[RFC ipsec 0/1] xfrm: set-mark default behavior changes

[RFC ipsec 0/1] xfrm: set-mark default behavior changes

[Benedict Wong]

A behavior change introduced in 9b42c1f179a6 (“xfrm: Extend the
output_mark to support input direction and masking”) results in a
change in:

1. Default outbound behavior with regards to route lookup marks, and
2. Inbound behavior for SAs used to decapsulate packets when the output
     mark (as specified in 4.14 to 4.18) is set.

This patch set restores the previous default outbound behavior,
resolving (1), but behavior change (2) will require more discussion.

Specifically, in (2), a SA with a "output mark" set will now have that
Mark imposed on the inbound packet (As opposed to the previous
output-mark behavior where the inbound packet's mark would not be
touched). This is less of a concern, as it is limited to the case where:

1. SA output mark is set
2. SA is using non-transport mode
3. SA is configured for inbound decapsulation (local dst IP)

Critically, conditions 1 and 3 imply a configuration that output mark
was not designed to support. The only valid use case for this seems
to be the loopback case (as IP addresses would apply bidirectionally).
As such, we believe that this behavioral change is acceptable as is.

Benedict Wong (1):
  xfrm: Make set-mark default behavior backward compatible

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

-- 
2.20.1.97.g81188d93c3-goog

[RFC ipsec 1/1] xfrm: Make set-mark default behavior backward compatible

[Benedict Wong]

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

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 934492bad8e0..5f574ede1332 100644
--- 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,
-- 
2.20.1.97.g81188d93c3-goog