UBUNTU-CVE-2026-64527: Medium severity ubuntu/linux-hwe-edge vulnerability
In the Linux kernel, the following vulnerability has been resolved: drm/hyperv: validate VMBus packet size in receive callback hypervreceivesub() reads msg->vidhdr.type and dispatches into one of four message-type branches without knowing how many bytes the host wrote into hv->recvbuf. The completion path then runs memcpy(hv->initbuf, msg, VMBUSMAXPACKETSIZE), so the consumer that wakes on waitforcompletiontimeout() can read up to 16 KiB of residue from a prior message as if it were the response payload. Pass bytesrecvd into hypervreceivesub() and reject any packet that does not cover the pipe + synthvid header. A single switch on msg->vidhdr.type then computes the type-specific payload size: the three completion-driving types (SYNTHVIDVERSIONRESPONSE, SYNTHVIDRESOLUTIONRESPONSE, SYNTHVIDVRAMLOCATIONACK) fall through to a shared exit that requires that size before memcpy/complete, while SYNTHVIDFEATURECHANGE validates its own payload and returns before reading isdirtneeded. Unknown types are dropped. SYNTHVIDRESOLUTIONRESPONSE is variable length: the host fills resolutioncount entries, not the full SYNTHVIDMAXRESOLUTIONCOUNT array. Validate the fixed prefix first so resolutioncount 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 vmbusrecvpacket() returned success. The memcpy length is bytesrecvd, which is bounded by VMBUSMAXPACKETSIZE only on a successful receive; on -ENOBUFS vmbusrecvpacket() instead reports the required length, which can exceed hv->recvbuf, so copying bytesrecvd 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 drmerrratelimited() rather than silently dropped, matching the CoCo-hardened pattern in hvkvponchannelcallback().
Affected Software
Remediation
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-28.28.1 - Compensating control
In hyperv_receive_sub(), pass bytes_recvd from vmbus_recvpacket(), invoke the sub-handler only when vmbus_recvpacket() returns success, and reject packets shorter than the pipe plus synthvid header. Report rejected and nonzero-return malformed messages with drm_err_ratelimited() instead of silently dropping them; do not attempt channel recovery for this case.
- Compensating control
Validate the message-type-specific payload size before memcpy() or completing the wait: require the calculated size for SYNTHVID_VERSION_RESPONSE, SYNTHVID_RESOLUTION_RESPONSE, and SYNTHVID_VRAM_LOCATION_ACK; validate SYNTHVID_FEATURE_CHANGE independently and return before reading is_dirt_needed. Copy only bytes_recvd after a successful receive, bounded by VMBUS_MAX_PACKET_SIZE.
- Compensating control
For SYNTHVID_RESOLUTION_RESPONSE, validate the fixed prefix before reading resolution_count, ensure resolution_count does not exceed the resolution-entry array bound, and require only the count-sized resolution array rather than the full SYNTHVID_MAX_RESOLUTION_COUNT array. Drop unknown message types.
Event History
Frequently Asked Questions
Which systems are in scope for this issue?
The issue applies to the listed Ubuntu kernel packages when the Hyper-V DRM receive path processes VMBus messages from the host. The affected code handles synthetic video protocol messages, including version, resolution, and VRAM-location responses.
What would exploitation require?
An attacker would need the ability to cause the host side to send a malformed or undersized VMBus packet to the guest's Hyper-V DRM receive callback. Packets that do not contain the required pipe and synthetic-video headers could otherwise be dispatched and copied as though they were valid responses.
What is the security impact of an undersized packet?
The completion path can copy up to VMBUS_MAX_PACKET_SIZE, 16 KiB, from the receive buffer even when the host supplied fewer bytes. A waiting consumer can then treat residual data from a previous message as response payload.
How can I verify that a kernel includes the fix?
The provided upstream fix is identified by commit 7f87763f47a3c22fb50265a00619ef10f2394b18. A fixed kernel validates the received packet length before dispatch, enforces type-specific payload sizes before copying and completing requests, and drops unknown message types.