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From: Vasiliy Kulikov on 9 Aug 2010 16:40
Hi folks,

I was reading batman-adv sources and noted:


1) Some incoming packets may cause a storm of error logs, such as at
routing.c:862


if (icmp_packet->msg_type != ECHO_REQUEST) {
pr_warning("Warning - can't forward icmp packet from %pM to "
"%pM: ttl exceeded\n", icmp_packet->orig,
icmp_packet->dst);

Any flooding bad guy is able to fill our disks with logs.
This should be logged only at some slow rate (e.g. 5 logs/sec) or as
pr_debug().


2) It seems to me that NF_HOOK() at hard-interface.c:458 is misused:

...
ret = NF_HOOK(PF_BRIDGE, NF_BR_LOCAL_IN, skb, dev, NULL,
batman_skb_recv_finish);
if (ret != 1)
goto err_out;

/* packet should hold at least type and version */
if (unlikely(skb_headlen(skb) < 2))
goto err_free;

/* expect a valid ethernet header here. */
if (unlikely(skb->mac_len != sizeof(struct ethhdr)
|| !skb_mac_header(skb)))
goto err_free;
...

static int batman_skb_recv_finish(struct sk_buff *skb)
{
return NF_ACCEPT;
}

As I understand, if there is any hook that returns NF_STOLEN, then skb
is leaked.


Thanks,
Vasiliy.
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From: Sven Eckelmann on 9 Aug 2010 17:00
Vasiliy Kulikov wrote:
> Hi folks,
>
> I was reading batman-adv sources and noted:

Thanks a lot.


> 1) Some incoming packets may cause a storm of error logs, such as at
> routing.c:862
>
>
> if (icmp_packet->msg_type != ECHO_REQUEST) {
> pr_warning("Warning - can't forward icmp packet from %pM to "
> "%pM: ttl exceeded\n", icmp_packet->orig,
> icmp_packet->dst);
>
> Any flooding bad guy is able to fill our disks with logs.
> This should be logged only at some slow rate (e.g. 5 logs/sec) or as
> pr_debug().

Correct. So you would prefer pr_warn_ratelimited?


> 2) It seems to me that NF_HOOK() at hard-interface.c:458 is misused:
>
> ...
> ret = NF_HOOK(PF_BRIDGE, NF_BR_LOCAL_IN, skb, dev, NULL,
> batman_skb_recv_finish);
> if (ret != 1)
> goto err_out;
>
> /* packet should hold at least type and version */
> if (unlikely(skb_headlen(skb) < 2))
> goto err_free;
>
> /* expect a valid ethernet header here. */
> if (unlikely(skb->mac_len != sizeof(struct ethhdr)
>
> || !skb_mac_header(skb)))
>
> goto err_free;
> ...
>
> static int batman_skb_recv_finish(struct sk_buff *skb)
> {
> return NF_ACCEPT;
> }
>
> As I understand, if there is any hook that returns NF_STOLEN, then skb
> is leaked.

@Linus Luessing: Could you please check that.

thanks,
Sven
From: Vasiliy Kulikov on 12 Aug 2010 08:50
On Mon, Aug 09, 2010 at 22:53 +0200, Sven Eckelmann wrote:
> Vasiliy Kulikov wrote:
> > 1) Some incoming packets may cause a storm of error logs, such as at
> > routing.c:862
> >
> >
> > if (icmp_packet->msg_type != ECHO_REQUEST) {
> > pr_warning("Warning - can't forward icmp packet from %pM to "
> > "%pM: ttl exceeded\n", icmp_packet->orig,
> > icmp_packet->dst);
> >
> > Any flooding bad guy is able to fill our disks with logs.
> > This should be logged only at some slow rate (e.g. 5 logs/sec) or as
> > pr_debug().
>
> Correct. So you would prefer pr_warn_ratelimited?

As I see in net/, such packets should be silently dropped with
drop_count++. Exceeded TTL is rather common situation and is not critical.
Also such buggy packets should be found out & dropped as fast as possible.
So IMO it should be debug output (if any) that does no overhead at nodebug
compilation.


3) Also there is no handler of online MTU change, at hard_if_event().


Is there any (un)official documentation/RFC/whatever of batman-adv
protocol? I found only expired RFC of batman that is using UDP at
www.open-mesh.org.


