[PATCH 4.20 44/50] xfrm: Make set-mark default behavior backward compatible

[Greg Kroah-Hartman]

4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Benedict Wong <benedictwong@xxxxxxxxxx>

commit e2612cd496e7b465711d219ea6118893d7253f52 upstream.

Fixes 9b42c1f179a6, which changed the default route lookup behavior for
tunnel mode SAs in the outbound direction to use the skb mark, whereas
previously mark=0 was used if the output mark was unspecified. In
mark-based routing schemes such as Androidâs, this change in default
behavior causes routing loops or lookup failures.

This patch restores the default behavior of using a 0 mark while still
incorporating the skb mark if the SET_MARK (and SET_MARK_MASK) is
specified.

Tested with additions to Android's kernel unit test suite:
https://android-review.googlesource.com/c/kernel/tests/+/860150

Fixes: 9b42c1f179a6 ("xfrm: Extend the output_mark to support input direction and masking")
Signed-off-by: Benedict Wong <benedictwong@xxxxxxxxxx>
Signed-off-by: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/xfrm/xfrm_policy.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -1628,7 +1628,10 @@ static struct dst_entry *xfrm_bundle_cre
 		dst_copy_metrics(dst1, dst);
 
 		if (xfrm[i]->props.mode != XFRM_MODE_TRANSPORT) {
-			__u32 mark = xfrm_smark_get(fl->flowi_mark, xfrm[i]);
+			__u32 mark = 0;
+
+			if (xfrm[i]->props.smark.v || xfrm[i]->props.smark.m)
+				mark = xfrm_smark_get(fl->flowi_mark, xfrm[i]);
 
 			family = xfrm[i]->props.family;
 			dst = xfrm_dst_lookup(xfrm[i], tos, fl->flowi_oif,