xfrm: Try to honor policy index if it's supplied by user

xfrm code always searches for unused policy index for
newly created policy regardless whether or not user
space policy index hint supplied.

This patch enables such feature so that using
"ip xfrm ... index=xxx" can be used by user to set
specific policy index.

Currently this beahvior is broken, so this patch make
it happen as expected.
Signed-off-by: NFan Du <fan.du@windriver.com>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -538,7 +538,7 @@ static void xfrm_hash_resize(struct work_struct *work)
 
 /* Generate new index... KAME seems to generate them ordered by cost
  * of an absolute inpredictability of ordering of rules. This will not pass. */
-static u32 xfrm_gen_index(struct net *net, int dir)
+static u32 xfrm_gen_index(struct net *net, int dir, u32 index)
 {
 	static u32 idx_generator;
 
@@ -548,8 +548,14 @@ static u32 xfrm_gen_index(struct net *net, int dir)
 		u32 idx;
 		int found;
 
+		if (!index) {
 			idx = (idx_generator | dir);
 			idx_generator += 8;
+		} else {
+			idx = index;
+			index = 0;
+		}
+
 		if (idx == 0)
 			idx = 8;
 		list = net->xfrm.policy_byidx + idx_hash(net, idx);
@@ -672,7 +678,7 @@ int xfrm_policy_insert(int dir, struct xfrm_policy *policy, int excl)
 		xfrm_policy_requeue(delpol, policy);
 		__xfrm_policy_unlink(delpol, dir);
 	}
-	policy->index = delpol ? delpol->index : xfrm_gen_index(net, dir);
+	policy->index = delpol ? delpol->index : xfrm_gen_index(net, dir, policy->index);
 	hlist_add_head(&policy->byidx, net->xfrm.policy_byidx+idx_hash(net, policy->index));
 	policy->curlft.add_time = get_seconds();
 	policy->curlft.use_time = 0;
@@ -1192,7 +1198,7 @@ int xfrm_sk_policy_insert(struct sock *sk, int dir, struct xfrm_policy *pol)
 	sk->sk_policy[dir] = pol;
 	if (pol) {
 		pol->curlft.add_time = get_seconds();
-		pol->index = xfrm_gen_index(net, XFRM_POLICY_MAX+dir);
+		pol->index = xfrm_gen_index(net, XFRM_POLICY_MAX+dir, 0);
 		__xfrm_policy_link(pol, XFRM_POLICY_MAX+dir);
 	}
 	if (old_pol) {

net/xfrm/xfrm_user.c

@@ -1189,6 +1189,8 @@ static int verify_policy_type(u8 type)
 
 static int verify_newpolicy_info(struct xfrm_userpolicy_info *p)
 {
+	int ret;
+
 	switch (p->share) {
 	case XFRM_SHARE_ANY:
 	case XFRM_SHARE_SESSION:
@@ -1224,7 +1226,13 @@ static int verify_newpolicy_info(struct xfrm_userpolicy_info *p)
 		return -EINVAL;
 	}
 
-	return verify_policy_dir(p->dir);
+	ret = verify_policy_dir(p->dir);
+	if (ret)
+		return ret;
+	if (p->index && ((p->index & XFRM_POLICY_MAX) != p->dir))
+		return -EINVAL;
+
+	return 0;
 }
 
 static int copy_from_user_sec_ctx(struct xfrm_policy *pol, struct nlattr **attrs)