netdev - Re: [PATCH 2/2] neighbour: allow NUD_NOARP entries to be forced GCed

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Date:   Mon, 19 Apr 2021 18:44:29 +0200
From:   Kasper Dupont <kasperd@...fm.28.feb.2009.kasperd.net>
To:     netdev@...r.kernel.org
Cc:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        davem@...emloft.net, kuba@...nel.org, dsahern@...nel.org,
        Kasper Dupont <kasperd@...wv.06.feb.2021.kasperd.net>
Subject: Re: [PATCH 2/2] neighbour: allow NUD_NOARP entries to be forced GCed

On 17/03/21 15.53, Thadeu Lima de Souza Cascardo wrote:
> IFF_POINTOPOINT interfaces use NUD_NOARP entries for IPv6. It's possible to
> fill up the neighbour table with enough entries that it will overflow for
> valid connections after that.
> 
> This behaviour is more prevalent after commit 58956317c8de ("neighbor:
> Improve garbage collection") is applied, as it prevents removal from
> entries that are not NUD_FAILED, unless they are more than 5s old.
> 
> Fixes: 58956317c8de (neighbor: Improve garbage collection)
> Reported-by: Kasper Dupont <kasperd@...wv.06.feb.2021.kasperd.net>
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
> ---
>  net/core/neighbour.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index bbc89c7ffdfd..be5ca411b149 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -256,6 +256,7 @@ static int neigh_forced_gc(struct neigh_table *tbl)
>  
>  		write_lock(&n->lock);
>  		if ((n->nud_state == NUD_FAILED) ||
> +		    (n->nud_state == NUD_NOARP) ||
>  		    (tbl->is_multicast &&
>  		     tbl->is_multicast(n->primary_key)) ||
>  		    time_after(tref, n->updated))
> -- 
> 2.27.0
> 

Is there any update regarding this change?

I noticed this regression when it was used in a DoS attack on one of
my servers which I had upgraded from Ubuntu 18.04 to 20.04.

I have verified that Ubuntu 18.04 is not subject to this attack and
Ubuntu 20.04 is vulnerable. I have also verified that the one-line
change which Cascardo has provided fixes the vulnerability on Ubuntu
20.04.

Kind regards
Kasper