selinux: normalize input to /sys/fs/selinux/enforce

At present, one can write any signed integer value to
/sys/fs/selinux/enforce and it will be stored,
e.g. echo -1 > /sys/fs/selinux/enforce or echo 2 >
/sys/fs/selinux/enforce. This makes no real difference
to the kernel, since it only ever cares if it is zero or non-zero,
but some userspace code compares it with 1 to decide if SELinux
is enforcing, and this could confuse it. Only a process that is
already root and is allowed the setenforce permission in SELinux
policy can write to /sys/fs/selinux/enforce, so this is not considered
to be a security issue, but it should be fixed.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/selinuxfs.c

@@ -163,6 +163,8 @@ static ssize_t sel_write_enforce(struct file *file, const char __user *buf,
 	if (sscanf(page, "%d", &new_value) != 1)
 		goto out;
 
+	new_value = !!new_value;
+
 	if (new_value != selinux_enforcing) {
 		length = task_has_security(current, SECURITY__SETENFORCE);
 		if (length)