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

[CMGS <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".

vxlan_fdb_replace(), which makes the same change through netlink,
already resets the cache. Reset it when snooping too.

Reproduced with network namespaces: a learning VXLAN device has one
peer VTEP on-link and another behind a router, and an inner MAC moves
from the first peer to the second. Without this patch, pings to the MAC
fail after the move until the route cache is flushed: in 5 of 5 runs
with one CPU, and in 1 of 3 runs with two CPUs, where it depends on
whether the sending CPU holds the stale entry. With the patch, they
succeed in all 8 runs.

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")
Signed-off-by: CMGS <ilskdw@gmail.com>
---
 drivers/net/vxlan/vxlan_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index d11028b37..6ee071cd2 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -1482,6 +1482,7 @@ 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;
 		vxlan_fdb_notify(vxlan, f, rdst, RTM_NEWNEIGH, true, NULL);
 	} else {
-- 
2.47.3