xfrm: Treat already-verified secpath entries as optional

This change allows inbound traffic through nested IPsec tunnels to
successfully match policies and templates, while retaining the secpath
stack trace as necessary for netfilter policies.

Specifically, this patch marks secpath entries that have already matched
against a relevant policy as having been verified, allowing it to be
treated as optional and skipped after a tunnel decapsulation (during
which the src/dst/proto/etc may have changed, and the correct policy
chain no long be resolvable).

This approach is taken as opposed to the iteration in b0355dbb,
where the secpath was cleared, since that breaks subsequent validations
that rely on the existence of the secpath entries (netfilter policies, or
transport-in-tunnel mode, where policies remain resolvable).

Fixes: b0355dbb ("Fix XFRM-I support for nested ESP tunnels")
Test: Tested against Android Kernel Unit Tests
Test: Tested against Android CTS
Signed-off-by: Benedict Wong <benedictwong@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1054,6 +1054,7 @@ struct xfrm_offload {
 struct sec_path {
 	int			len;
 	int			olen;
+	int			verified_cnt;
 
 	struct xfrm_state	*xvec[XFRM_MAX_DEPTH];
 	struct xfrm_offload	ovec[XFRM_MAX_OFFLOAD_DEPTH];

net/xfrm/xfrm_input.c

@@ -131,6 +131,7 @@ struct sec_path *secpath_set(struct sk_buff *skb)
 	memset(sp->ovec, 0, sizeof(sp->ovec));
 	sp->olen = 0;
 	sp->len = 0;
+	sp->verified_cnt = 0;
 
 	return sp;
 }

net/xfrm/xfrm_policy.c

@@ -3349,6 +3349,13 @@ xfrm_policy_ok(const struct xfrm_tmpl *tmpl, const struct sec_path *sp, int star
 		if (xfrm_state_ok(tmpl, sp->xvec[idx], family, if_id))
 			return ++idx;
 		if (sp->xvec[idx]->props.mode != XFRM_MODE_TRANSPORT) {
+			if (idx < sp->verified_cnt) {
+				/* Secpath entry previously verified, consider optional and
+				 * continue searching
+				 */
+				continue;
+			}
+
 			if (start == -1)
 				start = -2-idx;
 			break;
@@ -3723,6 +3730,9 @@ int __xfrm_policy_check(struct sock *sk, int dir, struct sk_buff *skb,
 		 * Order is _important_. Later we will implement
 		 * some barriers, but at the moment barriers
 		 * are implied between each two transformations.
+		 * Upon success, marks secpath entries as having been
+		 * verified to allow them to be skipped in future policy
+		 * checks (e.g. nested tunnels).
 		 */
 		for (i = xfrm_nr-1, k = 0; i >= 0; i--) {
 			k = xfrm_policy_ok(tpp[i], sp, k, family, if_id);
@@ -3741,6 +3751,8 @@ int __xfrm_policy_check(struct sock *sk, int dir, struct sk_buff *skb,
 		}
 
 		xfrm_pols_put(pols, npols);
+		sp->verified_cnt = k;
+
 		return 1;
 	}
 	XFRM_INC_STATS(net, LINUX_MIB_XFRMINPOLBLOCK);