[CVE-2026-64527][MODERATE 7.0] drm/hyperv: validate VMBus packet size in receive callback

[CVE-2026-64527][MODERATE 7.0] drm/hyperv: validate VMBus packet size in receive callback

[AL-KERNEL]

CVE: CVE-2026-64527
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: drm/hyperv: validate VMBus packet size in receive callback
Commit: 57d5d697642e05d5dd2d40660817765943dd709f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=57d5d697642e05d5dd2d40660817765943dd709f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64527
Analysis date: Sat, 25 Jul 2026 06:39:01 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malformed Hyper-V synthetic video VMBus packet can be processed without sufficient length validation, allowing stale receive-buffer data to be consumed as a valid response and potentially causing guest driver state corruption or denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-64527 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64527

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.

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

CVE-2026-64527	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-20;CWE-125;*CWE-200;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '7.6';DESCR 'A Hyper-V synthetic video VMBus packet can be processed before the guest driver verifies that the host supplied enough bytes for the pipe header, synthvid header, and type specific payload. This can make hyperv_receive_sub read message type or payload fields from stale recv_buf contents and can copy up to 16 KiB of residue from a previous message into the initialization buffer as if it were a valid response. For the CVSS the PR:N is used because the attacker does not need privileges inside the guest when the attacker controls the host side VMBus endpoint. The issue is adjacent rather than Internet reachable and is mainly relevant to host-to-guest or confidential computing threat models. Impact is at least guest driver state corruption or denial of service, with limited confidentiality or integrity impact possible through stale response data handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	INIT HARDWARE PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* **Firmware-mediated / host-mediated external influence: +1**
  The malformed input is supplied through the Hyper-V VMBus synthetic video channel by the host side endpoint, not by ordinary guest userspace.

* **Constrained kernel buffer / stale data misuse: +1**
  The old code could process `hv->recv_buf` without validating `bytes_recvd` and could copy stale residue from a previous message into `hv->init_buf`.

* **Cross-boundary isolation / policy concern: +1**
  This is a host-to-guest boundary issue and is especially relevant for confidential-computing style threat models where the guest should be robust against malformed host messages.

* **Integrity impact plausible: +1**
  Stale or incomplete response payloads may be interpreted as valid synthetic video protocol state, affecting resolution, VRAM acknowledgement, or feature-change handling.

* **Availability impact realistic: +1**
  Malformed VMBus packets can disrupt the Hyper-V DRM receive/init path. The commit explicitly notes malformed-message handling and no channel recovery.

Paranoid additional signal:

* **Weak / indirect corruption candidate or stronger DoS concern: +1**
  The patch context discusses avoiding paths where an oversized receive result could otherwise be misused, and the affected code runs in a trusted guest kernel driver callback. This does not prove arbitrary memory corruption, but it justifies manual-review sensitivity.

Not awarded:

* **No strong memory corruption +2**
  No UAF, double-free, attacker-controlled OOB write, arbitrary write, or type confusion is demonstrated.

* **No remote network +2**
  This is VMBus adjacent / host-mediated, not Internet or packet-network reachable.

* **No LPE +2**
  The patch does not show a guest-local privilege escalation primitive.

## 3. Reachability analysis

The trigger is a malformed Hyper-V synthetic video VMBus packet from the host side. A normal local user inside the guest is not the primary attacker. From the guest perspective, attacker privileges are not required if the attacker controls or compromises the host-side VMBus endpoint.

Namespaces and containers do not materially lower the trigger requirement because the affected path is not a normal container-exposed syscall or netlink API. The issue is relevant on Hyper-V guests using the synthetic video DRM driver, including cloud or virtualized environments where the host is part of the threat model. It is not directly reachable over the network unless an attacker first controls the host, hypervisor, or a management path capable of influencing VMBus traffic.

Call-site confidence: **high**, because the receive callback, sub-handler, length checks, and completion copy path are all visible in the patch.

## 4. Severity interpretation

This behaves like an **Actionable Moderate** issue, not an ordinary low-risk validation cleanup. The practical primitive is malformed host-controlled input causing stale buffer interpretation and guest driver state disruption. The realistic impact is limited confidentiality/integrity concern plus DoS in a virtualization boundary context.

It is **not a strong Important candidate by memory-corruption evidence alone**, because the patch does not demonstrate a controllable OOB write, UAF, reclaim primitive, callback corruption, or arbitrary write. The paranoid score is elevated because host-to-guest VMBus parsing bugs are security-sensitive and should not be auto-closed without manual review.

## 5. One-sentence report phrase

A malformed Hyper-V synthetic video VMBus packet can be processed without sufficient length validation, allowing stale receive-buffer data to be consumed as a valid response and potentially causing guest driver state corruption or denial of service.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a virtualization-boundary parsing bug with host-controlled input into a guest kernel driver, limited stale-data exposure, and plausible guest availability or state-integrity impact. It should not be auto-closed based only on the absence of a demonstrated arbitrary write or LPE primitive.

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

Patch: drm/hyperv: validate VMBus packet size in receive callback
Commit: 57d5d697642e05d5dd2d40660817765943dd709f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=57d5d697642e05d5dd2d40660817765943dd709f

Commit description:

hyperv_receive_sub() reads msg->vid_hdr.type and dispatches into one
of four message-type branches without knowing how many bytes the host
wrote into hv->recv_buf. The completion path then runs
memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE), so the consumer that
wakes on wait_for_completion_timeout() can read up to 16 KiB of
residue from a prior message as if it were the response payload.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet that
does not cover the pipe + synthvid header. A single switch on
msg->vid_hdr.type then computes the type-specific payload size: the
three completion-driving types (SYNTHVID_VERSION_RESPONSE,
SYNTHVID_RESOLUTION_RESPONSE, SYNTHVID_VRAM_LOCATION_ACK) fall through
to a shared exit that requires that size before memcpy/complete, while
SYNTHVID_FEATURE_CHANGE validates its own payload and returns before
reading is_dirt_needed. Unknown types are dropped.

SYNTHVID_RESOLUTION_RESPONSE is variable length: the host fills
resolution_count entries, not the full SYNTHVID_MAX_RESOLUTION_COUNT
array. Validate the fixed prefix first so resolution_count can be
read, bound it against the array, then require only the count-sized
array, so the shorter responses the host actually sends are accepted.

Only run the sub-handler when vmbus_recvpacket() returned success. The
memcpy length is bytes_recvd, which is bounded by VMBUS_MAX_PACKET_SIZE
only on a successful receive; on -ENOBUFS vmbus_recvpacket() instead
reports the required length, which can exceed hv->recv_buf, so copying
bytes_recvd would read and write past the 16 KiB buffers. Gating on the
success return keeps the copy bounded. The nonzero-return path is itself
a malformed-message case and is now logged rather than silently skipped;
channel recovery is not attempted.

Rejected packets are reported via drm_err_ratelimited() rather than
silently dropped, matching the CoCo-hardened pattern in
hv_kvp_onchannelcallback().

Fixes: 76c56a5 ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
Assisted-by: Claude:claude-opus-4-7 berkoc-pipeline
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
Link: https://patch.msgid.link/8200dbc199c7a9b75ac7e8af6c748d2189b5ebd5.1779542874.git.me@berkoc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/hyperv/hyperv_drm_proto.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=57d5d697642e05d5dd2d40660817765943dd709f

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

The original Linux kernel CVE announcement for CVE-2026-64527 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64527

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