[PATCH net v5 1/2] vxlan: reset the remote's dst cache when snooping moves a MAC

[Zhefu Peng <ilskdw@gmail.com>]

vxlan_snoop() handles a known MAC that now arrives from another remote
by rewriting the remote's IP address in place. It leaves the remote's
dst_cache alone, so the next packet to that MAC is routed with the dst
cached for the previous remote. When the two remotes have different
next hops, the packet leaves the wrong way. For example, if the
previous remote was on-link and the new one is behind a router, the
device resolves the new remote's address on the local link, the
neighbour stays incomplete, and traffic to the MAC is blackholed until
the cached route is invalidated, such as by "ip route flush cache".

The same branch also leaves remote_ifindex unchanged, although the
early return compares it and vxlan_xmit_one() uses it as the output
interface. When a MAC moves between an IPv6 link-local and another
address, the remote keeps the previous interface, and every later
packet from that remote enters the migration branch again.

vxlan_fdb_replace(), which makes the same change through netlink,
already resets the cache, updates remote_ifindex and clears offloaded.
Do the same when snooping.

vxlan_snoop() knows the interface only for IPv6 link-local sources and
uses 0 for all others, but the early return compares remote_ifindex for
every source. A packet from the remote of a dynamic entry that was
added with a "via" device therefore enters the migration branch as
well, which would now reset the device to 0. Compare remote_ifindex
only for link-local sources.

The selftest added in the next patch moves a MAC from an on-link IPv4
remote to one behind a router, and from a global IPv6 remote to a
link-local one. Both cases fail without this patch and pass with it.
A third case checks that the "via" device of a dynamic entry stays set
when its remote sends traffic.

This was found by our AI-driven automated live migration tests. The fix
and this changelog were written with the help of an AI coding assistant.

Fixes: 0c1d70af924b ("net: use dst_cache for vxlan device")
Fixes: 87613de95041 ("vxlan: fix snooping for link-local IPv6 addresses")
Signed-off-by: Zhefu Peng <ilskdw@gmail.com>
---
 drivers/net/vxlan/vxlan_core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index d11028b37..1cfb34fa2 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -1470,7 +1470,7 @@ static enum skb_drop_reason vxlan_snoop(struct net_device *dev,
 			return SKB_DROP_REASON_VXLAN_ENTRY_EXISTS;
 
 		if (likely(vxlan_addr_equal(&rdst->remote_ip, src_ip) &&
-			   rdst->remote_ifindex == ifindex))
+			   (!ifindex || rdst->remote_ifindex == ifindex)))
 			return SKB_NOT_DROPPED_YET;
 
 		/* Don't migrate static entries, drop packets */
@@ -1482,7 +1482,10 @@ static enum skb_drop_reason vxlan_snoop(struct net_device *dev,
 				    "%pM migrated from %pIS to %pIS\n",
 				    src_mac, &rdst->remote_ip.sa, &src_ip->sa);
 
+		dst_cache_reset(&rdst->dst_cache);
 		rdst->remote_ip = *src_ip;
+		rdst->remote_ifindex = ifindex;
+		rdst->offloaded = false;
 		vxlan_fdb_notify(vxlan, f, rdst, RTM_NEWNEIGH, true, NULL);
 	} else {
 		/* learned new entry */
-- 
2.54.0