Re: L2CAP ERTM compatibility problem

["Gustavo F. Padovan" <gustavo@padovan.org>]

Hi Nathan,

* Nathan Holstein <ngh@isomerica.net> [2010-06-08 13:20:37 -0400]:

> I'm currently testing an older set of Gustavo's ERTM fixes backported
> onto a 2.6.32 kernel.  I'm seeing a compatibility problem while
> attempting to establish a connection using ERTM.  This appears to be
> caused by commit 277ffbe in Gustavo's testing tree which adds minimum
> length checks to received ERTM and Streaming Mode packets.
> 
> Here's the incoming packet (hcidump -Xt):
> 
>         1276011301.453633 > ACL data: handle 12 flags 0x02 dlen 11
>             L2CAP(d): cid 0x0040 len 7 [psm 0]
>               0000: 02 01 07 00 06 2e 0b
> 
> This appears to be a valid packet containing a three byte sequence which
> should be passed up to the application.  However, the check added by
> commit 277ffbe causes the kernel to disconnect the socket since the data
> length is less than 4.
> 
> I don't see the point of the minimum length at this point in the code.
> The len variable already subtracts for the control, FCS and SAR fields
> if necessary.  It seems to me that the remaining length of the skb is
> purely socket data, and as such should be passed on to the application.

The checks comes from the section 3.3.7 of the L2CAP spec.

> 
> Here's my proposed fix:
> 
>         @@ -4189,19 +4193,23 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
>          
>          
>                         if (__is_iframe(control)) {
>         -                       if (len < 4) {
>         +                       if (len < 0) {
>                                         l2cap_send_disconn_req(pi->conn, sk);
>                                         goto drop;
>                                 }
>         @@ -4222,7 +4230,7 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
>                         if (pi->fcs == L2CAP_FCS_CRC16)
>                                 len -= 2;
>          
>         -               if (len > pi->mps || len < 4 || __is_sframe(control))
>         +               if (len > pi->mps || len < 0 || __is_sframe(control))
>                                 goto drop;
>          
>                         if (l2cap_check_fcs(pi, skb))
> 
> This lowers the minimum-packet length of I-frame and streaming mode
> packets put in place by commit 277ffbe.  It maintains the length check
> for S-frames.  I believe ensuring a non-negative length is necessary,
> but this check may need to be performed prior to the call to
> l2cap_check_fcs().

I agree with you that we should lower the value to 0. Please, send a proper
git-formatted patch so I can pick it.


Regards,

-- 
Gustavo F. Padovan
http://padovan.org