can, slip: Protect tty->disc_data in write_wakeup and close with RCU

write_wakeup can happen in parallel with close/hangup where tty->disc_data
is set to NULL and the netdevice is freed thus also freeing
disc_data. write_wakeup accesses disc_data so we must prevent close from
freeing the netdev while write_wakeup has a non-NULL view of
tty->disc_data.

We also need to make sure that accesses to disc_data are atomic. Which can
all be done with RCU.

This problem was found by Syzkaller on SLCAN, but the same issue is
reproducible with the SLIP line discipline using an LTP test based on the
Syzkaller reproducer.

A fix which didn't use RCU was posted by Hillf Danton.

Fixes: 2d2c15e49c75 ("slip: Fix deadlock in write_wakeup")
Fixes: 14dea43b5953 ("slcan: Port write_wakeup deadlock fix from slip")
Reported-by: syzbot+017e491ae13c0068598a@syzkaller.appspotmail.com
Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Tyler Hall <tylerwhall@gmail.com>
Cc: linux-can@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: syzkaller@googlegroups.com
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/can/slcan.c b/drivers/net/can/slcan.c
index 2e57122f02fb079c94e8e1fea197fe40286c133d..2f5c287eac95761f1a57110078875f6615b944f4 100644 (file)
--- a/drivers/net/can/slcan.c
+++ b/drivers/net/can/slcan.c
@@ -344,9 +344,16 @@ static void slcan_transmit(struct work_struct *work)
  */
 static void slcan_write_wakeup(struct tty_struct *tty)
 {
-       struct slcan *sl = tty->disc_data;
+       struct slcan *sl;
+
+       rcu_read_lock();
+       sl = rcu_dereference(tty->disc_data);
+       if (!sl)
+               goto out;
 
        schedule_work(&sl->tx_work);
+out:
+       rcu_read_unlock();
 }
 
 /* Send a can_frame to a TTY queue. */
@@ -644,10 +651,11 @@ static void slcan_close(struct tty_struct *tty)
                return;
 
        spin_lock_bh(&sl->lock);
-       tty->disc_data = NULL;
+       rcu_assign_pointer(tty->disc_data, NULL);
        sl->tty = NULL;
        spin_unlock_bh(&sl->lock);
 
+       synchronize_rcu();
        flush_work(&sl->tx_work);
 
        /* Flush network side */

diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c
index 2a91c192659ffb2762e9d1f821976ce362ac1272..61d7e0d1d77db8deaa8c7260ab0dde4945dae950 100644 (file)
--- a/drivers/net/slip/slip.c
+++ b/drivers/net/slip/slip.c
@@ -452,9 +452,16 @@ static void slip_transmit(struct work_struct *work)
  */
 static void slip_write_wakeup(struct tty_struct *tty)
 {
-       struct slip *sl = tty->disc_data;
+       struct slip *sl;
+
+       rcu_read_lock();
+       sl = rcu_dereference(tty->disc_data);
+       if (!sl)
+               goto out;
 
        schedule_work(&sl->tx_work);
+out:
+       rcu_read_unlock();
 }
 
 static void sl_tx_timeout(struct net_device *dev)
@@ -882,10 +889,11 @@ static void slip_close(struct tty_struct *tty)
                return;
 
        spin_lock_bh(&sl->lock);
-       tty->disc_data = NULL;
+       rcu_assign_pointer(tty->disc_data, NULL);
        sl->tty = NULL;
        spin_unlock_bh(&sl->lock);
 
+       synchronize_rcu();
        flush_work(&sl->tx_work);
 
        /* VSV = very important to remove timers */