[PATCH v2 1/4] media: dvb-core: Fix use-after-free due to race condition occurring in dvb_frontend

[Hyunwoo Kim <imv4bel@xxxxxxxxx>]

If the device node of dvb_frontend is open() and the device is
disconnected, many kinds of UAFs may occur when calling close()
on the device node.

The root cause of this is that wake_up() for dvbdev->wait_queue
is implemented in the dvb_frontend_release() function, but
wait_event() is not implemented in the dvb_frontend_stop() function.

So, implement wait_event() function in dvb_frontend_stop() and
add 'remove_mutex' which prevents race condition for 'fe->exit'.

Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
---
 drivers/media/dvb-core/dvb_frontend.c | 39 +++++++++++++++++++++++----
 include/media/dvb_frontend.h          |  6 ++++-
 2 files changed, 39 insertions(+), 6 deletions(-)

diff --git a/drivers/media/dvb-core/dvb_frontend.c b/drivers/media/dvb-core/dvb_frontend.c
index 48e735cdbe6b..b3556e3580c6 100644
--- a/drivers/media/dvb-core/dvb_frontend.c
+++ b/drivers/media/dvb-core/dvb_frontend.c
@@ -809,6 +809,8 @@ static void dvb_frontend_stop(struct dvb_frontend *fe)
 
 	dev_dbg(fe->dvb->device, "%s:\n", __func__);
 
+	mutex_lock(&fe->remove_mutex);
+
 	if (fe->exit != DVB_FE_DEVICE_REMOVED)
 		fe->exit = DVB_FE_NORMAL_EXIT;
 	mb();
@@ -818,6 +820,13 @@ static void dvb_frontend_stop(struct dvb_frontend *fe)
 
 	kthread_stop(fepriv->thread);
 
+	mutex_unlock(&fe->remove_mutex);
+
+	if (fepriv->dvbdev->users < -1) {
+		wait_event(fepriv->dvbdev->wait_queue,
+				fepriv->dvbdev->users == -1);
+	}
+
 	sema_init(&fepriv->sem, 1);
 	fepriv->state = FESTATE_IDLE;
 
@@ -2750,9 +2759,13 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
 	struct dvb_adapter *adapter = fe->dvb;
 	int ret;
 
+	mutex_lock(&fe->remove_mutex);
+
 	dev_dbg(fe->dvb->device, "%s:\n", __func__);
-	if (fe->exit == DVB_FE_DEVICE_REMOVED)
+	if (fe->exit == DVB_FE_DEVICE_REMOVED) {
+		mutex_unlock(&fe->remove_mutex);
 		return -ENODEV;
+	}
 
 	if (adapter->mfe_shared) {
 		mutex_lock(&adapter->mfe_lock);
@@ -2773,8 +2786,10 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
 			while (mferetry-- && (mfedev->users != -1 ||
 					      mfepriv->thread)) {
 				if (msleep_interruptible(500)) {
-					if (signal_pending(current))
+					if (signal_pending(current)) {
+						mutex_unlock(&fe->remove_mutex);
 						return -EINTR;
+					}
 				}
 			}
 
@@ -2786,6 +2801,7 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
 				if (mfedev->users != -1 ||
 				    mfepriv->thread) {
 					mutex_unlock(&adapter->mfe_lock);
+					mutex_unlock(&fe->remove_mutex);
 					return -EBUSY;
 				}
 				adapter->mfe_dvbdev = dvbdev;
@@ -2845,6 +2861,8 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
 
 	if (adapter->mfe_shared)
 		mutex_unlock(&adapter->mfe_lock);
+
+	mutex_unlock(&fe->remove_mutex);
 	return ret;
 
 err3:
@@ -2866,6 +2884,8 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
 err0:
 	if (adapter->mfe_shared)
 		mutex_unlock(&adapter->mfe_lock);
+
+	mutex_unlock(&fe->remove_mutex);
 	return ret;
 }
 
@@ -2876,6 +2896,8 @@ static int dvb_frontend_release(struct inode *inode, struct file *file)
 	struct dvb_frontend_private *fepriv = fe->frontend_priv;
 	int ret;
 
+	mutex_lock(&fe->remove_mutex);
+
 	dev_dbg(fe->dvb->device, "%s:\n", __func__);
 
 	if ((file->f_flags & O_ACCMODE) != O_RDONLY) {
@@ -2897,11 +2919,17 @@ static int dvb_frontend_release(struct inode *inode, struct file *file)
 		}
 		mutex_unlock(&fe->dvb->mdev_lock);
 #endif
-		if (fe->exit != DVB_FE_NO_EXIT)
-			wake_up(&dvbdev->wait_queue);
 		if (fe->ops.ts_bus_ctrl)
 			fe->ops.ts_bus_ctrl(fe, 0);
-	}
+
+		if (fe->exit != DVB_FE_NO_EXIT) {
+			mutex_unlock(&fe->remove_mutex);
+			wake_up(&dvbdev->wait_queue);
+		} else
+			mutex_unlock(&fe->remove_mutex);
+
+	} else
+		mutex_unlock(&fe->remove_mutex);
 
 	dvb_frontend_put(fe);
 
@@ -3000,6 +3028,7 @@ int dvb_register_frontend(struct dvb_adapter *dvb,
 	fepriv = fe->frontend_priv;
 
 	kref_init(&fe->refcount);
+	mutex_init(&fe->remove_mutex);
 
 	/*
 	 * After initialization, there need to be two references: one
diff --git a/include/media/dvb_frontend.h b/include/media/dvb_frontend.h
index e7c44870f20d..411ec32cd8df 100644
--- a/include/media/dvb_frontend.h
+++ b/include/media/dvb_frontend.h
@@ -686,7 +686,10 @@ struct dtv_frontend_properties {
  * @id:			Frontend ID
  * @exit:		Used to inform the DVB core that the frontend
  *			thread should exit (usually, means that the hardware
- *			got disconnected.
+ *			got disconnected.)
+ * @remove_mutex:	mutex that avoids a race condition between a callback
+ *			called when the hardware is disconnected and the
+ *			file_operations of dvb_frontend
  */
 
 struct dvb_frontend {
@@ -704,6 +707,7 @@ struct dvb_frontend {
 	int (*callback)(void *adapter_priv, int component, int cmd, int arg);
 	int id;
 	unsigned int exit;
+	struct mutex remove_mutex;
 };
 
 /**
-- 
2.25.1