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Feature #4532

fix writing the various PLMN files

Added by laforge 7 months ago. Updated 7 months ago.

Status:
In Progress
Priority:
High
Assignee:
Target version:
-
Start date:
05/05/2020
Due date:
% Done:

70%

Spec Reference:

Description

Changing the MCC/MNC prefix of the IMSI is not sufficient anymore for making phones assume that the new MCC/MNC is their home network. In addition, a variety of files with "PLMN" in their name may need updating:
  • PLMNsel
  • PLMNwAcT
  • OPLMNwAcT
  • HPLMNAcT

However, related updates don't always happen on all cards, as they should.

mksim_sja2.sh mksim_sja2.sh 326 Bytes dexter, 05/11/2020 11:38 AM
sysmo_usim_sja2_after_write.txt sysmo_usim_sja2_after_write.txt 5.65 KB dexter, 05/11/2020 11:38 AM
sysmo_usim_sja2_before_write.txt sysmo_usim_sja2_before_write.txt 5.62 KB dexter, 05/11/2020 11:38 AM
mobilcom_sim.txt mobilcom_sim.txt 4.8 KB dexter, 05/12/2020 04:57 PM

Checklist

  • verify sysmoISIM-SJA2 fix
  • ensure we have a test covering all PLMN files are written correctly
  • port fix to WavemobileSIM (if needed, maybe it doesn't haven have that EF?)

History

#1 Updated by laforge 7 months ago

  • Checklist item verify sysmoISIM-SJA2 fix added
  • Checklist item ensure we have a test covering all PLMN files are written correctly added
  • Checklist item port fix to WavemobileSIM (if needed, maybe it doesn't haven have that EF?) added
  • Subject changed from support writing the various PLMN files to fix writing the various PLMN files
  • Assignee set to dexter
  • Priority changed from Low to High

Contrary to my original assumption, we actually do have code to write them. This code is used in both SysmoUSIMSJS1 and FairwavesSIM. However, the code is missing from the sysmoISIMSJA2 (and also from WavemobileSIM).

dexter, I created a trivial fix for the sysmoISIMSJA2, see https://gerrit.osmocom.org/c/pysim/+/18051

Please verify this patch, make sure we have test coverage (our tests didn't discover this bug, they should!) and also consider expanding the fix to WavemobileSIM

#2 Updated by laforge 7 months ago

  • Description updated (diff)

#3 Updated by dexter 7 months ago

I have checked the problem back and from what I can see PLMNsel, PLMNwAcT, OPLMNwAcT, HPLMNAcT are updated. I have attached the output of pySim-Read.py before and after the write. So from what I can see the patch is working fine.

#4 Updated by laforge 7 months ago

There are some open questions on the correctness of the encoding of the files (see related sysmocom customer support inquiry)

#5 Updated by dexter 7 months ago

The implementation of enc_plmn() in utils.py seems have a problem with padding unused digits correctly.

See 3GPP TS 51.011, 10.3.4 gives an example, lets try encoding it manually and see if the result matches up

246 81 MCC/MNC
246F81 Padd unused digit with 0xF
42F618 Swap nibbles (matches also output of enc_plmn()

And now lets try the PLMN we try to use

262 02
262F02
62F220

When calling enc_plmn() manually with 262 02, we get: 62f22f

I think the way enc_plmn handles leading zeros is wrong.

#6 Updated by dexter 7 months ago

I had a look at a simcard from a productive network. There the leading zeros of the MNC are also preserved but what confuses me is that the PLMNsel file does not start with 62f210, which would be the home network (262 01).

Here is what what the beginning of my PLMNsel looks like:

32f23012f47022f21002f8100

32f230 => 232f03 => 232 03
12f470 => 214f07 => 214 07

Attached one can find the dump from the simcard I used for this test.

#7 Updated by laforge 7 months ago

In case this is not obvious: MNCs come as 2-digit MNC or as 3-digit MNC.

Hence, 262-02 and 262-002 are different networks.

The encoding as per TS 04.08 is implemented in libosmocore for more than a decade and we use it pretty much anywhre on the radio interface, whether in RR or MM or other sub-layers such as BSSMAP. They python code should just simply replicate what the C code is doing.

The encoding of the above two examples should be:
  • 262-02 -> 62F220
  • 262-002 -> 620220

Furthermore, it is not surprising that EF_PLMNsel doesn't list the home operator (derived from the IMSI prefix). In classic 2G SIM cards, the home network is always derived from the IMSI prefix, and it cannot be overridden. Only in USIM, it is possible that the HPLMN MCC/MNC is explicitly set different (e.g. like the EF_HPLMNwAcT

#8 Updated by laforge 7 months ago

It seems like pySim unfortunately treats mnc as integer. So there is no way to know if the user entererd 02 or 002 (referring to the above example). I suggest to
  • treat MNC as a string
  • verify that it only contains decimal digits
  • pad it to three characters length (if it is 2 only) using a leading F
  • then concatenate + nibble-swap with the MCC in enc_plmn()

I hacked this up in https://gerrit.osmocom.org/c/pysim/+/18225 and it seemed to work locally with both 2-digit and 3-digit MNC as well as 1/2/3 digit MCC.

#9 Updated by laforge 7 months ago

  • % Done changed from 20 to 70

patch has been merged. dexter, please check the two missing checklist items in this ticket and take any action, if required.

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