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

Thanks. Just to clarify, the vhost-user-gpu check was already there and
compares the return value of iov_to_buf(), not iov_size(). != and < should
be equivalent for vhost-user-gpu check so I left it unchanged.

The new virtio-gpu check looks at the total iov size, which can be larger
than the header for valid commands, so it needs to use <. Would you still
prefer changing the vhost-user-gpu condition to < for consistency?

Regards,
Ankur Saini

On Wed, Jul 29, 2026 at 9:31 AM Akihiko Odaki <[email protected]>
wrote:

> On 2026/07/29 5:44, 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.
> >
> > 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.
>
> vhost-user-gpu and virtio-gpu has slightly different conditions:
>
> - vhost-user-gpu rejects: iov_size() != sizeof(cmd->cmd_hdr)
> - virtio-gpu rejects: iov_size() < sizeof(cmd->cmd_hdr)
>
> Ideally they should be consistent.
>
> Regards,
> Akihiko Odaki
>
> >
> > 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,
>
>