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Subject[PATCH net 2/2] ax25: Fix NULL pointer dereferences in ax25 timers
Date
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 uses ax25_disconnect() to delete timers before we set null to
ax25_cb->ax25_dev in ax25_kill_by_device(). Function ax25_disconnect()
will not return until all timers are stopped, because we have changed
del_timer() to del_timer_sync(). What`s more, we add condition check in
ax25_destroy_socket(), because ax25_stop_heartbeat() will not return,
if there is still heartbeat.

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
net/ax25/af_ax25.c | 7 ++++---
net/ax25/ax25_timer.c | 10 +++++-----
2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index fc564b87acc..46e4d0cc878 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -90,18 +90,18 @@ 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;
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);
sock_put(sk);
@@ -310,7 +310,8 @@ void ax25_destroy_socket(ax25_cb *ax25)

ax25_cb_del(ax25);

- ax25_stop_heartbeat(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);
diff --git a/net/ax25/ax25_timer.c b/net/ax25/ax25_timer.c
index 85865ebfdfa..99af3d1aeec 100644
--- a/net/ax25/ax25_timer.c
+++ b/net/ax25/ax25_timer.c
@@ -78,27 +78,27 @@ void ax25_start_idletimer(ax25_cb *ax25)

void ax25_stop_heartbeat(ax25_cb *ax25)
{
- del_timer(&ax25->timer);
+ del_timer_sync(&ax25->timer);
}

void ax25_stop_t1timer(ax25_cb *ax25)
{
- del_timer(&ax25->t1timer);
+ del_timer_sync(&ax25->t1timer);
}

void ax25_stop_t2timer(ax25_cb *ax25)
{
- del_timer(&ax25->t2timer);
+ del_timer_sync(&ax25->t2timer);
}

void ax25_stop_t3timer(ax25_cb *ax25)
{
- del_timer(&ax25->t3timer);
+ del_timer_sync(&ax25->t3timer);
}

void ax25_stop_idletimer(ax25_cb *ax25)
{
- del_timer(&ax25->idletimer);
+ del_timer_sync(&ax25->idletimer);
}

int ax25_t1timer_running(ax25_cb *ax25)
--
2.17.1
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