security/apparmor: use kfree_sensitive() in unpack_secmark()

The unpack_secmark() function currently uses kfree() to release memory
allocated for secmark structures and their labels. However, if a failure
occurs after partially parsing secmark, sensitive data may remain in
memory, posing a security risk.

To mitigate this, replace kfree() with kfree_sensitive() for freeing
secmark structures and their labels, aligning with the approach used
in free_ruleset().

I am submitting this as an RFC to seek freedback on whether this change
is appropriate and aligns with the subsystem's expectations. If
confirmed to be helpful, I will send a formal patch.

Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: GoodLuck612

security/apparmor/policy_unpack.c

@@ -599,8 +599,8 @@ static bool unpack_secmark(struct aa_ext *e, struct aa_ruleset *rules)
 fail:
 	if (rules->secmark) {
 		for (i = 0; i < size; i++)
-			kfree(rules->secmark[i].label);
-		kfree(rules->secmark);
+			kfree_sensitive(rules->secmark[i].label);
+		kfree_sensitive(rules->secmark);
 		rules->secmark_count = 0;
 		rules->secmark = NULL;
 	}