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

Michael S. Tsirkin
2022-03-02T13:35:23Z

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@redhat.com>
>
> [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.

> >
> > Cc: <stable@vger.kernel.org>
> > Reported-by: syzbot+adc3cb32385586bec859@syzkaller.appspotmail.com
> > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > ---
> > drivers/vhost/vhost.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 59edb5a1ffe28..bbaff6a5e21b8 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -693,6 +693,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> > int i;
> >
> > for (i = 0; i < dev->nvqs; ++i) {
> > + mutex_lock(&dev->vqs[i]->mutex);
> > if (dev->vqs[i]->error_ctx)
> > eventfd_ctx_put(dev->vqs[i]->error_ctx);
> > if (dev->vqs[i]->kick)
> > @@ -700,6 +701,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> > if (dev->vqs[i]->call_ctx.ctx)
> > eventfd_ctx_put(dev->vqs[i]->call_ctx.ctx);
> > vhost_vq_reset(dev, dev->vqs[i]);
> > + mutex_unlock(&dev->vqs[i]->mutex);
> > }
> > vhost_dev_free_iovecs(dev);
> > if (dev->log_ctx)
> > --
> > 2.35.1.574.g5d30c73bfb-goog
> >