netdev - [PATCH ipsec] xfrm: Preserve xfrm interface secpath for packets forwarded

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Date:   Tue, 11 Apr 2023 09:25:02 +0200
From:   Martin Willi <martin@...ongswan.org>
To:     Steffen Klassert <steffen.klassert@...unet.com>,
        Benedict Wong <benedictwong@...gle.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: [PATCH ipsec] xfrm: Preserve xfrm interface secpath for packets forwarded

The commit referenced below clears the secpath on packets received via
xfrm interfaces to support nested IPsec tunnels. This breaks Netfilter
policy matching using xt_policy in the FORWARD chain, as the secpath
is missing during forwarding. INPUT matching is not affected, as it is
done before secpath reset.

A work-around could use XFRM input interface matching for such rules,
but this does not work if the XFRM interface is part of a VRF; the
Netfilter input interface is replaced by the VRF interface, making a
sufficient match for IPsec-protected packets difficult.

So instead, limit the secpath reset to packets that are targeting the
local host, in the default or a specific VRF. This should allow nested
tunnels, but keeps the secpath intact on packets that are passed to
Netfilter chains with potential IPsec policy matches.

Fixes: b0355dbbf13c ("Fix XFRM-I support for nested ESP tunnels")
Signed-off-by: Martin Willi <martin@...ongswan.org>
---
 include/net/xfrm.h     | 10 ++++++++++
 net/xfrm/xfrm_policy.c |  2 +-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 3e1f70e8e424..f16df2f07a83 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -1349,6 +1349,16 @@ void xfrm_flowi_addr_get(const struct flowi *fl,
 	}
 }

+static inline bool xfrm_flowi_is_forwarding(struct net *net,
+					    const struct flowi *fl)
+{
+	if (fl->flowi_oif == LOOPBACK_IFINDEX)
+		return false;
+	if (netif_index_is_l3_master(net, fl->flowi_oif))
+		return false;
+	return true;
+}
+
 static __inline__ int
 __xfrm4_state_addr_check(const struct xfrm_state *x,
 			 const xfrm_address_t *daddr, const xfrm_address_t *saddr)
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 5c61ec04b839..4f49698eb29f 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -3745,7 +3745,7 @@ int __xfrm_policy_check(struct sock *sk, int dir, struct sk_buff *skb,
 			goto reject;
 		}

-		if (if_id)
+		if (if_id && !xfrm_flowi_is_forwarding(net, &fl))
 			secpath_reset(skb);

 		xfrm_pols_put(pols, npols);
-- 
2.34.1