[CVE-2026-64456][MODERATE 7.0] hwrng: virtio: clamp device-reported used.len at copy_data()

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

CVE: CVE-2026-64456
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: hwrng: virtio: clamp device-reported used.len at copy_data()
Commit: 3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64456
Analysis date: Sat, 25 Jul 2026 11:36:02 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious or buggy virtio-rng backend can over-report used.len and cause the guest kernel hwrng path to read beyond vi->data into adjacent slab memory, potentially exposing guest kernel heap bytes through /dev/hwrng under a hostile-backend threat model.

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

The original Linux kernel CVE announcement for CVE-2026-64456 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64456

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-2026-64456	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L';CWE-125;CWE-126;*CWE-20;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '7.6';DESCR 'virtio-rng trusts the backend reported used len and copy_data can copy from vi data after data_idx without clamping the effective range to sizeof(vi data). A malicious or buggy virtio rng backend can over report used len and make the guest driver read past the inline entropy buffer into adjacent kmalloc slab memory. The immediate effect is an out-of-bounds read inside the guest kernel. The leaked bytes can contaminate the guest RNG and may be exposed to guest root through /dev/hwrng, which is especially relevant in confidential compute or hostile host threat models. For the CVSS the PR:N is used because the attacker does not need privileges inside the guest once the virtio rng backend is controlled or malicious. The issue is not Internet network reachable. It is reachable across the virtio device boundary from host or backend to guest. Impact is at least guest kernel memory disclosure risk and possible denial of service in hardened builds, with no direct evidence of an arbitrary write or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)  SKIP CVE-2026-64456 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: f7f510ec195781c857ab76366a3e1c59e1caae42 YES  NO NO unknown 	MAYBE	READ KASAN OOB VIRT LEAK SPECTRE HARDWARE LINUS  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* +1 Firmware-mediated external influence: the attacker model is a malicious or buggy virtio-rng backend or hypervisor-controlled device behavior, not ordinary Internet reachability.
* +1 Confidentiality impact plausible: unchecked used.len can make the guest kernel copy adjacent kmalloc slab bytes into the hwrng path, and guest root may observe them via /dev/hwrng.
* +1 Broad/default/common subsystem: virtio-rng is common in virtualized Linux guests, even though exposure depends on VM configuration.
* +1 Privileged kernel/device-management memory safety path: device-reported length is trusted inside a guest kernel driver and reaches memcpy from an inline buffer.
* +1 Practical exploit chain strongly implied: host or backend over-reports used.len, guest hwrng path copies OOB slab bytes, guest root can read /dev/hwrng.

Paranoid additional signal:

* +1 Cross-boundary isolation concern: in confidential-compute or hostile-host threat models, this can become a guest-kernel heap disclosure channel across the virtio device boundary, although the practical path is narrow.

Not counted:

* No remote/network-triggerable +2, because this is virtio backend mediated, not Internet/network reachable.
* No strong memory corruption +2, because the demonstrated primitive is OOB read, not OOB write, UAF, double free, or arbitrary write.
* No privilege escalation +2, because the patch does not show reclaim, overwrite, type confusion, callback control, or refcount takeover.
* No reliable DoS +1 in the conservative score, because the KASAN crash confirms memory-safety violation but normal production behavior is more clearly disclosure/RNG contamination than guaranteed crash.

## 3. Reachability analysis

The bug is triggered by a malicious or buggy virtio-rng backend that reports used.len larger than the posted inline buffer size. Guest userspace is not required to first trigger the OOB read because hwrng_fillfn can run after device probe. To observe leaked bytes through /dev/hwrng, guest root or an equivalent privileged guest process is typically needed. Namespaces do not materially reduce the backend-side requirement, but guest-side observation depends on device-node permissions. The path is common in VM deployments using virtio-rng, but confidential-compute guests often disable virtio-rng, which narrows the highest-impact threat model.

Call-site confidence: high. The patch and commit describe the direct path from random_recv_done to data_avail, then copy_data, memcpy, hwrng_fillfn, and /dev/hwrng observation.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a KASAN-confirmed guest kernel slab OOB read driven by a device-reported length across a virtio boundary. It does not look like an Important LPE candidate on the provided evidence because there is no write primitive, UAF reclaim, type confusion, or callback control. Realistic impact is guest kernel memory disclosure risk and RNG contamination under a malicious backend model. The paranoid score treats the confidential-compute cross-boundary disclosure angle as requiring manual review, but not as proof of arbitrary code execution or direct privilege escalation.

