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

[Jason Wang]

On Thu, Jul 18, 2024 at 2:28 AM Steven Sistare
<steven.sistare@xxxxxxxxxx> wrote:
>
> On 7/16/2024 1:16 AM, Jason Wang wrote:
> > On Mon, Jul 15, 2024 at 10:27 PM Steven Sistare
> > <steven.sistare@xxxxxxxxxx> wrote:
> >>
> >> On 7/14/2024 10:26 PM, Jason Wang wrote:
> >>> 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.
> >>
> >> The original owner must send the file descriptor to the new owner.
> >
> > This seems not to be in the steps you put in the cover letter.
>
> It's there:
>    "The vdpa device descriptor, fd, remains open across the exec."
>
> But, I can say more about how fd visibility constitutes permission to changer
> owner in this commit message.

Yes, that would be great.

>
> >> That constitutes permission to take ownership.
> >
> > This seems like a relaxed version of the reset_owner:
> >
> > Currently, reset_owner have the following check:
>
> Not relaxed, just different.  A process cannot do anything with fd if it
> is not the owner, *except* for becoming the new owner.  Holding the fd is
> like holding a key.

I meant it kind of "defeats" the effort of VHOST_NET_RESET_OWNER ...

Thanks

>
> - Steve
>
> > /* Caller should have device mutex */
> > long vhost_dev_check_owner(struct vhost_dev *dev)
> > {
> >          /* Are you the owner? If not, I don't think you mean to do that */
> >          return dev->mm == current->mm ? 0 : -EPERM;
> > }
> > EXPORT_SYMBOL_GPL(vhost_dev_check_owner);
> >
> > It means even if the fd is passed to some other process, the reset
> > owner won't work there.
> >
> > Thanks
> >
> >>
> >>>> +       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.
> >>
> >> vhost_detach mm drops dev->mm.
> >> vhost_attach_mm grabs current->mm.
> >>
> >> - Steve
> >>
> >
>