tun: add IFF_TUN_EXCL flag to avoid opening a persistent device.

When creating a certain types of VPN, NetworkManager will first attempt
to find an available tun device by iterating through 'vpn%d' until it
finds one that isn't already busy. Then it'll set that to be persistent
and owned by the otherwise unprivileged user that the VPN dæmon itself
runs as.

There's a race condition here -- during the period where the vpn%d
device is created and we're waiting for the VPN dæmon to actually
connect and use it, if we try to create _another_ device we could end up
re-using the same one -- because trying to open it again doesn't get
-EBUSY as it would while it's _actually_ busy.

So solve this, we add an IFF_TUN_EXCL flag which causes tun_set_iff() to
fail if it would be opening an existing persistent tundevice -- so that
we can make sure we're getting an entirely _new_ device.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 589f0ca668d6a45f9100af7d39f46ef63929439a..94622e5fb936d8263f5c7bc5bb50c7f433d7230c 100644 (file)
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -874,6 +874,8 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 
        dev = __dev_get_by_name(net, ifr->ifr_name);
        if (dev) {
+               if (ifr->ifr_flags & IFF_TUN_EXCL)
+                       return -EBUSY;
                if ((ifr->ifr_flags & IFF_TUN) && dev->netdev_ops == &tun_netdev_ops)
                        tun = netdev_priv(dev);
                else if ((ifr->ifr_flags & IFF_TAP) && dev->netdev_ops == &tap_netdev_ops)

diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h
index 049d6c9428db60b78dbb49a595e774da001effb0..915ba5789f0eb62c02c1c2664e7ef66b1e631ab5 100644 (file)
--- a/include/linux/if_tun.h
+++ b/include/linux/if_tun.h
@@ -55,6 +55,7 @@
 #define IFF_NO_PI      0x1000
 #define IFF_ONE_QUEUE  0x2000
 #define IFF_VNET_HDR   0x4000
+#define IFF_TUN_EXCL   0x8000
 
 /* Features for GSO (TUNSETOFFLOAD). */
 #define TUN_F_CSUM     0x01    /* You can hand me unchecksummed packets. */