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

Thanks. I found the similar path in contrib/vhost-user-gpu and will include
a minimal fix for that in v2.

On Wed, Jul 29, 2026 at 1:22 AM Akihiko Odaki <[email protected]>
wrote:

> On 2026/07/28 23:16, Ankur Saini 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.
> >
> > 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]>
>
> Reviewed-by: Akihiko Odaki <[email protected]>
>
> Please also fix vhost-user-gpu.
>
> Regards,
> Akihiko Odaki
>
> > ---
> > 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,
>
>