QEMU virtio-gpu fix blocks guest data leak via short headers

QEMU’s virtio-gpu device will reject control requests with short or truncated headers, closing CVE-2026-18054, a flaw that could leak guest data back to the guest.

A short control request left command data only partly filled in. Guest-controlled flags in the common header could then cause stale fence metadata from prior state to be returned as if it belonged to the current request. The fill helpers noticed the short copy but only logged it and returned, so the malformed command was still completed successfully with a no-data OK response.

Ankur Saini’s patch clears any partially copied object and finishes the request with an invalid-parameter error instead, for both the in-process virtio-gpu path and vhost-user-gpu. Truncated type-specific commands are rejected the same way. Akihiko Odaki suggested the approach and reviewed the change.

Hosts that expose virtio-gpu to untrusted guests should take the fix once it lands in a release.