Re: [PATCH v2] virtio-gpu: reject requests with short/truncated control headers

On Wed, Jul 29, 2026 at 12:45 AM Ankur Saini <[email protected]> wrote:
>
> 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.

yeah, at least it had a warning ;) (and a bit less exploitable than
full-blown qemu)

>
> 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]>

Reviewed-by: Marc-André Lureau <[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]>
>
>