Thanks,
Vasiliy.
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From: Vasiliy Kulikov on 13 Aug 2010 14:20
On Tue, Aug 10, 2010 at 00:34 +0400, Vasiliy Kulikov wrote:
> 2) It seems to me that NF_HOOK() at hard-interface.c:458 is misused:
>
> ...
> ret = NF_HOOK(PF_BRIDGE, NF_BR_LOCAL_IN, skb, dev, NULL,
> batman_skb_recv_finish);
> if (ret != 1)
> goto err_out;
>
> /* packet should hold at least type and version */
> if (unlikely(skb_headlen(skb) < 2))
> goto err_free;
>
> /* expect a valid ethernet header here. */
> if (unlikely(skb->mac_len != sizeof(struct ethhdr)
> || !skb_mac_header(skb)))
> goto err_free;
> ...
>
> static int batman_skb_recv_finish(struct sk_buff *skb)
> {
> return NF_ACCEPT;
> }
>
> As I understand, if there is any hook that returns NF_STOLEN, then skb
> is leaked.

New ideas ;)

a) Currently processing of tables does not confirm to its names.

From `man ebtables`:
...
filter is the default table and contains three built-in
chains: INPUT (for frames destined for the bridge itself,
on the level of the MAC destination address), OUTPUT (for
locally-generated or (b)routed frames) and FORWARD (for
frames being forwarded by the bridge).
nat is mostly used to change the mac addresses and contains
three built-in chains: PREROUTING (for altering frames as
soon as they come in), OUTPUT (for altering locally gener‐
ated or (b)routed frames before they are bridged) and
POSTROUTING (for altering frames as they are about to go
out). A small note on the naming of chains PREROUTING and
POSTROUTING: it would be more accurate to call them PREFOR‐
WARDING and POSTFORWARDING, but for all those who come from
the iptables world to ebtables it is easier to have the
same names. Note that you can change the name (-E) if you
don't like the default.
...

Second argument to this NF_HOOK() should be NF_BR_PRE_ROUTING as it is not
know yet whether this packet is for local machine.

NF_BR_LOCAL_IN should locate in interface_rx just before netif_rx [*] - see below
NF_BR_LOCAL_OUT in interface_tx before big 'if (is_bcast ...)' [*]
NF_BR_POST_ROUTING in send_skb_packet instead of current NF_BR_LOCAL_OUT
NF_BR_FORWARD somewhere in recv_{bat,icmp,unicast,bcast}_packet() if
packet is being forwarded [*]


b) Why do you use bridge tables at all? This layer does not know
anything about batman layer, only ethernet that is only a tunnel for
batman. So, it is able to hook traffic from concrete prev-hop routers,
but not from original sources of packets. I think it is not enough for
network filter.
Also if you want to process [*] cases you have to append fake
ethernet headers before network header as NF_HOOK() would use ethernet
header.

So, I see 2 solutions:
1) write your own table layer similar to ebtables & userland tool :) It
is costly, but you'll be able to fully filter/hook of batman traffic.
2) write ebtables module to check batman header fields. It is slightly
slower, but potentially may do the same as (1).

However, I think it is not so important and may be rated as feature.


c) Maybe it's better to give user an ability to tune some network
parameters? Like ttl, whether answer to icmp, ratelimit of generating
output icmps, etc.


d) Why do you send icmp TTL exceeded for the icmp itself? E.g. in case
of loop or/and small default TTL you'll probably get a storm of icmps.
Exactly in this case IP silently drops TTL exceeded icmps ;)


Hope this information will be usefull.


Thanks,
Vasiliy.
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From: Sven Eckelmann on 13 Aug 2010 19:30
Vasiliy Kulikov wrote:
> On Tue, Aug 10, 2010 at 00:34 +0400, Vasiliy Kulikov wrote:
> > 2) It seems to me that NF_HOOK() at hard-interface.c:458 is misused:
> > ...
> >
> > ret = NF_HOOK(PF_BRIDGE, NF_BR_LOCAL_IN, skb, dev, NULL,
> >
> > batman_skb_recv_finish);
> >
> > if (ret != 1)
> >
> > goto err_out;
> >
> > /* packet should hold at least type and version */
> > if (unlikely(skb_headlen(skb) < 2))
> >
> > goto err_free;
> >
> > /* expect a valid ethernet header here. */
> > if (unlikely(skb->mac_len != sizeof(struct ethhdr)
> >
> > || !skb_mac_header(skb)))
> >
> > goto err_free;
> >
> > ...
> >
> > static int batman_skb_recv_finish(struct sk_buff *skb)
> > {
> >
> > return NF_ACCEPT;
> >
> > }
> >
> > As I understand, if there is any hook that returns NF_STOLEN, then skb
> > is leaked.
>
[...]

> b) Why do you use bridge tables at all? This layer does not know
> anything about batman layer, only ethernet that is only a tunnel for
> batman. So, it is able to hook traffic from concrete prev-hop routers,
> but not from original sources of packets. I think it is not enough for
> network filter.
> Also if you want to process [*] cases you have to append fake
> ethernet headers before network header as NF_HOOK() would use ethernet
> header.

Because a different person (no one from the actual development team) wanted to
have it for testing purposes. Maybe we just drop it again.

thanks,
Sven
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