kprobes: handle empty/invalid input to debugfs "enabled" file

When writing invalid input to 'debug/kprobes/enabled' it'll silently be
ignored.  Even worse, when writing an empty string to this file, the
outcome is purely random as the switch statement will make its decision
based on the value of an uninitialized stack variable.

Fix this by handling invalid/empty input as error returning -EINVAL.
Signed-off-by: NMathias Krause <minipli@googlemail.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/kprobes.c

@@ -2332,6 +2332,7 @@ static ssize_t write_enabled_file_bool(struct file *file,
 	if (copy_from_user(buf, user_buf, buf_size))
 		return -EFAULT;
 
+	buf[buf_size] = '\0';
 	switch (buf[0]) {
 	case 'y':
 	case 'Y':
@@ -2343,6 +2344,8 @@ static ssize_t write_enabled_file_bool(struct file *file,
 	case '0':
 		disarm_all_kprobes();
 		break;
+	default:
+		return -EINVAL;
 	}
 
 	return count;