netdev - [PATCH net] ax25: fix UAF bug in ax25_send

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Date:   Mon, 28 Mar 2022 11:57:26 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-hams@...r.kernel.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        pabeni@...hat.com, kuba@...nel.org, davem@...emloft.net,
        ralf@...ux-mips.org, jreuter@...na.de, dan.carpenter@...cle.com,
        Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH net] ax25: fix UAF bug in ax25_send_control()

If we don`t called ax25_bind() before ax25_kill_by_device(), the free
operation of ax25_dev and net_device in ax25_kill_by_device() will not
be executed, because we could not find ax25_cb in ax25_list.

Then, we call ax25_bind() and ax25_connect() to set AX25_STATE_1 to
ax25->state.

Finally, when we call ax25_release(), ax25_dev and net_device will be
freed before dereference sites in ax25_send_control().

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()                |
 dev_put_track() //(4) FREE   |
 ax25_dev_put()  //(4) FREE   |
 case AX25_STATE_1:           |
  ax25_send_control()         |
   alloc_skb()       //USE    |

The refcount of ax25_dev and net_device increase in position (1) and (2),
and decrease in position (3) and (4).

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 reorders 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@....edu.cn>
---
 net/ax25/af_ax25.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 992b6e5d85d..f5686c463bc 100644
--- 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);
-- 
2.17.1