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

[Jason Wang <jasowang@redhat.com>]

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 <jasowang@redhat.com>
---
 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))
+		/* 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;
+			}
+		}
+
 	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)
+		return -EINVAL;
+
+	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;
+
+	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)) {
+				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