net: neigh: disallow transition to NUD_STALE if lladdr is unchanged in neigh_update()

NUD_STALE is used when the caller(e.g. arp_process()) can't guarantee
neighbour reachability. If the entry was NUD_VALID and lladdr is unchanged,
the entry state should not be changed.

Currently the code puts an extra "NUD_CONNECTED" condition. So if old state
was NUD_DELAY or NUD_PROBE (they are NUD_VALID but not NUD_CONNECTED), the
state can be changed to NUD_STALE.

This may cause problem. Because NUD_STALE lladdr doesn't guarantee
reachability, when we send traffic, the state will be changed to
NUD_DELAY. In normal case, if we get no confirmation (by dst_confirm()),
we will change the state to NUD_PROBE and send probe traffic. But now the
state may be reset to NUD_STALE again(e.g. by broadcast ARP packets),
so the probe traffic will not be sent. This situation may happen again and
again, and packets will be sent to an non-reachable lladdr forever.

The fix is to remove the "NUD_CONNECTED" condition. After that the
"NEIGH_UPDATE_F_WEAK_OVERRIDE" condition (used by IPv6) in that branch will
be redundant, so remove it.

This change may increase probe traffic, but it's essential since NUD_STALE
lladdr is unreliable. To ensure correctness, we prefer to resolve lladdr,
when we can't get confirmation, even while remote packets try to set
NUD_STALE state.

Signed-off-by: Chunhui He <hchunhui@mail.ustc.edu.cn>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 5cdc62a..cf26e04 100644 (file)
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1060,8 +1060,6 @@ static void neigh_update_hhs(struct neighbour *neigh)
        NEIGH_UPDATE_F_WEAK_OVERRIDE will suspect existing "connected"
                                lladdr instead of overriding it
                                if it is different.
-                               It also allows to retain current state
-                               if lladdr is unchanged.
        NEIGH_UPDATE_F_ADMIN    means that the change is administrative.
 
        NEIGH_UPDATE_F_OVERRIDE_ISROUTER allows to override existing
@@ -1150,10 +1148,7 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
                        } else
                                goto out;
                } else {
-                       if (lladdr == neigh->ha && new == NUD_STALE &&
-                           ((flags & NEIGH_UPDATE_F_WEAK_OVERRIDE) ||
-                            (old & NUD_CONNECTED))
-                           )
+                       if (lladdr == neigh->ha && new == NUD_STALE)
                                new = old;
                }
        }