ima: ima_write_policy() limit locking

There is no need to hold the ima_write_mutex for so long.  We only need it
around ima_parse_add_rule().

Changelog:
- The return path now takes into account failed kmalloc() call.
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Petko Manolov <petkan@mip-labs.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/ima/ima_fs.c

@@ -261,13 +261,8 @@ static const struct file_operations ima_ascii_measurements_ops = {
 static ssize_t ima_write_policy(struct file *file, const char __user *buf,
 				size_t datalen, loff_t *ppos)
 {
-	char *data = NULL;
+	char *data;
 	ssize_t result;
-	int res;
-
-	res = mutex_lock_interruptible(&ima_write_mutex);
-	if (res)
-		return res;
 
 	if (datalen >= PAGE_SIZE)
 		datalen = PAGE_SIZE - 1;
@@ -286,14 +281,19 @@ static ssize_t ima_write_policy(struct file *file, const char __user *buf,
 
 	result = -EFAULT;
 	if (copy_from_user(data, buf, datalen))
-		goto out;
+		goto out_free;
 
+	result = mutex_lock_interruptible(&ima_write_mutex);
+	if (result < 0)
+		goto out_free;
 	result = ima_parse_add_rule(data);
+	mutex_unlock(&ima_write_mutex);
+
+out_free:
+	kfree(data);
 out:
 	if (result < 0)
 		valid_policy = 0;
-	kfree(data);
-	mutex_unlock(&ima_write_mutex);
 
 	return result;
 }