[PATCH V5 0/2] Fix refcount leak and NPD bugs in ax25

[PATCH V5 0/2] Fix refcount leak and NPD bugs in ax25

[Duoming Zhou]

The first patch fixes refcount leak in ax25 that could cause 
ax25-ex-connected-session-now-listening-state-bug.

The second patch fixes NPD bugs in ax25 timers.

Duoming Zhou (2):
  ax25: Fix refcount leaks caused by ax25_cb_del()
  ax25: Fix NULL pointer dereferences in ax25 timers

 net/ax25/af_ax25.c   | 18 +++++++++++++-----
 net/ax25/ax25_subr.c | 20 ++++++++++++++------
 2 files changed, 27 insertions(+), 11 deletions(-)

-- 
2.17.1

[PATCH V5 1/2] ax25: Fix refcount leaks caused by ax25_cb_del()

[Duoming Zhou]

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>
---
Changes in V5:
  - Use sk->sk_socket to check in ax25_kill_by_device().

 net/ax25/af_ax25.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 6bd09718077..cf8847cfc66 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -98,8 +98,10 @@ static void ax25_kill_by_device(struct net_device *dev)
 			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) {
-- 
2.17.1

[PATCH V6 2/2] ax25: Fix NULL pointer dereferences in ax25 timers

[Duoming Zhou]

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>
---
Changes in V6:
  - Add Fixes tag.
  - Make commit message more clearer.

 net/ax25/af_ax25.c   |  4 ++--
 net/ax25/ax25_subr.c | 20 ++++++++++++++------
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index cf8847cfc66..992b6e5d85d 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -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_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);
diff --git a/net/ax25/ax25_subr.c b/net/ax25/ax25_subr.c
index 15ab812c4fe..3a476e4f6cd 100644
--- a/net/ax25/ax25_subr.c
+++ b/net/ax25/ax25_subr.c
@@ -261,12 +261,20 @@ void ax25_disconnect(ax25_cb *ax25, int reason)
 {
 	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;
 
-- 
2.17.1

Re: [PATCH V5 0/2] Fix refcount leak and NPD bugs in ax25

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (master)
by David S. Miller <davem@davemloft.net>:

On Fri, 18 Mar 2022 08:54:03 +0800 you wrote:
> The first patch fixes refcount leak in ax25 that could cause
> ax25-ex-connected-session-now-listening-state-bug.
> 
> The second patch fixes NPD bugs in ax25 timers.
> 
> Duoming Zhou (2):
>   ax25: Fix refcount leaks caused by ax25_cb_del()
>   ax25: Fix NULL pointer dereferences in ax25 timers
> 
> [...]

Here is the summary with links:
  - [V5,1/2] ax25: Fix refcount leaks caused by ax25_cb_del()
    https://git.kernel.org/netdev/net/c/9fd75b66b8f6
  - [V6,2/2] ax25: Fix NULL pointer dereferences in ax25 timers
    https://git.kernel.org/netdev/net/c/fc6d01ff9ef0

You are awesome, thank you!
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