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

From: Greg KH <[email-masked]>
Date: Mon, 7 Mar 2022 20:33:27 +0100

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: <[email-masked]>
> Reported-by: [email-masked]
> Signed-off-by: Lee Jones <[email-masked]>
> ---
>  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