linux-kernel - Re: [PATCH v9 3/4] vhost: add VHOST_FORK_FROM_OWNER ioctl and validate inherit

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Message-ID: <zd4njyzxo6jnmrthhsgo26t7t44tjfkkd7aghzd237s2h5pwf6@nmwhp43dvlj6>
Date: Tue, 22 Apr 2025 15:45:56 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Cindy Lu <lulu@...hat.com>
Cc: jasowang@...hat.com, mst@...hat.com, michael.christie@...cle.com, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v9 3/4] vhost: add VHOST_FORK_FROM_OWNER ioctl and
 validate inherit_owner

On Mon, Apr 21, 2025 at 10:44:09AM +0800, Cindy Lu wrote:
>Add a new UAPI to configure the vhost device to use the kthread mode.
>The userspace application can use IOCTL VHOST_FORK_FROM_OWNER
>to choose between owner and kthread mode if necessary.
>This setting must be applied before VHOST_SET_OWNER, as the worker
>will be created in the VHOST_SET_OWNER function.
>
>In addition, the VHOST_NEW_WORKER requires the inherit_owner
>setting to be true. So we need to add a check for this.
>
>Signed-off-by: Cindy Lu <lulu@...hat.com>
>---
> drivers/vhost/vhost.c      | 29 +++++++++++++++++++++++++++--
> include/uapi/linux/vhost.h | 16 ++++++++++++++++
> 2 files changed, 43 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index be97028a8baf..fb0c7fb43f78 100644
>--- a/drivers/vhost/vhost.c
>+++ b/drivers/vhost/vhost.c
>@@ -1018,6 +1018,13 @@ long vhost_worker_ioctl(struct vhost_dev *dev, unsigned int ioctl,
> 	switch (ioctl) {
> 	/* dev worker ioctls */
> 	case VHOST_NEW_WORKER:
>+		/*
>+		 * vhost_tasks will account for worker threads under the parent's
>+		 * NPROC value but kthreads do not. To avoid userspace overflowing
>+		 * the system with worker threads inherit_owner must be true.
>+		 */
>+		if (!dev->inherit_owner)
>+			return -EFAULT;
> 		ret = vhost_new_worker(dev, &state);
> 		if (!ret && copy_to_user(argp, &state, sizeof(state)))
> 			ret = -EFAULT;
>@@ -1134,7 +1141,7 @@ void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem)
> 	int i;
>
> 	vhost_dev_cleanup(dev);
>-

nit: I'd avoid removing this empty line, it helps while reading code.

>+	dev->inherit_owner = true;
> 	dev->umem = umem;
> 	/* We don't need VQ locks below since vhost_dev_cleanup makes sure
> 	 * VQs aren't running.
>@@ -2287,7 +2294,25 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> 		r = vhost_dev_set_owner(d);
> 		goto done;
> 	}
>-
>+	if (ioctl == VHOST_FORK_FROM_OWNER) {
>+		u8 inherit_owner;
>+		/*inherit_owner can only be modified before owner is set*/
>+		if (vhost_dev_has_owner(d)) {
>+			r = -EBUSY;
>+			goto done;
>+		}
>+		if (copy_from_user(&inherit_owner, argp, sizeof(u8))) {
>+			r = -EFAULT;
>+			goto done;
>+		}
>+		if (inherit_owner > 1) {
>+			r = -EINVAL;
>+			goto done;
>+		}
>+		d->inherit_owner = (bool)inherit_owner;
>+		r = 0;
>+		goto done;
>+	}

Ditto, an empty like should help to separate to the next step.

> 	/* You must be the owner to do anything else */
> 	r = vhost_dev_check_owner(d);
> 	if (r)
>diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
>index b95dd84eef2d..1ae0917bfeca 100644
>--- a/include/uapi/linux/vhost.h
>+++ b/include/uapi/linux/vhost.h
>@@ -235,4 +235,20 @@
>  */
> #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
> 					      struct vhost_vring_state)
>+
>+/**
>+ * VHOST_FORK_FROM_OWNER - Set the inherit_owner flag for the vhost device,
>+ * This ioctl must called before VHOST_SET_OWNER.

Shoud we mention that this IOCTL is only supported if 
CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is y?

>+ *
>+ * @param inherit_owner: An 8-bit value that determines the vhost thread mode
>+ *
>+ * When inherit_owner is set to 1(default value):
>+ *   - Vhost will create tasks similar to processes forked from the owner,
>+ *     inheriting all of the owner's attributes.
>+ *
>+ * When inherit_owner is set to 0:
>+ *   - Vhost will create tasks as kernel thread.
>+ */
>+#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
>+
> #endif
>-- 
>2.45.0
>