Subject: Re: [PATCH 5/6] vhost_net: fix use after free of vq->ubufs

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 16 Apr 2012 16:28:41 +0300

On Mon, Apr 16, 2012 at 02:08:25PM +0800, Jason Wang wrote:
> When zerocopy socket is used, ubufs pointer were used in handle_tx()
> without any validation. This would cause NULL pointer deference after
> it has been freed in vhost_net_set_backend(). Fix this by check the
> pointer before using it.
> 
> Signed-off-by: Jason Wang <[email-masked]>

OK so it's NULL dereference and not user after free :)
Also could you clarify how does this happen pls?
Don't we always initialize ubufs when vhost_sock_zcopy is set?

> ---
>  drivers/vhost/net.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f0da2c3..29abd65 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -163,10 +163,15 @@ static void handle_tx(struct vhost_net *net)
>  	mutex_lock(&vq->mutex);
>  	vhost_disable_notify(&net->dev, vq);
>  
> +	zcopy = vhost_sock_zcopy(sock);
> +	if (zcopy && !vq->ubufs) {
> +		mutex_unlock(&vq->mutex);
> +		return;
> +	}
> +
>  	if (wmem < sock->sk->sk_sndbuf / 2)
>  		tx_poll_stop(net);
>  	hdr_size = vq->vhost_hlen;
> -	zcopy = vhost_sock_zcopy(sock);
>  
>  	for (;;) {
>  		/* Release DMAs done buffers first */