drivers: net: slip: fix NPD bug in sl_tx_timeout()

stable inclusion
from stable-v5.10.112
commit ca24c5e8f0ac3d43ec0cff29e1c861be73aff165
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5HL0X

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ca24c5e8f0ac3d43ec0cff29e1c861be73aff165

--------------------------------

[ Upstream commit ec4eb8a8 ]

When a slip driver is detaching, the slip_close() will act to
cleanup necessary resources and sl->tty is set to NULL in
slip_close(). Meanwhile, the packet we transmit is blocked,
sl_tx_timeout() will be called. Although slip_close() and
sl_tx_timeout() use sl->lock to synchronize, we don`t judge
whether sl->tty equals to NULL in sl_tx_timeout() and the
null pointer dereference bug will happen.

   (Thread 1)                 |      (Thread 2)
                              | slip_close()
                              |   spin_lock_bh(&sl->lock)
                              |   ...
...                           |   sl->tty = NULL //(1)
sl_tx_timeout()               |   spin_unlock_bh(&sl->lock)
  spin_lock(&sl->lock);       |
  ...                         |   ...
  tty_chars_in_buffer(sl->tty)|
    if (tty->ops->..) //(2)   |
    ...                       |   synchronize_rcu()

We set NULL to sl->tty in position (1) and dereference sl->tty
in position (2).

This patch adds check in sl_tx_timeout(). If sl->tty equals to
NULL, sl_tx_timeout() will goto out.
Signed-off-by: NDuoming Zhou <duoming@zju.edu.cn>
Reviewed-by: NJiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20220405132206.55291-1-duoming@zju.edu.cnSigned-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

drivers/net/slip/slip.c

@@ -468,7 +468,7 @@ static void sl_tx_timeout(struct net_device *dev, unsigned int txqueue)
 	spin_lock(&sl->lock);
 
 	if (netif_queue_stopped(dev)) {
-		if (!netif_running(dev))
+		if (!netif_running(dev) || !sl->tty)
 			goto out;
 
 		/* May be we must check transmitter timeout here ?