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

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Date:   Wed, 2 Mar 2022 08:30:54 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     jasowang@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, stable@...r.kernel.org,
        syzbot+adc3cb32385586bec859@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared
 whilst still in use

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
> 
> Cc: <stable@...r.kernel.org>
> Reported-by: syzbot+adc3cb32385586bec859@...kaller.appspotmail.com
> Signed-off-by: Lee Jones <lee.jones@...aro.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);
>  	}

So this is a mitigation plan but the bug is still there though
we don't know exactly what it is.  I would prefer adding something like
WARN_ON(mutex_is_locked(vqs[i]->mutex) here - does this make sense?



>  	vhost_dev_free_iovecs(dev);
>  	if (dev->log_ctx)
> -- 
> 2.35.1.574.g5d30c73bfb-goog