Re: [PATCH] l2tp: Add protocol field decompression

[Guillaume Nault]

On Fri, Dec 14, 2018 at 07:59:21PM +0200, Sam Protsenko wrote:
> When Protocol Field Compression (PFC) is enabled, the "Protocol" field
> in PPP packet will be received without leading 0x00. See section 6.5 in
> RFC 1661 for details. So let's decompress protocol field if needed, the
> same way it's done in drivers/net/ppp/pptp.c.
> 
> In case when "nopcomp" pppd option is not enabled, PFC (pcomp) can be
> negotiated during LCP handshake, and L2TP driver in kernel will receive
> PPP packets with compressed Protocol field, which in turn leads to next
> error:
> 
>     Protocol Rejected (unsupported protocol 0x2145)
> 
> because instead of Protocol=0x0021 in PPP packet there will be
> Protocol=0x21. This patch unwraps it back to 0x0021, which fixes the
> issue.
> 
> Sending the compressed Protocol field will be implemented in subsequent
> patch, this one is self-sufficient.
> 
> Signed-off-by: Sam Protsenko <semen.protsenko@xxxxxxxxxx>
> ---
>  net/l2tp/l2tp_ppp.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
> index 04d9946dcdba..c03c6461f236 100644
> --- a/net/l2tp/l2tp_ppp.c
> +++ b/net/l2tp/l2tp_ppp.c
> @@ -236,6 +236,10 @@ static void pppol2tp_recv(struct l2tp_session *session, struct sk_buff *skb, int
>  	    skb->data[1] == PPP_UI)
>  		skb_pull(skb, 2);
>  
> +	/* Decompress protocol field if PFC is enabled */
> +	if ((*skb->data) & 0x1)
> +		*(u8 *)skb_push(skb, 1) = 0;
> +
Sorry, but PFC is PPP stuff. It's absolutely independent from the
transport layer. Therefore, it belongs to ppp_generic.c and L2TP has
nothing do with it.

I know some other transports got it wrong, but let's stop
re-implementing PPP features in every lower layer. Teaching ppp_input()
about PFC is all that is needed to get the feature work correctly on
all transports. As a bonus, it'd bring PFC support to PPPoE and would
let us drop the duplicated code from pptp.c, ppp_async.c and
ppp_synctty.c.

As an example of the problems caused by mixing the PPP and L2TP
layers, consider the case of L2TP sockets that aren't PPPOX_BOUND.
They're supposed to receive the original PPP frames. Now with your
patch if the protocol was compressed, the L2TP socket receives a fake
header. User space didn't ask for that and has no way to figure out
what was the original packet. To make things worse, that behaviour now
changes depending on the kernel version.

If you all agree, can we please revert this patch and properly
implement PFC in ppp_generic.c?