[NET]: Allow group ownership of TUN/TAP devices.

Introduce a new syscall TUNSETGROUP for group ownership setting of tap
devices. The user now is allowed to send packages if either his euid or
his egid matches the one specified via tunctl (via -u or -g
respecitvely). If both, gid and uid, are set via tunctl, both have to
match.

Signed-off-by: Guido Guenther <[e-mail]>
Signed-off-by: Jeff Dike <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 	init_waitqueue_head(&tun->read_wait);
 
 	tun->owner = -1;
+	tun->group = -1;
 
 	SET_MODULE_OWNER(dev);
 	dev->open = tun_net_open;
···
 			return -EBUSY;
 
 		/* Check permissions */
-		if (tun->owner != -1 &&
-		    current->euid != tun->owner && !capable(CAP_NET_ADMIN))
+		if (((tun->owner != -1 &&
+		      current->euid != tun->owner) ||
+		     (tun->group != -1 &&
+		      current->egid != tun->group)) &&
+		     !capable(CAP_NET_ADMIN))
 			return -EPERM;
 	}
 	else if (__dev_get_by_name(ifr->ifr_name))
···
 		tun->owner = (uid_t) arg;
 
 		DBG(KERN_INFO "%s: owner set to %d\n", tun->dev->name, tun->owner);
+		break;
+
+	case TUNSETGROUP:
+		/* Set group of the device */
+		tun->group= (gid_t) arg;
+
+		DBG(KERN_INFO "%s: group set to %d\n", tun->dev->name, tun->group);
 		break;
 
 	case TUNSETLINK:

include/linux/if_tun.h

···
 	unsigned long 		flags;
 	int			attached;
 	uid_t			owner;
+	gid_t			group;
 
 	wait_queue_head_t	read_wait;
 	struct sk_buff_head	readq;
···
 #define TUNSETPERSIST _IOW('T', 203, int) 
 #define TUNSETOWNER   _IOW('T', 204, int)
 #define TUNSETLINK    _IOW('T', 205, int)
+#define TUNSETGROUP   _IOW('T', 206, int)
 
 /* TUNSETIFF ifr flags */
 #define IFF_TUN		0x0001