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

[Stefano Garzarella]

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;


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