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

Greg KH
2022-03-07T19:33:37Z

On Mon, Mar 07, 2022 at 07:17:57PM +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 here is take the same lock
> during virtqueue clean-up and we mitigate the reported issues.
>
> Also WARN() as a precautionary measure. The purpose of this is to
> capture possible future race conditions which may pop up over time.
>
> Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00
>
> 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 | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 59edb5a1ffe28..ef7e371e3e649 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -693,6 +693,15 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> int i;
>
> for (i = 0; i < dev->nvqs; ++i) {
> + /* No workers should run here by design. However, races have
> + * previously occurred where drivers have been unable to flush
> + * all work properly prior to clean-up. Without a successful
> + * flush the guest will malfunction, but avoiding host memory
> + * corruption in those cases does seem preferable.
> + */
> + WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));

So you are trading one syzbot triggered issue for another one in the
future? :)

If this ever can happen, handle it, but don't log it with a WARN_ON() as
that will trigger the panic-on-warn boxes, as well as syzbot. Unless
you want that to happen?

And what happens if the mutex is locked _RIGHT_ after you checked it?
You still have a race...

thanks,

greg k-h