Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[Josef Bacik <josef@toxicpanda.com>]

On 1/17/20 6:50 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.
> 
> To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl
> 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.

I don't doubt what you are seeing, but what exactly are we NULL pointer 
dereferencing?  I can't quite figure it out from the steps.

> 
> 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);

The else is superfluous here.  Thanks,

Josef