Re: [PATCH v2] net: neigh: disallow transition to NUD_STALE if lladdr is unchanged in neigh_update()

[David Miller]

From: Chunhui He <hchunhui@xxxxxxxxxxxxxxxx>
Date: Tue, 26 Jul 2016 06:16:52 +0000

> 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@xxxxxxxxxxxxxxxx>

Applied, thanks.