Input: evdev - limit amount of data for writes

Limit amount of data that can be written into an evdev instance at
a given time to 4096 bytes (170 input events) to avoid holding
evdev->mutex for too long and starving other users.
Reviewed-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Link: https://lore.kernel.org/r/Zr5L8TUzkJcB9HcF@google.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/evdev.c

@@ -498,6 +498,13 @@ static ssize_t evdev_write(struct file *file, const char __user *buffer,
 	struct input_event event;
 	int retval = 0;
 
+	/*
+	 * Limit amount of data we inject into the input subsystem so that
+	 * we do not hold evdev->mutex for too long. 4096 bytes corresponds
+	 * to 170 input events.
+	 */
+	count = min(count, 4096);
+
 	if (count != 0 && count < input_event_size())
 		return -EINVAL;