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

On 19/04/21 10.10, David Ahern wrote:

On 4/19/21 9:44 AM, Kasper Dupont wrote:

quoted

On 17/03/21 15.53, Thadeu Lima de Souza Cascardo wrote:

quoted

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 <redacted>
Signed-off-by: Thadeu Lima de Souza Cascardo <redacted>
---
 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.