Subject: Re: [PATCH net-next rfc 2/2] tuntap: allow unpriveledge user to enable and disable queues

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 11 Dec 2012 14:30:12 +0200

On Tue, Dec 11, 2012 at 07:03:47PM +0800, Jason Wang wrote:
> Currently, when a file is attached to tuntap through TUNSETQUEUE, the uid/gid
> and CAP_NET_ADMIN were checked, and we use this ioctl to create and destroy
> queues. Sometimes, userspace such as qemu need to the ability to enable and
> disable a specific queue without priveledge since guest operating system may
> change the number of queues it want use.
> 
> To support this kind of ability, this patch introduce a flag enabled which is
> used to track whether the queue is enabled by userspace. And also restrict that
> only one deivce could be used for a queue to attach. With this patch, the DAC
> checking when adding queues through IFF_ATTACH_QUEUE is still done and after
> this, IFF_DETACH_QUEUE/IFF_ATTACH_QUEUE  could be used to disable/enable this
> queue.
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/tun.c |   81 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 files changed, 73 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index d593f56..43831a7 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -138,6 +138,7 @@ struct tun_file {
>  	/* only used for fasnyc */
>  	unsigned int flags;
>  	u16 queue_index;
> +	bool enabled;
>  };
>  
>  struct tun_flow_entry {
> @@ -345,9 +346,11 @@ unlock:
>  static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb)
>  {
>  	struct tun_struct *tun = netdev_priv(dev);
> +	struct tun_file *tfile;
>  	struct tun_flow_entry *e;
>  	u32 txq = 0;
>  	u32 numqueues = 0;
> +	int i;
>  
>  	rcu_read_lock();
>  	numqueues = tun->numqueues;
> @@ -366,6 +369,19 @@ static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb)
>  			txq -= numqueues;
>  	}
>  
> +	tfile = rcu_dereference(tun->tfiles[txq]);
> +	if (unlikely(!tfile->enabled))

This unlikely tag is suspicious. It should be perfectly
legal to use less queues than created.

> +		/* tun_detach() should make sure there's at least one queue
> +		 * could be used to do the tranmission.
> +		 */
> +		for (i = 0; i < numqueues; i++) {
> +			tfile = rcu_dereference(tun->tfiles[i]);
> +			if (tfile->enabled) {
> +				txq = i;
> +				break;
> +			}
> +		}
> +

Worst case this will do a linear scan over all queueus on each packet.
Instead, I think we need a list of all queues and only install
the active ones in the array.

>  	rcu_read_unlock();
>  	return txq;
>  }
> @@ -386,6 +402,36 @@ static void tun_set_real_num_queues(struct tun_struct *tun)
>  	netif_set_real_num_rx_queues(tun->dev, tun->numqueues);
>  }
>  
> +static int tun_enable(struct tun_file *tfile)
> +{
> +	if (tfile->enabled == true)

simply if (tfile->enabled)

> +		return -EINVAL;

Actually it's better to have operations be
idempotent. If it's enabled, enabling should
be a NOP not an error.

> +
> +	tfile->enabled = true;
> +	return 0;
> +}
> +
> +static int tun_disable(struct tun_file *tfile)
> +{
> +	struct tun_struct *tun = rcu_dereference_protected(tfile->tun,
> +							   lockdep_rtnl_is_held());
> +	u16 index = tfile->queue_index;
> +
> +	if (!tun)
> +		return -EINVAL;
> +
> +	if (tun->numqueues == 1)
> +		return -EINVAL;

So if there's a single queue we can't disable it,
but if there are > 1 we can disable them all.
This seems arbitrary.

> +
> +	BUG_ON(index >= tun->numqueues);
> +	tfile->enabled = false;
> +
> +	synchronize_net();
> +	tun_flow_delete_by_queue(tun, index);
> +
> +	return 0;
> +}
> +
>  static void __tun_detach(struct tun_file *tfile, bool clean)
>  {
>  	struct tun_file *ntfile;
> @@ -446,6 +492,7 @@ static void tun_detach_all(struct net_device *dev)
>  		BUG_ON(!tfile);
>  		wake_up_all(&tfile->wq.wait);
>  		rcu_assign_pointer(tfile->tun, NULL);
> +		tfile->enabled = false;
>  		--tun->numqueues;
>  	}
>  	BUG_ON(tun->numqueues != 0);
> @@ -490,6 +537,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
>  	rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
>  	sock_hold(&tfile->sk);
>  	tun->numqueues++;
> +	tfile->enabled = true;
>  
>  	tun_set_real_num_queues(tun);
>  
> @@ -672,6 +720,10 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  	if (txq >= tun->numqueues)
>  		goto drop;
>  
> +	/* Drop packet if the queue was not enabled */
> +	if (!tfile->enabled)
> +		goto drop;
> +
>  	tun_debug(KERN_INFO, tun, "tun_net_xmit %d\n", skb->len);
>  
>  	BUG_ON(!tfile);
> @@ -1010,6 +1062,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  	bool zerocopy = false;
>  	int err;
>  
> +	if (!tfile->enabled)
> +		return -EINVAL;
> +
>  	if (!(tun->flags & TUN_NO_PI)) {
>  		if ((len -= sizeof(pi)) > total_len)
>  			return -EINVAL;
> @@ -1199,6 +1254,9 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  	struct tun_pi pi = { 0, skb->protocol };
>  	ssize_t total = 0;
>  
> +	if (!tfile->enabled)
> +		return -EINVAL;
> +
>  	if (!(tun->flags & TUN_NO_PI)) {
>  		if ((len -= sizeof(pi)) < 0)
>  			return -EINVAL;
> @@ -1769,15 +1827,21 @@ static int tun_set_queue(struct file *file, struct ifreq *ifr)
>  		if (dev->netdev_ops != &tap_netdev_ops &&
>  			dev->netdev_ops != &tun_netdev_ops)
>  			ret = -EINVAL;
> -		else if (tun_not_capable(tun))
> -			ret = -EPERM;
> -		/* TUNSETIFF is needed to do permission checking */
> -		else if (tun->numqueues == 0)
> -			ret = -EPERM;
> -		else
> -			ret = tun_attach(tun, file);
> +		else {
> +			if (!rcu_dereference(tfile->tun)) {

Should be rcu_dereference_protected.

> +				if (tun_not_capable(tun) ||
> +				    tun->numqueues == 0)
> +					ret = -EPERM;
> +				else
> +					ret = tun_attach(tun, file);
> +			}
> +			else {
> +				/* FIXME: permission check? */
> +				ret = tun_enable(tfile);
> +			}
> +		}
>  	} else if (ifr->ifr_flags & IFF_DETACH_QUEUE)
> -		__tun_detach(tfile, false);
> +		tun_disable(tfile);
>  	else
>  		ret = -EINVAL;
>  
> @@ -2085,6 +2149,7 @@ static int tun_chr_open(struct inode *inode, struct file * file)
>  	tfile->socket.file = file;
>  	tfile->socket.ops = &tun_socket_ops;
>  
> +	tfile->enabled = false;
>  	sock_init_data(&tfile->socket, &tfile->sk);
>  	sk_change_net(&tfile->sk, tfile->net);
>  
> -- 
> 1.7.1