counter: Fix use-after-free race condition for events_queue_size write

A race condition is possible when writing to events_queue_size where the
events kfifo is freed during the execution of a kfifo_in(), resulting in
a use-after-free. This patch prevents such a scenario by protecting the
events queue in operation with a spinlock and locking before performing
the events queue size adjustment.

The existing events_lock mutex is renamed to events_out_lock to reflect
that it only protects events queue out operations. Because the events
queue in operations can occur in an interrupt context, a new
events_in_lock spinlock is introduced and utilized.

Fixes: feff17a5 ("counter: Implement events_queue_size sysfs attribute")
Cc: David Lechner <david@lechnology.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/20211021103540.955639-1-vilhelm.gray@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/counter/counter-chrdev.c

@@ -81,10 +81,10 @@ static ssize_t counter_chrdev_read(struct file *filp, char __user *buf,
 				return -ENODEV;
 		}
 
-		if (mutex_lock_interruptible(&counter->events_lock))
+		if (mutex_lock_interruptible(&counter->events_out_lock))
 			return -ERESTARTSYS;
 		err = kfifo_to_user(&counter->events, buf, len, &copied);
-		mutex_unlock(&counter->events_lock);
+		mutex_unlock(&counter->events_out_lock);
 		if (err < 0)
 			return err;
 	} while (!copied);
@@ -436,7 +436,8 @@ int counter_chrdev_add(struct counter_device *const counter)
 	spin_lock_init(&counter->events_list_lock);
 	mutex_init(&counter->n_events_list_lock);
 	init_waitqueue_head(&counter->events_wait);
-	mutex_init(&counter->events_lock);
+	spin_lock_init(&counter->events_in_lock);
+	mutex_init(&counter->events_out_lock);
 
 	/* Initialize character device */
 	cdev_init(&counter->chrdev, &counter_fops);
@@ -559,7 +560,8 @@ void counter_push_event(struct counter_device *const counter, const u8 event,
 		ev.watch.component = comp_node->component;
 		ev.status = -counter_get_data(counter, comp_node, &ev.value);
 
-		copied += kfifo_in(&counter->events, &ev, 1);
+		copied += kfifo_in_spinlocked_noirqsave(&counter->events, &ev,
+							1, &counter->events_in_lock);
 	}
 
 exit_early:

drivers/counter/counter-sysfs.c

@@ -11,6 +11,8 @@
 #include <linux/kfifo.h>
 #include <linux/kstrtox.h>
 #include <linux/list.h>
+#include <linux/mutex.h>
+#include <linux/spinlock.h>
 #include <linux/string.h>
 #include <linux/sysfs.h>
 #include <linux/types.h>
@@ -796,6 +798,7 @@ static int counter_events_queue_size_write(struct counter_device *counter,
 {
 	DECLARE_KFIFO_PTR(events, struct counter_event);
 	int err;
+	unsigned long flags;
 
 	/* Allocate new events queue */
 	err = kfifo_alloc(&events, val, GFP_KERNEL);
@@ -803,8 +806,12 @@ static int counter_events_queue_size_write(struct counter_device *counter,
 		return err;
 
 	/* Swap in new events queue */
+	mutex_lock(&counter->events_out_lock);
+	spin_lock_irqsave(&counter->events_in_lock, flags);
 	kfifo_free(&counter->events);
 	counter->events.kfifo = events.kfifo;
+	spin_unlock_irqrestore(&counter->events_in_lock, flags);
+	mutex_unlock(&counter->events_out_lock);
 
 	return 0;
 }

include/linux/counter.h

@@ -296,7 +296,8 @@ struct counter_ops {
  * @n_events_list_lock:	lock to protect Counter next events list operations
  * @events:		queue of detected Counter events
  * @events_wait:	wait queue to allow blocking reads of Counter events
- * @events_lock:	lock to protect Counter events queue read operations
+ * @events_in_lock:	lock to protect Counter events queue in operations
+ * @events_out_lock:	lock to protect Counter events queue out operations
  * @ops_exist_lock:	lock to prevent use during removal
  */
 struct counter_device {
@@ -323,7 +324,8 @@ struct counter_device {
 	struct mutex n_events_list_lock;
 	DECLARE_KFIFO_PTR(events, struct counter_event);
 	wait_queue_head_t events_wait;
-	struct mutex events_lock;
+	spinlock_t events_in_lock;
+	struct mutex events_out_lock;
 	struct mutex ops_exist_lock;
 };