[PATCH v2 ipsec] Clear secpath on loopback_xmit

[Benedict Wong <benedictwong@google.com>]

This patch clears the skb->sp when transmitted over loopback. This
ensures that the loopback-ed packet does not have any secpath
information from the outbound transforms.

At present, this causes XFRM tunnel mode packets to be dropped with
XFRMINNOPOLS, due to the outbound state being in the secpath, without
a matching inbound policy. Clearing the secpath ensures that all states
added to the secpath are exclusively from the inbound processing.

Tests: xfrm tunnel mode tests added for loopback:
    https://android-review.googlesource.com/c/kernel/tests/+/777328
Fixes: 8fe7ee2ba983 ("[IPsec]: Strengthen policy checks")
Signed-off-by: Benedict Wong <benedictwong@google.com>
---
 drivers/net/loopback.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 30612497643c..a6bf54df94bd 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -50,6 +50,7 @@
 #include <linux/ethtool.h>
 #include <net/sock.h>
 #include <net/checksum.h>
+#include <net/xfrm.h>
 #include <linux/if_ether.h>	/* For the statistics structure. */
 #include <linux/if_arp.h>	/* For ARPHRD_ETHER */
 #include <linux/ip.h>
@@ -82,6 +83,9 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
 	 */
 	skb_dst_force(skb);
 
+	// Clear secpath to ensure xfrm policy check not tainted by outbound SAs.
+	secpath_reset(skb);
+
 	skb->protocol = eth_type_trans(skb, dev);
 
 	/* it's OK to use per_cpu_ptr() because BHs are off */
-- 
2.19.0.605.g01d371f741-goog