[PATCH] virtio-gpu: reject requests with short/truncated control headers

A control request shorter than virtio_gpu_ctrl_hdr can leave cmd_hdr
partially initialized. If the supplied bytes set the fence flag, stale
fence metadata may later be returned to the guest.

Validate the common header length before dispatch. Clear cmd_hdr and
complete malformed requests with ERR_INVALID_PARAMETER so stale fields
cannot reach the response.

Fixes: CVE-2026-18054
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4094
Reported-by: Ankur Saini <[email protected]>
Signed-off-by: Ankur Saini <[email protected]>
---
A control request shorter than virtio_gpu_ctrl_hdr can leave cmd_hdr
partially initialized. If the supplied bytes set the fence flag, stale
fence metadata may later be returned to the guest.

Validate the common header length before dispatch. Clear cmd_hdr and
complete malformed requests with ERR_INVALID_PARAMETER so stale fields
cannot reach the response.
---
 hw/display/virtio-gpu.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index 4d46a4eb10..15a845eff6 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -1105,8 +1105,14 @@ void virtio_gpu_process_cmdq(VirtIOGPU *g)
             break;
         }
 
-        /* process command */
-        vgc->process_cmd(g, cmd);
+        if (unlikely(iov_size(cmd->elem.out_sg, cmd->elem.out_num) <
+                     sizeof(cmd->cmd_hdr))) {
+            memset(&cmd->cmd_hdr, 0, sizeof(cmd->cmd_hdr));
+            virtio_gpu_ctrl_response_nodata(
+                g, cmd, VIRTIO_GPU_RESP_ERR_INVALID_PARAMETER);
+        } else {
+            vgc->process_cmd(g, cmd);
+        }
 
         /* command suspended */
         if (!cmd->finished && !(cmd->cmd_hdr.flags & VIRTIO_GPU_FLAG_FENCE)) {

---
base-commit: 299e7557ed15a9a325620698add379a3ce2d1d95
change-id: 20260728-virtio-gpu-short-header-476aa1dae4f7

Best regards,
-- 
Ankur Saini <[email protected]>