Subject: [PATCH v2] tty: hvc: Fix data abort due to race in hvc_open

From: Raghavendra Rao Ananta <[email-masked]>
Date: Tue, 19 May 2020 23:47:08 -0700

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, do a NULL check at the beginning, before proceeding ahead.

The issue can be easily reproduced by launching two tasks simultaneously
that does an open() call on /dev/hvcX.
For example:
$ cat /dev/hvc0 & cat /dev/hvc0 &

Cc: [email-masked]
Signed-off-by: Raghavendra Rao Ananta <[email-masked]>
---
 drivers/tty/hvc/hvc_console.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 436cc51c92c3..80709f754cc8 100644
--- a/drivers/tty/hvc/hvc_console.c
+++ b/drivers/tty/hvc/hvc_console.c
@@ -350,6 +350,9 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
 	unsigned long flags;
 	int rc = 0;

+	if (!hp)
+		return -ENODEV;
+
 	spin_lock_irqsave(&hp->port.lock, flags);
 	/* Check and then increment for fast path open. */
 	if (hp->port.count++ > 0) {
--
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