[X][B][D][E][F][SRU][PATCH 1/1] can, slip: Protect tty->disc_data in write_wakeup and close with RCU

[Po-Hsu Lin]

From: Richard Palethorpe <rpalethorpe at suse.com>

BugLink: https://bugs.launchpad.net/bugs/1862114

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: 661f7fda21b1 ("slip: Fix deadlock in write_wakeup")
Fixes: a8e83b17536a ("slcan: Port write_wakeup deadlock fix from slip")
Reported-by: syzbot+017e491ae13c0068598a at syzkaller.appspotmail.com
Signed-off-by: Richard Palethorpe <rpalethorpe at suse.com>
Cc: Wolfgang Grandegger <wg at grandegger.com>
Cc: Marc Kleine-Budde <mkl at pengutronix.de>
Cc: "David S. Miller" <davem at davemloft.net>
Cc: Tyler Hall <tylerwhall at gmail.com>
Cc: linux-can at vger.kernel.org
Cc: netdev at vger.kernel.org
Cc: linux-kernel at vger.kernel.org
Cc: syzkaller at googlegroups.com
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 0ace17d56824165c7f4c68785d6b58971db954dd)
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 drivers/net/can/slcan.c | 12 ++++++++++--
 drivers/net/slip/slip.c | 12 ++++++++++--
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/net/can/slcan.c b/drivers/net/can/slcan.c
index cf0769a..b2e5bca 100644
--- a/drivers/net/can/slcan.c
+++ b/drivers/net/can/slcan.c
@@ -343,9 +343,16 @@ 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. */
@@ -640,10 +647,11 @@ 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 2a91c19..61d7e0d 100644
--- 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 */
-- 
2.7.4