Hi Harald,

For the most part, you need to see the UNION definitions in src/mme/mme_context.h as below.

348 union {
349 struct {
350 ED3(uint8_t spare;,
351 uint16_t overflow;,
352 uint8_t sqn;)
353 } __attribute__ ((packed));
354 uint32_t i32;
355 } ul_count;

Therefore, increasing SQN will also increase ul_count.i32.

Let me add the comments in src/mme/nas_security.c

/* If SQN exeeds 1 bytes and turn in once again,

Overflow is increased */

197 if (mme_ue->ul_count.sqn > h->sequence_number)
198 mme_ue->ul_count.overflow++;

/* Always set SQN from UE's SQN as below */

199 mme_ue->ul_count.sqn = h->sequence_number;

Let me know if I have misunderstood anything.

Thanks a lot!

Best Regards,

Sukchan

On Sat, Jul 13, 2019 at 10:19 PM Harald Welte <laforge@gnumonks.org> wrote:

Hi Sukchan and list,

I'm currently reading the MME code and I'm having some trouble understanding
the handling of the uplink counter for NAS security.

I only see mme_ue->ul_count.i32 ever being set to '0' when a new security
context is used. But I don't see it ever being incremented? I only see
ul_count.sqn incremented, but then the nas_mac_calculate() always gets
ul_count.i32 passed as input.

So I'm somehow not understanding how the MAC can ever verify on any uplink
message beyond/after the first one which establishes a new security context.

What am I missing? Thanks for your insight!

--
- Harald Welte <laforge@gnumonks.org> http://laforge.gnumonks.org/
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