tun: Fix races in tun_set_iff

It is possible for two different tasks with access to the same file
descriptor to call tun_set_iff on it at the same time and race to
attach to a tap device.  Prevent this by placing all of the logic to
attach to a file descriptor in one function and testing the file
descriptor to be certain it is not already attached to another tun
device.

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

1 files changed, 32 insertions, 16 deletions

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 17923a50853..20ef14dc560 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -106,6 +106,31 @@ struct tun_struct {
 #endif
 };
 
+static int tun_attach(struct tun_struct *tun, struct file *file)
+{
+        const struct cred *cred = current_cred();
+
+        ASSERT_RTNL();
+
+        if (file->private_data)
+                return -EINVAL;
+
+        if (tun->attached)
+                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;
+
+        file->private_data = tun;
+        tun->attached = 1;
+        get_net(dev_net(tun->dev));
+
+        return 0;
+}
+
 /* TAP filterting */
 static void addr_hash_set(u32 *mask, const u8 *addr)
 {
@@ -695,7 +720,6 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 {
         struct tun_struct *tun;
         struct net_device *dev;
-        const struct cred *cred = current_cred();
         int err;
 
         dev = __dev_get_by_name(net, ifr->ifr_name);
@@ -707,17 +731,9 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
                 else
                         return -EINVAL;
 
-                if (tun->attached)
-                        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;
-                }
+                err = tun_attach(tun, file);
+                if (err < 0)
+                        return err;
         }
         else {
                 char *name;
@@ -766,6 +782,10 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
                 err = register_netdevice(tun->dev);
                 if (err < 0)
                         goto err_free_dev;
+
+                err = tun_attach(tun, file);
+                if (err < 0)
+                        goto err_free_dev;
         }
 
         DBG(KERN_INFO "%s: tun_set_iff\n", tun->dev->name);
@@ -785,10 +805,6 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
         else
                 tun->flags &= ~TUN_VNET_HDR;
 
-        file->private_data = tun;
-        tun->attached = 1;
-        get_net(dev_net(tun->dev));
-
         /* Make sure persistent devices do not get stuck in
          * xoff state.
          */