On 2026/07/29 16:28, Ankur Saini wrote:
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 fact that the checks compare different values made me look at how
virtio-gpu copies commands.
For commands with type-specific fields, processing copies the request
in two stages:
1. Copy the common header to determine the command type.
2. Copy the complete type-specific command.
vhost-user-gpu open-codes the first step in vg_handle_ctrl() and uses
VUGPU_FILL_CMD() for the second. virtio-gpu uses VIRTIO_GPU_FILL_CMD()
for both steps.
Both fill macros already detect a short copy, but only log and return.
They should instead complete the malformed request with
ERR_INVALID_PARAMETER. For a common-header copy, the partially copied
header must also be cleared before forming the response.
Once VIRTIO_GPU_FILL_CMD() handles this failure, the new check in
virtio_gpu_process_cmdq() will be unnecessary because the macro covers
the first step. Fixing both macros will also reject truncated
type-specific commands.
Regards,
Akihiko Odaki
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]
tokyo.ac.jp <mailto:[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
<https://gitlab.com/qemu-project/qemu/-/work_items/4094>
> Reported-by: Ankur Saini <[email protected]
<mailto:[email protected]>>
> Suggested-by: Akihiko Odaki <[email protected]
<mailto:[email protected]>>
> Signed-off-by: Ankur Saini <[email protected]
<mailto:[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/20260728-virtio-
[email protected] <https://
lore.kernel.org/qemu-devel/20260728-virtio-gpu-short-header-v1-1-
[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,
