Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use

[Michael S. Tsirkin]

On Wed, Mar 02, 2022 at 03:11:21PM +0100, Stefano Garzarella wrote:
> On Wed, Mar 02, 2022 at 08:35:08AM -0500, Michael S. Tsirkin wrote:
> > On Wed, Mar 02, 2022 at 10:34:46AM +0100, Stefano Garzarella wrote:
> > > On Wed, Mar 02, 2022 at 07:54:21AM +0000, Lee Jones wrote:
> > > > vhost_vsock_handle_tx_kick() already holds the mutex during its call
> > > > to vhost_get_vq_desc().  All we have to do is take the same lock
> > > > during virtqueue clean-up and we mitigate the reported issues.
> > > >
> > > > Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00
> > > 
> > > This issue is similar to [1] that should be already fixed upstream by [2].
> > > 
> > > However I think this patch would have prevented some issues, because
> > > vhost_vq_reset() sets vq->private to NULL, preventing the worker from
> > > running.
> > > 
> > > Anyway I think that when we enter in vhost_dev_cleanup() the worker should
> > > be already stopped, so it shouldn't be necessary to take the mutex. But in
> > > order to prevent future issues maybe it's better to take them, so:
> > > 
> > > Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > 
> > > [1]
> > > https://syzkaller.appspot.com/bug?id=993d8b5e64393ed9e6a70f9ae4de0119c605a822
> > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a58da53ffd70294ebea8ecd0eb45fd0d74add9f9
> > 
> > 
> > Right. I want to queue this but I would like to get a warning
> > so we can detect issues like [2] before they cause more issues.
> 
> I agree, what about moving the warning that we already have higher up, right
> at the beginning of the function?
> 
> I mean something like this:
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 59edb5a1ffe2..1721ff3f18c0 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -692,6 +692,8 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>  {
>         int i;
> +       WARN_ON(!llist_empty(&dev->work_list));
> +
>         for (i = 0; i < dev->nvqs; ++i) {
>                 if (dev->vqs[i]->error_ctx)
>                         eventfd_ctx_put(dev->vqs[i]->error_ctx);
> @@ -712,7 +714,6 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>         dev->iotlb = NULL;
>         vhost_clear_msg(dev);
>         wake_up_interruptible_poll(&dev->wait, EPOLLIN | EPOLLRDNORM);
> -       WARN_ON(!llist_empty(&dev->work_list));
>         if (dev->worker) {
>                 kthread_stop(dev->worker);
>                 dev->worker = NULL;
> 

Hmm I'm not sure why it matters.

> And maybe we can also check vq->private and warn in the loop, because the
> work_list may be empty if the device is doing nothing.
> 
> Thanks,
> Stefano