[PATCH v2] 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.

The vhost-user-gpu backend has the same issue: it logs a short header
copy but continues to process the partially initialized command.

Validate the common header length before dispatch in both paths. 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]>
Suggested-by: Akihiko Odaki <[email protected]>
Signed-off-by: Ankur Saini <[email protected]>
---
Changes in v2:
- Also reject short/truncated control headers in vhost-user-gpu.
- Expand the commit message to describe both affected paths.
- Link to v1: https://lore.kernel.org/qemu-devel/[email protected]
---
 contrib/vhost-user-gpu/vhost-user-gpu.c | 21 ++++++++++++---------
 hw/display/virtio-gpu.c                 | 10 ++++++++--
 2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/contrib/vhost-user-gpu/vhost-user-gpu.c b/contrib/vhost-user-gpu/vhost-user-gpu.c
index bb41758e34..8149834745 100644
--- a/contrib/vhost-user-gpu/vhost-user-gpu.c
+++ b/contrib/vhost-user-gpu/vhost-user-gpu.c
@@ -924,16 +924,19 @@ vg_handle_ctrl(VuDev *dev, int qidx)
         if (len != sizeof(cmd->cmd_hdr)) {
             g_warning("%s: command size incorrect %zu vs %zu\n",
                       __func__, len, sizeof(cmd->cmd_hdr));
-        }
-
-        virtio_gpu_ctrl_hdr_bswap(&cmd->cmd_hdr);
-        g_debug("%d %s\n", cmd->cmd_hdr.type,
-                vg_cmd_to_string(cmd->cmd_hdr.type));
-
-        if (vg->virgl) {
-            vg_virgl_process_cmd(vg, cmd);
+            memset(&cmd->cmd_hdr, 0, sizeof(cmd->cmd_hdr));
+            vg_ctrl_response_nodata(
+                vg, cmd, VIRTIO_GPU_RESP_ERR_INVALID_PARAMETER);
         } else {
-            vg_process_cmd(vg, cmd);
+            virtio_gpu_ctrl_hdr_bswap(&cmd->cmd_hdr);
+            g_debug("%d %s\n", cmd->cmd_hdr.type,
+                    vg_cmd_to_string(cmd->cmd_hdr.type));
+
+            if (vg->virgl) {
+                vg_virgl_process_cmd(vg, cmd);
+            } else {
+                vg_process_cmd(vg, cmd);
+            }
         }
 
         if (cmd->state != VG_CMD_STATE_FINISHED) {
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]>