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 <[e-mail]>
Signed-off-by: Julian Anastasov <[e-mail]>
Reviewed-by: Hannes Frederic Sowa <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/neighbour.c

···
 	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
···
 			} 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;
 		}
 	}