[REGRESSION] virtio: reject shm region if length is zero

[Alyssa Ross]

On Mon, May 12, 2025 at 07:51:53AM +0930, samiuddinsami.md...@gmail.com wrote:
> From: Sami Uddin <sami.md...@gmail.com>
>
> Prevent usage of shared memory regions where the length is zero,
> as such configurations are not valid and may lead to unexpected behavior.
>
> Signed-off-by: Sami Uddin <sami.md...@gmail.com>
> ---
> v3:
> - Use idiomatic 'if (!region->len)' as suggested by reviewer
> v2:
> - Fixed coding style issue: added space after 'if' statement
>
>  include/linux/virtio_config.h | 2 ++
>  1 file changed, 2 insertions(+)

Hi, I'm sorry to be the bearer of bad news, but since this patch my VM
no longer works.  The system is running wayland-proxy-virtwl[1] inside
a crosvm[2] VM, using crosvm's virtio-gpu device to do cross-domain
Wayland forwarding.

Since this change, wayland-proxy-virtwl crashes with the following log
message:

        wl-proxy [WARNING]: Error handling client: Unix.Unix_error(Unix.EINVAL, 
"DRM_IOCTL_VIRTGPU_RESOURCE_CREATE_BLOB", "")

I'm pretty confused by what this change was supposed to do in the first
place…  Looking at how virtio_get_shm_region() is used in
virtio_gpu_init(), it's called with a pointer to zeroed memory, and then
the get_shm_region() implementation is supposed to write to the region,
without ever reading from it as far as I can tell.  Why is the initial
value of an out parameter being checked at all?  How does this prevent
using zero-length shared memory regions?

[1]: https://crosvm.dev/
[2]: https://github.com/talex5/wayland-proxy-virtwl

#regzbot introduced: 206cc44588f72b49ad4d7e21a7472ab2a72a83df

> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index 169c7d367fac..b3e1d30c765b 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -329,6 +329,8 @@ static inline
>  bool virtio_get_shm_region(struct virtio_device *vdev,
>                          struct virtio_shm_region *region, u8 id)
>  {
> +     if (!region->len)
> +             return false;
>       if (!vdev->config->get_shm_region)
>               return false;
>       return vdev->config->get_shm_region(vdev, region, id);
> --
> 2.34.1
>

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