[PATCH 4.4 021/135] tty: hvc: Fix data abort due to race in hvc_open

[Sasha Levin]

From: Raghavendra Rao Ananta <rananta@xxxxxxxxxxxxxx>

[ Upstream commit e2bd1dcbe1aa34ff5570b3427c530e4332ecf0fe ]

Potentially, hvc_open() can be called in parallel when two tasks calls
open() on /dev/hvcX. In such a scenario, if the hp->ops->notifier_add()
callback in the function fails, where it sets the tty->driver_data to
NULL, the parallel hvc_open() can see this NULL and cause a memory abort.
Hence, serialize hvc_open and check if tty->private_data is NULL before
proceeding ahead.

The issue can be easily reproduced by launching two tasks simultaneously
that does nothing but open() and close() on /dev/hvcX.
For example:
$ ./simple_open_close /dev/hvc0 & ./simple_open_close /dev/hvc0 &

Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200428032601.22127-1-rananta@xxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/tty/hvc/hvc_console.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index acf6d143c7531..81f23af8beca9 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -89,6 +89,8 @@ static LIST_HEAD(hvc_structs);
  */
 static DEFINE_SPINLOCK(hvc_structs_lock);
 
+/* Mutex to serialize hvc_open */
+static DEFINE_MUTEX(hvc_open_mutex);
 /*
  * This value is used to assign a tty->index value to a hvc_struct based
  * upon order of exposure via hvc_probe(), when we can not match it to
@@ -333,16 +335,24 @@ static int hvc_install(struct tty_driver *driver, struct tty_struct *tty)
  */
 static int hvc_open(struct tty_struct *tty, struct file * filp)
 {
-	struct hvc_struct *hp = tty->driver_data;
+	struct hvc_struct *hp;
 	unsigned long flags;
 	int rc = 0;
 
+	mutex_lock(&hvc_open_mutex);
+
+	hp = tty->driver_data;
+	if (!hp) {
+		rc = -EIO;
+		goto out;
+	}
+
 	spin_lock_irqsave(&hp->port.lock, flags);
 	/* Check and then increment for fast path open. */
 	if (hp->port.count++ > 0) {
 		spin_unlock_irqrestore(&hp->port.lock, flags);
 		hvc_kick();
-		return 0;
+		goto out;
 	} /* else count == 0 */
 	spin_unlock_irqrestore(&hp->port.lock, flags);
 
@@ -371,6 +381,8 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
 	/* Force wakeup of the polling thread */
 	hvc_kick();
 
+out:
+	mutex_unlock(&hvc_open_mutex);
 	return rc;
 }
 
-- 
2.25.1