xfrm: Make set-mark default behavior backward compatible

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 Android’s, 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@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 8cfd75b623969364441ff4a66c6c6c5288472bdf..ba0a4048c846bbbbde6e0d3320f6d4cfa46da424 100644 (file)
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2600,7 +2600,10 @@ static struct dst_entry *xfrm_bundle_create(struct xfrm_policy *policy,
                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,