[PATCH] USB: cdc-acm: fix possible null pointer in acm_tty_hangup

[Vincent Palatin]

Sometimes, acm_tty_hangup and acm_tty_close are called concurrently.
This results in acm pointer being null in acm_tty_hangup and panics when
it is dereferenced.

Signed-off-by: Vincent Palatin <vpalatin@xxxxxxxxxxxx>
---
 drivers/usb/class/cdc-acm.c |   16 +++++++++++-----
 1 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index dac7676..3d389f3 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -539,7 +539,6 @@ static void acm_port_down(struct acm *acm)
 {
 	int i;
 
-	mutex_lock(&open_mutex);
 	if (acm->dev) {
 		usb_autopm_get_interface(acm->control);
 		acm_set_control(acm, acm->ctrlout = 0);
@@ -551,14 +550,19 @@ static void acm_port_down(struct acm *acm)
 		acm->control->needs_remote_wakeup = 0;
 		usb_autopm_put_interface(acm->control);
 	}
-	mutex_unlock(&open_mutex);
 }
 
 static void acm_tty_hangup(struct tty_struct *tty)
 {
-	struct acm *acm = tty->driver_data;
-	tty_port_hangup(&acm->port);
-	acm_port_down(acm);
+	struct acm *acm;
+
+	mutex_lock(&open_mutex);
+	acm = tty->driver_data;
+	if (acm) {
+		tty_port_hangup(&acm->port);
+		acm_port_down(acm);
+	}
+	mutex_unlock(&open_mutex);
 }
 
 static void acm_tty_close(struct tty_struct *tty, struct file *filp)
@@ -579,7 +583,9 @@ static void acm_tty_close(struct tty_struct *tty, struct file *filp)
 		mutex_unlock(&open_mutex);
 		return;
 	}
+	mutex_lock(&open_mutex);
 	acm_port_down(acm);
+	mutex_unlock(&open_mutex);
 	tty_port_close_end(&acm->port, tty);
 	tty_port_tty_set(&acm->port, NULL);
 }
-- 
1.7.3.1

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