Subject: Re: [PATCH v3 7/9] vhost: Add new UAPI to support change to task mode

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Nov 2024 02:31:31 -0500

On Tue, Nov 05, 2024 at 03:25:26PM +0800, Cindy Lu wrote:
> Add a new UAPI to enable setting the vhost device to task mode.
> The userspace application can use VHOST_SET_INHERIT_FROM_OWNER
> to configure the mode if necessary.
> This setting must be applied before VHOST_SET_OWNER, as the worker
> will be created in the VHOST_SET_OWNER function
> 
> Signed-off-by: Cindy Lu <[email-masked]>
> ---
>  drivers/vhost/vhost.c      | 15 ++++++++++++++-
>  include/uapi/linux/vhost.h |  2 ++
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index c17dc01febcc..70c793b63905 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2274,8 +2274,9 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>  {
>  	struct eventfd_ctx *ctx;
>  	u64 p;
> -	long r;
> +	long r = 0;
>  	int i, fd;
> +	bool inherit_owner;
>  
>  	/* If you are not the owner, you can become one */
>  	if (ioctl == VHOST_SET_OWNER) {
> @@ -2332,6 +2333,18 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>  		if (ctx)
>  			eventfd_ctx_put(ctx);
>  		break;
> +	case VHOST_SET_INHERIT_FROM_OWNER:
> +		/*inherit_owner can only be modified before owner is set*/

bad coding style

> +		if (vhost_dev_has_owner(d))
> +			break;

is this silently failing? should return EBUSY or something like this.

> +
> +		if (copy_from_user(&inherit_owner, argp,
> +				   sizeof(inherit_owner))) {

not good, 

> +			r = -EFAULT;
> +			break;
> +		}
> +		d->inherit_owner = inherit_owner;

> +		break;
>  	default:
>  		r = -ENOIOCTLCMD;
>  		break;

This means any task can break out of jail
and steal root group system time by setting inherit_owner to 0
even if system is configured to inherit by default.

we need a modparam to block this.

> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index b95dd84eef2d..1e192038633d 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -235,4 +235,6 @@
>   */
>  #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
>  					      struct vhost_vring_state)
> +
> +#define VHOST_SET_INHERIT_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, bool)

do not put bool in interfaces. something like u8 and validate it is 0 or
1.

>  #endif
> -- 
> 2.45.0