47057033f19af7e52ec464b6

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

The original Linux kernel CVE announcement for CVE-2026-64457 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64457

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: 0
  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).