Re: [PATCH 5/5] gtp: let userspace handle packets for invalid tunnels

[Andreas Schultz <aschultz@tpip.net>]

Hi Pablo,

----- On Jan 24, 2017, at 8:03 PM, pablo pablo@netfilter.org wrote:

> Hi Andreas,
> 
> On Tue, Jan 24, 2017 at 06:24:02PM +0100, Andreas Schultz wrote:
>> enable userspace to send error replies for invalid tunnels
>> 
>> Signed-off-by: Andreas Schultz <aschultz@tpip.net>
>> ---
>>  drivers/net/gtp.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
>> index 912721e..c607333 100644
>> --- a/drivers/net/gtp.c
>> +++ b/drivers/net/gtp.c
>> @@ -198,12 +198,12 @@ static int gtp0_udp_encap_recv(struct gtp_dev *gtp, struct
>> sk_buff *skb,
>>  	pctx = gtp0_pdp_find(gtp, be64_to_cpu(gtp0->tid));
>>  	if (!pctx) {
>>  		netdev_dbg(gtp->dev, "No PDP ctx to decap skb=%p\n", skb);
>> -		return -1;
>> +		return 1;
>>  	}
>>  
>>  	if (!gtp_check_src_ms(skb, pctx, hdrlen)) {
>>  		netdev_dbg(gtp->dev, "No PDP ctx for this MS\n");
>> -		return -1;
>> +		return 1;
> 
> So userspace gets the packet that we cannot forward. I guess your
> userspace codebase performs this sanity checks again so you can send
> the appropriate error reply?

For TEID /= 0, the only reply is a T-PDU of type error indication. There
is no cause specified. So I don't actually have to repeat the check.
TEID == 0 is more interesting, this tells userspace that it tried to
send on an invalid tunnel and should tear it down.

If you like, you can have a look at the userspace code. The relevant piece
is at https://github.com/travelping/gtp_u_kmod/blob/master/src/gtp_u_kmod_port.erl#L231

But be warned, it's written in Erlang ;-)

Andreas

> 
>>  	}
>>  
>>  	/* Get rid of the GTP + UDP headers. */
>> @@ -247,12 +247,12 @@ static int gtp1u_udp_encap_recv(struct gtp_dev *gtp,
>> struct sk_buff *skb,
>>  	pctx = gtp1_pdp_find(gtp, ntohl(gtp1->tid));
>>  	if (!pctx) {
>>  		netdev_dbg(gtp->dev, "No PDP ctx to decap skb=%p\n", skb);
>> -		return -1;
>> +		return 1;
>>  	}
>>  
>>  	if (!gtp_check_src_ms(skb, pctx, hdrlen)) {
>>  		netdev_dbg(gtp->dev, "No PDP ctx for this MS\n");
>> -		return -1;
>> +		return 1;
>>  	}
>>  
>>  	/* Get rid of the GTP + UDP headers. */
>> --
>> 2.10.2