ax25: fix UAF bug in ax25_send_control()

There are UAF bugs in ax25_send_control(), when we call ax25_release()
to deallocate ax25_dev. The possible race condition is shown below:

      (Thread 1)              |     (Thread 2)
ax25_dev_device_up() //(1)    |
                              | ax25_kill_by_device()
ax25_bind()          //(2)    |
ax25_connect()                | ...
 ax25->state = AX25_STATE_1   |
 ...                          | ax25_dev_device_down() //(3)

      (Thread 3)
ax25_release()                |
 ax25_dev_put()  //(4) FREE   |
 case AX25_STATE_1:           |
  ax25_send_control()         |
   alloc_skb()       //USE    |

The refcount of ax25_dev increases in position (1) and (2), and
decreases in position (3) and (4). The ax25_dev will be freed
before dereference sites in ax25_send_control().

The following is part of the report:

[  102.297448] BUG: KASAN: use-after-free in ax25_send_control+0x33/0x210
[  102.297448] Read of size 8 at addr ffff888009e6e408 by task ax25_close/602
[  102.297448] Call Trace:
[  102.303751]  ax25_send_control+0x33/0x210
[  102.303751]  ax25_release+0x356/0x450
[  102.305431]  __sock_release+0x6d/0x120
[  102.305431]  sock_close+0xf/0x20
[  102.305431]  __fput+0x11f/0x420
[  102.305431]  task_work_run+0x86/0xd0
[  102.307130]  get_signal+0x1075/0x1220
[  102.308253]  arch_do_signal_or_restart+0x1df/0xc00
[  102.308253]  exit_to_user_mode_prepare+0x150/0x1e0
[  102.308253]  syscall_exit_to_user_mode+0x19/0x50
[  102.308253]  do_syscall_64+0x48/0x90
[  102.308253]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  102.308253] RIP: 0033:0x405ae7

This patch defers the free operation of ax25_dev and net_device after
all corresponding dereference sites in ax25_release() to avoid UAF.

Fixes: 9fd75b66b8f6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 992b6e5..f5686c4 100644 (file)
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -991,10 +991,6 @@ static int ax25_release(struct socket *sock)
        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) {
@@ -1056,6 +1052,10 @@ static int ax25_release(struct socket *sock)
                sk->sk_state_change(sk);
                ax25_destroy_socket(ax25);
        }
+       if (ax25_dev) {
+               dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
+               ax25_dev_put(ax25_dev);
+       }
 
        sock->sk   = NULL;
        release_sock(sk);