From ec4eb8a86ade4d22633e1da2a7d85a846b7d1798 Mon Sep 17 00:00:00 2001
From: Duoming Zhou <duoming@zju.edu.cn>
Date: Tue, 5 Apr 2022 21:22:06 +0800
Subject: [PATCH] drivers: net: slip: fix NPD bug in sl_tx_timeout()
Git-commit: ec4eb8a86ade4d22633e1da2a7d85a846b7d1798
Patch-mainline: v5.18-rc2
References: bsc#1205671 CVE-2022-41858

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: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20220405132206.55291-1-duoming@zju.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Vasant Karasulli <vkarasulli@suse.de>

---
 drivers/net/slip/slip.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c
index 88396ff99f03..6865d32270e5 100644
--- a/drivers/net/slip/slip.c
+++ b/drivers/net/slip/slip.c
@@ -469,7 +469,7 @@ 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 ?
--
2.34.1