## 5. One-sentence report phrase

A malicious or buggy virtio-rng backend can over-report used.len and cause the guest kernel hwrng path to read beyond vi->data into adjacent slab memory, potentially exposing guest kernel heap bytes through /dev/hwrng under a hostile-backend threat model.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed OOB read in a virtio host-to-guest device path with plausible guest kernel heap disclosure and confidential-compute relevance, even though there is no demonstrated write primitive or privilege-escalation path.

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

Patch: hwrng: virtio: clamp device-reported used.len at copy_data()
Commit: 3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5

Commit description:

random_recv_done() stores the device-reported used.len directly into
vi->data_avail.  copy_data() then indexes vi->data[] using
vi->data_idx (advanced by previous copy_data() calls) and issues a
memcpy() without re-validating either value against the posted
buffer size sizeof(vi->data) (SMP_CACHE_BYTES bytes, typically 32
or 64).

A malicious or buggy virtio-rng backend can set used.len beyond
sizeof(vi->data), steering the memcpy() past the end of the inline
array into adjacent kmalloc-1k slab bytes.  hwrng_fillfn() mixes
those bytes into the guest RNG, and guest root can also observe
them directly via /dev/hwrng.

Concrete impact is inside the guest:

 - Memory-safety / hardening: any virtio-rng backend that
   over-reports used.len causes the driver to read past vi->data
   into unrelated slab contents.  hwrng_fillfn() is a kernel thread
   that runs as soon as the device is probed; no guest userspace
   interaction is required to first-trigger the OOB.

 - Cross-boundary leak (confidential-compute threat model): a
   malicious hypervisor cooperating with a malicious or compromised
   guest root userspace can use /dev/hwrng as a leak channel for
   guest-kernel heap data.  The host sets a large used.len, guest
   root reads /dev/hwrng, and the returned bytes contain guest
   kernel slab contents that were adjacent to vi->data.  In
   practice, confidential-compute guests (SEV-SNP, TDX) usually
   disable virtio-rng entirely, so this path is narrow, but the
   fix is still worth carrying because the underlying
   memory-safety bug contaminates the guest RNG on any host.

KASAN confirms the OOB on a 7.1-rc4 guest whose virtio-rng backend
has been patched to report used.len = 0x10000:

  BUG: KASAN: slab-out-of-bounds in virtio_read+0x394/0x5d0
  Read of size 64 at addr ffff88800ae0ba20 by task hwrng/52
  Call Trace:
   __asan_memcpy+0x23/0x60
   virtio_read+0x394/0x5d0
   hwrng_fillfn+0xb2/0x470
   kthread+0x2cc/0x3a0
  Allocated by task 1:
   probe_common+0xa5/0x660
   virtio_dev_probe+0x549/0xbc0
  The buggy address belongs to the object at ffff88800ae0b800
   which belongs to the cache kmalloc-1k of size 1024
  The buggy address is located 0 bytes to the right of
   allocated 544-byte region [ffff88800ae0b800, ffff88800ae0ba20)

Same class of bug as commit c04db81 ("net/9p: Fix buffer
overflow in USB transport layer"), which hardened
usb9pfs_rx_complete() against unchecked device-reported length in
the USB 9p transport.

With the clamp at point of use and array_index_nospec() in place,
the same harness boots cleanly: copy_data() returns zero for the
bogus report, the device-supplied bytes after data_idx are
discarded, and the driver issues a fresh request.

Fixes: f7f510e ("virtio: An entropy device, as suggested by hpa.")
Cc: stable@vger.kernel.org
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260531142251.2792061-1-michael.bommarito@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/char/hw_random/virtio-rng.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=3aa3e89cf80721c8d382b4c1a2b70a0449dad4a5

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

The original Linux kernel CVE announcement for CVE-2026-64456 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64456

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:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L
  The Best / paranoid CVSS score: 7.6

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: 5
  Paranoid ActionableScore: 6

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).