Bug #3806: OsmoBSC accepts BSSAP with wrong length field - OsmoBSC - Open Source Mobile Communications

Bug #3806

OsmoBSC accepts BSSAP with wrong length field

Added by laforge over 1 year ago. Updated about 2 months ago.

Status:

Stalled

Priority:

Normal

Assignee:

Category:

A interface

Target version:

Start date:

02/18/2019

Due date:

% Done:

40%

Spec Reference:

Description

As seen in #3805, OsmoBSC would happily accept BSSMAP CLEAR COMMAND messages with IEs that extend beyond the length field of the BSSAP header.

This is definitely wrong. We should

parse the length field
ensure we have a minimum of that number of bytes of payload as specified by the length field
truncate the msgb to a payload length as specified

This way any additional garbage at the end of a message would simply be ignored, with us only parsing the specified "length" number of bytes.

Let's also make sure to add TTCN-3 tests for this, intentionally sending length field values too large and too short.

Once implemented in OsmoBSC, we should also implement it on the MSC side.

bssap_adapter.diff

774 Bytes

laforge, 03/26/2019 10:11 AM

Related issues

History

#1 Updated by laforge over 1 year ago

Related to Bug #3805: OsmoMSC sends invalid BSSMAP length field on CSFB CLEAR COMMAND added

#2 Updated by dexter over 1 year ago

Status changed from New to In Progress
% Done changed from 0 to 40

I have now integrated checking+truncating of the bssmap message length, there is no TTCN3 test yet.

https://gerrit.osmocom.org/#/c/osmo-bsc/+/13306 osmo_bsc_bssap: check bssamp length field
https://gerrit.osmocom.org/#/c/osmo-msc/+/13307 a_iface_bssap: check bssamp length field

#3 Updated by dexter over 1 year ago

The two patches for the length check, which I proposed look good in review. However, I have problems creating with TTCN3 for this since TTCN3 seems to let me only generate valid CLEAR COMMANDs. The best would be if I could somehow send a hand crafted SCCP payload, however, I do not know if this is possible.

#4 Updated by laforge over 1 year ago

File bssap_adapter.diff bssap_adapter.diff added

On Tue, Mar 26, 2019 at 08:40:40AM +0000, dexter [REDMINE] wrote:

The two patches for the length check, which I proposed look good in review. However, I have problems creating with TTCN3 for this since TTCN3 seems to let me only generate valid CLEAR COMMANDs. The best would be if I could somehow send a hand crafted SCCP payload, however, I do not know if this is possible.

1) Normal option

Normally, You'd need to use the SCCP_Emulation directly, without the BSSMAP_Emulation on top.

2) Hackish option

Alternatively, you could extend the BSSAP_Conn_PT (port betwene BSSMAP_Emulation and ConnHdlr)
to accept something like 'octetstring' (in addition to PDU_BSSAP, etc.) and extend the
BSSMAP_Emulation.main() function with a

    [] CLIENT.receive(octetstring:?) -> value oct {
        BSSAP.send(oct);
    }

you'd also have to extend BSSAP_CODEC_PT similarly, implementing somethin like a
"type record BSSAP_N_DATA_RAW_req" which uses "octetstring" instead of PDU_BSSAP.

However, I guess at that point it all becomes too complex. So I'd suggest to simply
go for yet another way:

3) BSSAP_Adapter without "ops"

When you call f_bssmap_init with an "omit" argument as BssmapOps, then you basically
get the entire SIGTRAN stack up to SCCP, but without any BSSAP/BSSMAP on top.

You would then have something like a

modulepar {
    BSSAP_Configuration mp_bssap_cfg := { ... }; /* like BSC_Tests.ttcn */
}

component RAW_SCCP_CT {
    BSSAP_Adapter g_ba;
    port SCCPasp_PT SCCP;
}

function f_init() runs on RAW_SCCP_CT {
    f_bssap_init(g_ba, mp_cfg, "RAW_SCCP", omit);
    connect(self:SCCP, g_ba.vc_SCCP:SCCP_SP_PORT);
}

from that point onwards, you can then use things like

testcase TC_foo() runs on RAW_SCCP_CT {
    f_init();
    SCCP.send(t_ASP_N_CONNECT_req(called, calling, omit, omit, '01020304'O, conn_id, omit));
    SCCP.receive(tr_ASP_N_CONNECT_res ....

You may need the attached patch to fix a bug in BSSAP_Adapter.ttcn