[PATCH 4.19 51/53] ax25: Fix NULL pointer dereferences in ax25 timers

Greg Kroah-Hartman
2022-04-26T08:40:18Z

From: Duoming Zhou <duoming@zju.edu.cn>

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: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
[OP: backport to 4.19: adjust context]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ax25/af_ax25.c | 4 ++--
net/ax25/ax25_subr.c | 20 ++++++++++++++------
2 files changed, 16 insertions(+), 8 deletions(-)

--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -92,20 +92,20 @@ again:
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);
--- a/net/ax25/ax25_subr.c
+++ b/net/ax25/ax25_subr.c
@@ -264,12 +264,20 @@ void ax25_disconnect(ax25_cb *ax25, int
{
ax25_clear_queues(ax25);

- 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);
+ 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;