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Commit 40cb8b3b authored by Duoming Zhou's avatar Duoming Zhou Committed by Greg Kroah-Hartman
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ax25: Fix NULL pointer dereferences in ax25 timers 



commit fc6d01ff9ef03b66d4a3a23b46fc3c3d8cf92009 upstream.

The previous commit 7ec02f5ac8a5 ("ax25: fix NPD bug in ax25_disconnect")
move ax25_disconnect into lock_sock() in order to prevent NPD bugs. But
there are race conditions that may lead to null pointer dereferences in
ax25_heartbeat_expiry(), ax25_t1timer_expiry(), ax25_t2timer_expiry(),
ax25_t3timer_expiry() and ax25_idletimer_expiry(), when we use
ax25_kill_by_device() to detach the ax25 device.

One of the race conditions that cause null pointer dereferences can be
shown as below:

      (Thread 1)                    |      (Thread 2)
ax25_connect()                      |
 ax25_std_establish_data_link()     |
  ax25_start_t1timer()              |
   mod_timer(&ax25->t1timer,..)     |
                                    | ax25_kill_by_device()
   (wait a time)                    |  ...
                                    |  s->ax25_dev = NULL; //(1)
   ax25_t1timer_expiry()            |
    ax25->ax25_dev->values[..] //(2)|  ...
     ...                            |

We set null to ax25_cb->ax25_dev in position (1) and dereference
the null pointer in position (2).

The corresponding fail log is shown below:
===============================================================
BUG: kernel NULL pointer dereference, address: 0000000000000050
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.17.0-rc6-00794-g45690b7d0
RIP: 0010:ax25_t1timer_expiry+0x12/0x40
...
Call Trace:
 call_timer_fn+0x21/0x120
 __run_timers.part.0+0x1ca/0x250
 run_timer_softirq+0x2c/0x60
 __do_softirq+0xef/0x2f3
 irq_exit_rcu+0xb6/0x100
 sysvec_apic_timer_interrupt+0xa2/0xd0
...

This patch moves ax25_disconnect() before s->ax25_dev = NULL
and uses del_timer_sync() to delete timers in ax25_disconnect().
If ax25_disconnect() is called by ax25_kill_by_device() or
ax25->ax25_dev is NULL, the reason in ax25_disconnect() will be
equal to ENETUNREACH, it will wait all timers to stop before we
set null to s->ax25_dev in ax25_kill_by_device().

Fixes: 7ec02f5ac8a5 ("ax25: fix NPD bug in ax25_disconnect")
Signed-off-by: default avatarDuoming Zhou <duoming@zju.edu.cn>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
[OP: backport to 5.4: adjust context]
Signed-off-by: default avatarOvidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent d2be5b56

net/ax25/af_ax25.c

+2 −2
Original line number Diff line number Diff line
@@ -89,20 +89,20 @@ static void ax25_kill_by_device(struct net_device *dev) 
			sk = s->sk;

			if (!sk) {

				spin_unlock_bh(&ax25_list_lock);

				s->ax25_dev = NULL;

				ax25_disconnect(s, ENETUNREACH);

				s->ax25_dev = NULL;

				spin_lock_bh(&ax25_list_lock);

				goto again;

			}

			sock_hold(sk);

			spin_unlock_bh(&ax25_list_lock);

			lock_sock(sk);

			ax25_disconnect(s, ENETUNREACH);

			s->ax25_dev = NULL;

			if (sk->sk_socket) {

				dev_put(ax25_dev->dev);

				ax25_dev_put(ax25_dev);

			}

			ax25_disconnect(s, ENETUNREACH);

			release_sock(sk);

			spin_lock_bh(&ax25_list_lock);

			sock_put(sk);

net/ax25/ax25_subr.c

+14 −6
Original line number Diff line number Diff line
@@ -261,12 +261,20 @@ void ax25_disconnect(ax25_cb *ax25, int reason) 
{

	ax25_clear_queues(ax25);



	if (reason == ENETUNREACH) {

		del_timer_sync(&ax25->timer);

		del_timer_sync(&ax25->t1timer);

		del_timer_sync(&ax25->t2timer);

		del_timer_sync(&ax25->t3timer);

		del_timer_sync(&ax25->idletimer);

	} else {

		if (!ax25->sk || !sock_flag(ax25->sk, SOCK_DESTROY))

			ax25_stop_heartbeat(ax25);

		ax25_stop_t1timer(ax25);

		ax25_stop_t2timer(ax25);

		ax25_stop_t3timer(ax25);

		ax25_stop_idletimer(ax25);

	}



	ax25->state = AX25_STATE_0;