linux-kernel - Re: [v3] nbd: fix potential NULL pointer fault in nbd_genl

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Date:   Mon, 10 Feb 2020 11:05:38 -0600
From:   Mike Christie <mchristi@...hat.com>
To:     Sun Ke <sunke32@...wei.com>, josef@...icpanda.com, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, nbd@...er.debian.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v3] nbd: fix potential NULL pointer fault in nbd_genl_disconnect

On 02/10/2020 01:32 AM, Sun Ke wrote:
> Open /dev/nbdX first, the config_refs will be 1 and
> the pointers in nbd_device are still null. Disconnect
> /dev/nbdX, then reference a null recv_workq. The
> protection by config_refs in nbd_genl_disconnect is useless.
> 
> To fix it, just add a check for a non null task_recv in
> nbd_genl_disconnect.
> 
> Signed-off-by: Sun Ke <sunke32@...wei.com>
> ---
> v1 -> v2:
> Add an omitted mutex_unlock.
> 
> v2 -> v3:
> Add nbd->config_lock, suggested by Josef.
> ---
>  drivers/block/nbd.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index b4607dd96185..870b3fd0c101 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -2008,12 +2008,20 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
>  		       index);
>  		return -EINVAL;
>  	}
> +	mutex_lock(&nbd->config_lock);
>  	if (!refcount_inc_not_zero(&nbd->refs)) {
> +		mutex_unlock(&nbd->config_lock);
>  		mutex_unlock(&nbd_index_mutex);
>  		printk(KERN_ERR "nbd: device at index %d is going down\n",
>  		       index);
>  		return -EINVAL;
>  	}
> +	if (!nbd->recv_workq) {
> +		mutex_unlock(&nbd->config_lock);
> +		mutex_unlock(&nbd_index_mutex);
> +		return -EINVAL;
> +	}
> +	mutex_unlock(&nbd->config_lock);
>  	mutex_unlock(&nbd_index_mutex);
>  	if (!refcount_inc_not_zero(&nbd->config_refs)) {
>  		nbd_put(nbd);
>

With my other patch then we will not need this right? It handles your
case by just being integrated with the existing checks in:

nbd_disconnect_and_put->nbd_clear_sock->sock_shutdown

...

static void sock_shutdown(struct nbd_device *nbd)
{

....

        if (config->num_connections == 0)
                return;


num_connections is zero for your case since we never did a
nbd_genl_disconnect so we would return here.