tun: Fix unregister race

It is possible for tun_chr_close to race with dellink on the
a tun device.  In which case if __tun_get runs before dellink
but dellink runs before tun_chr_close calls unregister_netdevice
we will attempt to unregister the netdevice after it is already
gone.  

The two cases are already serialized on the rtnl_lock, so I have
gone for the cheap simple fix of moving rtnl_lock to cover __tun_get
in tun_chr_close.  Eliminating the possibility of the tun device
being unregistered between __tun_get and unregister_netdevice in
tun_chr_close.

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Tested-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d83ccb8c982b..811d3517fce0 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1322,21 +1322,22 @@ static int tun_chr_open(struct inode *inode, struct file * file)
 static int tun_chr_close(struct inode *inode, struct file *file)
 {
         struct tun_file *tfile = file->private_data;
-        struct tun_struct *tun = __tun_get(tfile);
+        struct tun_struct *tun;
 
 
+        rtnl_lock();
+        tun = __tun_get(tfile);
         if (tun) {
                 DBG(KERN_INFO "%s: tun_chr_close\n", tun->dev->name);
 
-                rtnl_lock();
                 __tun_detach(tun);
 
                 /* If desireable, unregister the netdevice. */
                 if (!(tun->flags & TUN_PERSIST))
                         unregister_netdevice(tun->dev);
 
-                rtnl_unlock();
         }
+        rtnl_unlock();
 
         tun = tfile->tun;
         if (tun)