ax25: Fix refcount leaks caused by ax25_cb_del()

The previous commit d01ffb9eee4a ("ax25: add refcount in ax25_dev to
avoid UAF bugs") and commit feef318c855a ("ax25: fix UAF bugs of
net_device caused by rebinding operation") increase the refcounts of
ax25_dev and net_device in ax25_bind() and decrease the matching refcounts
in ax25_kill_by_device() in order to prevent UAF bugs, but there are
reference count leaks.

The root cause of refcount leaks is shown below:

     (Thread 1)                      |      (Thread 2)
ax25_bind()                          |
 ...                                 |
 ax25_addr_ax25dev()                 |
  ax25_dev_hold()   //(1)            |
  ...                                |
 dev_hold_track()   //(2)            |
 ...                                 | ax25_destroy_socket()
                                     |  ax25_cb_del()
                                     |   ...
                                     |   hlist_del_init() //(3)
                                     |
                                     |
     (Thread 3)                      |
ax25_kill_by_device()                |
 ...                                 |
 ax25_for_each(s, &ax25_list) {      |
  if (s->ax25_dev == ax25_dev) //(4) |
   ...                               |

Firstly, we use ax25_bind() to increase the refcount of ax25_dev in
position (1) and increase the refcount of net_device in position (2).
Then, we use ax25_cb_del() invoked by ax25_destroy_socket() to delete
ax25_cb in hlist in position (3) before calling ax25_kill_by_device().
Finally, the decrements of refcounts in ax25_kill_by_device() will not
be executed, because no s->ax25_dev equals to ax25_dev in position (4).

This patch adds decrements of refcounts in ax25_release() and use
lock_sock() to do synchronization. If refcounts decrease in ax25_release(),
the decrements of refcounts in ax25_kill_by_device() will not be
executed and vice versa.

Fixes: d01ffb9eee4a ("ax25: add refcount in ax25_dev to avoid UAF bugs")
Fixes: 87563a043cef ("ax25: fix reference count leaks of ax25_dev")
Fixes: feef318c855a ("ax25: fix UAF bugs of net_device caused by rebinding operation")
Reported-by: Thomas Osterried <thomas@osterried.de>
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 6bd0971..cf8847c 100644 (file)
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -98,8 +98,10 @@ again:
                        spin_unlock_bh(&ax25_list_lock);
                        lock_sock(sk);
                        s->ax25_dev = NULL;
-                       dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
-                       ax25_dev_put(ax25_dev);
+                       if (sk->sk_socket) {
+                               dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
+                               ax25_dev_put(ax25_dev);
+                       }
                        ax25_disconnect(s, ENETUNREACH);
                        release_sock(sk);
                        spin_lock_bh(&ax25_list_lock);
@@ -979,14 +981,20 @@ static int ax25_release(struct socket *sock)
 {
        struct sock *sk = sock->sk;
        ax25_cb *ax25;
+       ax25_dev *ax25_dev;
 
        if (sk == NULL)
                return 0;
 
        sock_hold(sk);
-       sock_orphan(sk);
        lock_sock(sk);
+       sock_orphan(sk);
        ax25 = sk_to_ax25(sk);
+       ax25_dev = ax25->ax25_dev;
+       if (ax25_dev) {
+               dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
+               ax25_dev_put(ax25_dev);
+       }
 
        if (sk->sk_type == SOCK_SEQPACKET) {
                switch (ax25->state) {