tun: Make tun_net_xmit atomic wrt tun_attach && tun_detach

Currently this small race allows for a packet to be received when we
detach from an tun device and still be enqueued.  Not especially
important but not what the code is trying to do.

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

1 files changed, 17 insertions, 7 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index dfbf58659771..fa93160bf522 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -115,25 +115,33 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
 {
         struct tun_file *tfile = file->private_data;
         const struct cred *cred = current_cred();
+        int err;
 
         ASSERT_RTNL();
 
-        if (tfile->tun)
-                return -EINVAL;
-
-        if (tun->tfile)
-                return -EBUSY;
-
         /* Check permissions */
         if (((tun->owner != -1 && cred->euid != tun->owner) ||
              (tun->group != -1 && cred->egid != tun->group)) &&
                 !capable(CAP_NET_ADMIN))
                 return -EPERM;
 
+        netif_tx_lock_bh(tun->dev);
+
+        err = -EINVAL;
+        if (tfile->tun)
+                goto out;
+
+        err = -EBUSY;
+        if (tun->tfile)
+                goto out;
+
+        err = 0;
         tfile->tun = tun;
         tun->tfile = tfile;
 
-        return 0;
+out:
+        netif_tx_unlock_bh(tun->dev);
+        return err;
 }
 
 static void __tun_detach(struct tun_struct *tun)
@@ -141,8 +149,10 @@ static void __tun_detach(struct tun_struct *tun)
         struct tun_file *tfile = tun->tfile;
 
         /* Detach from net device */
+        netif_tx_lock_bh(tun->dev);
         tfile->tun = NULL;
         tun->tfile = NULL;
+        netif_tx_unlock_bh(tun->dev);
 
         /* Drop read queue */
         skb_queue_purge(&tun->readq);