Re: [PATCH 2/6] vhost: validate MMU notifier registration

[Jason Wang]

On 2019/7/23 äå5:17, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:14AM -0400, Jason Wang wrote:
> > The return value of mmu_notifier_register() is not checked in
> > vhost_vring_set_num_addr(). This will cause an out of sync between mm
> > and MMU notifier thus a double free. To solve this, introduce a
> > boolean flag to track whether MMU notifier is registered and only do
> > unregistering when it was true.
> >
> > Reported-and-tested-by:
> > syzbot+e58112d71f77113ddb7b@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> Right. This fixes the bug.
> But it's not great that simple things like
> setting vq address put pressure on memory allocator.
> Also, if we get a single during processing
> notifier register will fail, disabling optimization permanently.


Yes, but do we really care about this case. E.g we even fail for -ENOMEM 
for set owner.


> In fact, see below:
>
>
> > ---
> >   drivers/vhost/vhost.c | 19 +++++++++++++++----
> >   drivers/vhost/vhost.h |  1 +
> >   2 files changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 34c0d970bcbc..058191d5efad 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -630,6 +630,7 @@ void vhost_dev_init(struct vhost_dev *dev,
> >   	dev->iov_limit = iov_limit;
> >   	dev->weight = weight;
> >   	dev->byte_weight = byte_weight;
> > +	dev->has_notifier = false;
> >   	init_llist_head(&dev->work_list);
> >   	init_waitqueue_head(&dev->wait);
> >   	INIT_LIST_HEAD(&dev->read_list);
> > @@ -731,6 +732,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
> >   	if (err)
> >   		goto err_mmu_notifier;
> >   #endif
> > +	dev->has_notifier = true;
> >   
> >   	return 0;
> >   
> I just noticed that set owner now fails if we get a signal.
> Userspace could retry in theory but it does not:
> this is userspace abi breakage since it used to only
> fail on invalid input.


Well, at least kthread_create() and vhost_dev_alloc_iovecs() will 
allocate memory.

Thanks


> > @@ -960,7 +962,11 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >   	}
> >   	if (dev->mm) {
> >   #if VHOST_ARCH_CAN_ACCEL_UACCESS
> > -		mmu_notifier_unregister(&dev->mmu_notifier, dev->mm);
> > +		if (dev->has_notifier) {
> > +			mmu_notifier_unregister(&dev->mmu_notifier,
> > +						dev->mm);
> > +			dev->has_notifier = false;
> > +		}
> >   #endif
> >   		mmput(dev->mm);
> >   	}
> > @@ -2065,8 +2071,10 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
> >   	/* Unregister MMU notifer to allow invalidation callback
> >   	 * can access vq->uaddrs[] without holding a lock.
> >   	 */
> > -	if (d->mm)
> > +	if (d->has_notifier) {
> >   		mmu_notifier_unregister(&d->mmu_notifier, d->mm);
> > +		d->has_notifier = false;
> > +	}
> >   
> >   	vhost_uninit_vq_maps(vq);
> >   #endif
> > @@ -2086,8 +2094,11 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
> >   	if (r == 0)
> >   		vhost_setup_vq_uaddr(vq);
> >   
> > -	if (d->mm)
> > -		mmu_notifier_register(&d->mmu_notifier, d->mm);
> > +	if (d->mm) {
> > +		r = mmu_notifier_register(&d->mmu_notifier, d->mm);
> > +		if (!r)
> > +			d->has_notifier = true;
> > +	}
> >   #endif
> >   
> >   	mutex_unlock(&vq->mutex);
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 819296332913..a62f56a4cf72 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -214,6 +214,7 @@ struct vhost_dev {
> >   	int iov_limit;
> >   	int weight;
> >   	int byte_weight;
> > +	bool has_notifier;
> >   };
> >   
> >   bool vhost_exceeds_weight(struct vhost_virtqueue *vq, int pkts, int total_len);
> > --
> > 2.18.1