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

[Kasper Dupont <kasperd@gczfm.28.feb.2009.kasperd.net>]

On 19/04/21 10.10, David Ahern wrote:
> On 4/19/21 9:44 AM, Kasper Dupont wrote:
> > 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@gjkwv.06.feb.2021.kasperd.net>
> >> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.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.
> > 
> 
> your testing included both patches or just this one?

I applied only this one line change on top of the kernel in Ubuntu
20.04. The behavior I observed was that without the patch the kernel
was vulnerable and with that patch I was unable to reproduce the
problem.

The other longer patch is for a different issue which Cascardo
discovered while working on the one I had reported. I don't have an
environment set up where I can reproduce the issue addressed by that
larger patch.