Re: [PATCH V2 3/7] vhost-vdpa: VHOST_NEW_OWNER

[Jason Wang]

On Fri, Jul 12, 2024 at 9:19 PM Steve Sistare <steven.sistare@xxxxxxxxxx> wrote:
>
> Add an ioctl to transfer file descriptor ownership and pinned memory
> accounting from one process to another.
>
> This is more efficient than VHOST_RESET_OWNER followed by VHOST_SET_OWNER,
> as that would unpin all physical pages, requiring them to be repinned in
> the new process.  That would cost multiple seconds for large memories, and
> be incurred during a virtual machine's pause time during live update.
>
> Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> ---
>  drivers/vhost/vdpa.c       | 41 ++++++++++++++++++++++++++++++++++++++
>  drivers/vhost/vhost.c      | 15 ++++++++++++++
>  drivers/vhost/vhost.h      |  1 +
>  include/uapi/linux/vhost.h | 10 ++++++++++
>  4 files changed, 67 insertions(+)
>
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index b49e5831b3f0..5cf55ca4ec02 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -632,6 +632,44 @@ static long vhost_vdpa_resume(struct vhost_vdpa *v)
>         return ret;
>  }
>
> +static long vhost_vdpa_new_owner(struct vhost_vdpa *v)
> +{
> +       int r;
> +       struct vhost_dev *vdev = &v->vdev;
> +       struct mm_struct *mm_old = vdev->mm;
> +       struct mm_struct *mm_new = current->mm;
> +       long pinned_vm = v->pinned_vm;
> +       unsigned long lock_limit = PFN_DOWN(rlimit(RLIMIT_MEMLOCK));
> +
> +       if (!mm_old)
> +               return -EINVAL;
> +       mmgrab(mm_old);
> +
> +       if (!v->vdpa->use_va &&
> +           pinned_vm + atomic64_read(&mm_new->pinned_vm) > lock_limit) {
> +               r = -ENOMEM;
> +               goto out;
> +       }

So this seems to allow an arbitrary process to execute this. Seems to be unsafe.

I wonder if we need to add some checks here, maybe PID or other stuff
to only allow the owner process to do this.

> +       r = vhost_vdpa_bind_mm(v, mm_new);
> +       if (r)
> +               goto out;
> +
> +       r = vhost_dev_new_owner(vdev);
> +       if (r) {
> +               vhost_vdpa_bind_mm(v, mm_old);
> +               goto out;
> +       }
> +
> +       if (!v->vdpa->use_va) {
> +               atomic64_sub(pinned_vm, &mm_old->pinned_vm);
> +               atomic64_add(pinned_vm, &mm_new->pinned_vm);
> +       }
> +
> +out:
> +       mmdrop(mm_old);
> +       return r;
> +}
> +
>  static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
>                                    void __user *argp)
>  {
> @@ -876,6 +914,9 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
>         case VHOST_VDPA_RESUME:
>                 r = vhost_vdpa_resume(v);
>                 break;
> +       case VHOST_NEW_OWNER:
> +               r = vhost_vdpa_new_owner(v);
> +               break;
>         default:
>                 r = vhost_dev_ioctl(&v->vdev, cmd, argp);
>                 if (r == -ENOIOCTLCMD)
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index b60955682474..ab40ae50552f 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -963,6 +963,21 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
>  }
>  EXPORT_SYMBOL_GPL(vhost_dev_set_owner);
>
> +/* Caller should have device mutex */
> +long vhost_dev_new_owner(struct vhost_dev *dev)
> +{
> +       if (dev->mm == current->mm)
> +               return -EBUSY;
> +
> +       if (!vhost_dev_has_owner(dev))
> +               return -EINVAL;
> +
> +       vhost_detach_mm(dev);
> +       vhost_attach_mm(dev);

This seems to do nothing unless I miss something.

Thanks