xfrm: policy: check reinserted policies match their node

validate the re-inserted policies match the lookup node.
Policies that fail this test won't be returned in the candidate set.

This is enabled by default for now, it should not cause noticeable
reinsert slow down.

Such reinserts are needed when we have to merge an existing node
(e.g. for 10.0.0.0/28 because a overlapping subnet was added (e.g.
10.0.0.0/24), so whenever this happens existing policies have to
be placed on the list of the new node.
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -806,10 +806,16 @@ static void xfrm_policy_inexact_list_reinsert(struct net *net,
 					      struct xfrm_pol_inexact_node *n,
 					      u16 family)
 {
+	unsigned int matched_s, matched_d;
 	struct hlist_node *newpos = NULL;
 	struct xfrm_policy *policy, *p;
 
+	matched_s = 0;
+	matched_d = 0;
+
 	list_for_each_entry_reverse(policy, &net->xfrm.policy_all, walk.all) {
+		bool matches_s, matches_d;
+
 		if (!policy->bydst_reinsert)
 			continue;
 
@@ -827,6 +833,32 @@ static void xfrm_policy_inexact_list_reinsert(struct net *net,
 			hlist_add_behind(&policy->bydst, newpos);
 		else
 			hlist_add_head(&policy->bydst, &n->hhead);
+
+		/* paranoia checks follow.
+		 * Check that the reinserted policy matches at least
+		 * saddr or daddr for current node prefix.
+		 *
+		 * Matching both is fine, matching saddr in one policy
+		 * (but not daddr) and then matching only daddr in another
+		 * is a bug.
+		 */
+		matches_s = xfrm_policy_addr_delta(&policy->selector.saddr,
+						   &n->addr,
+						   n->prefixlen,
+						   family) == 0;
+		matches_d = xfrm_policy_addr_delta(&policy->selector.daddr,
+						   &n->addr,
+						   n->prefixlen,
+						   family) == 0;
+		if (matches_s && matches_d)
+			continue;
+
+		WARN_ON_ONCE(!matches_s && !matches_d);
+		if (matches_s)
+			matched_s++;
+		if (matches_d)
+			matched_d++;
+		WARN_ON_ONCE(matched_s && matched_d);
 	}
 }