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Subject: Re: [PATCH] nbd: fix potential NULL pointer fault in connect and
 disconnect process
To:     Sun Ke <sunke32@huawei.com>, josef@toxicpanda.com, axboe@kernel.dk
References: <20200117115005.37006-1-sunke32@huawei.com>
Cc:     linux-block@vger.kernel.org, nbd@other.debian.org,
        linux-kernel@vger.kernel.org, Xiubo Li <xiubli@redhat.com>
From:   Mike Christie <mchristi@redhat.com>
Message-ID: <5E21EF96.1010204@redhat.com>
Date:   Fri, 17 Jan 2020 11:32:06 -0600
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On 01/17/2020 05:50 AM, Sun Ke wrote:
> Connect and disconnect a nbd device repeatedly, will cause
> NULL pointer fault.
> 
> It will appear by the steps:
> 1. Connect the nbd device and disconnect it, but now nbd device
>    is not disconnected totally.
> 2. Connect the same nbd device again immediately, it will fail
>    in nbd_start_device with a EBUSY return value.
> 3. Wait a second to make sure the last config_refs is reduced
>    and run nbd_config_put to disconnect the nbd device totally.
> 4. Start another process to open the nbd_device, config_refs
>    will increase and at the same time disconnect it.

Just to make sure I understood this, for step 4 the process is doing:

open(/dev/nbdX);
ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)

?

There is no successful NBD_DO_IT / nbd_genl_connect between the open and
disconnect calls at step #4, because it would normally be done at #2 and
that failed. nbd_disconnect_and_put could then reference a null
recv_workq. If we are also racing with a close() then that could free
the device/config from under nbd_disconnect_and_put.

> 
> To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl

I'm not sure if we need the new bit. We could just add a check for a non
null task_recv in nbd_genl_disconnect like how nbd_start_device and
nbd_genl_disconnect do.

The new bit might be more clear which is nice. If we got this route,
should the new bit be a runtime_flag like other device state bits?


> and nbd_genl_connect if nbd device is started successfully.
> Clear it in nbd_config_put. Test it in nbd_genl_disconnect and
> nbd_genl_reconfigure.
> 
> Signed-off-by: Sun Ke <sunke32@huawei.com>
> ---
>  drivers/block/nbd.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index b4607dd96185..ddd364e208ab 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -83,6 +83,7 @@ struct link_dead_args {
>  
>  #define NBD_DESTROY_ON_DISCONNECT	0
>  #define NBD_DISCONNECT_REQUESTED	1
> +#define NBD_HAS_STARTED				2
>  
>  struct nbd_config {
>  	u32 flags;
> @@ -1215,6 +1216,7 @@ static void nbd_config_put(struct nbd_device *nbd)
>  		nbd->disk->queue->limits.discard_alignment = 0;
>  		blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
>  		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> +		clear_bit(NBD_HAS_STARTED, &nbd->flags);
>  
>  		mutex_unlock(&nbd->config_lock);
>  		nbd_put(nbd);
> @@ -1290,6 +1292,8 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
>  	ret = nbd_start_device(nbd);
>  	if (ret)
>  		return ret;
> +	else
> +		set_bit(NBD_HAS_STARTED, &nbd->flags);
>  
>  	if (max_part)
>  		bdev->bd_invalidated = 1;
> @@ -1961,6 +1965,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>  	mutex_unlock(&nbd->config_lock);
>  	if (!ret) {
>  		set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
> +		set_bit(NBD_HAS_STARTED, &nbd->flags);
>  		refcount_inc(&nbd->config_refs);
>  		nbd_connect_reply(info, nbd->index);
>  	}
> @@ -2008,6 +2013,14 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
>  		       index);
>  		return -EINVAL;
>  	}
> +
> +	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
> +		mutex_unlock(&nbd_index_mutex);
> +		printk(KERN_ERR "nbd: device at index %d failed to start\n",
> +		       index);
> +		return -EBUSY;
> +	}
> +
>  	if (!refcount_inc_not_zero(&nbd->refs)) {
>  		mutex_unlock(&nbd_index_mutex);
>  		printk(KERN_ERR "nbd: device at index %d is going down\n",
> @@ -2049,6 +2062,14 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
>  		       index);
>  		return -EINVAL;
>  	}
> +
> +	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
> +		mutex_unlock(&nbd_index_mutex);
> +		printk(KERN_ERR "nbd: device at index %d failed to start\n",
> +		       index);
> +		return -EBUSY;
> +	}
> +
>  	if (!refcount_inc_not_zero(&nbd->refs)) {
>  		mutex_unlock(&nbd_index_mutex);
>  		printk(KERN_ERR "nbd: device at index %d is going down\n",
>