MME not handling Emergency Attach correctly

[spencersevilla]

When a UE sends an Attach Request, the normal/expected values for "EPS Attach Type" in NAS message are 0x1 (EPS attach) or 0x2 (combined EPS/IMSI attach). Interestingly, our code does not appear to check this value and simply copies it into the "EPS Attach Result" field in the Attach Accept message. This logic works for those normal values above, but fails for request type 0x110 ("EPS Emergency Attach") in attach request, which maps to "Reserved" in attach response. When we send this Attach accept message, we then receive an EMM Status error.

Copying the value when we make the attach accept could make sense if we write in different path-handling for Emergency Attach. We need to read the 3GPP and figure out how to handle these emergency attach messages, since this is important. I don't think we want to (or can) handle emergency calls so we should probably respond with an Attach Reject or something along those lines.

Refs:

• src/mme/emm_build.c:60 is where we just copy the value over
• pg 236 of https://www.arib.or.jp/english/html/overview/doc/STD-T63V12_20/5_Appendix/Rel9/24/24301-9b0.pdf shows the values
• eNB_ID 136 in the attached PCAP (which is labeled as .zip to fool GitHub).
  emergency_attach.zip

[spencersevilla]

@pathiratk I think this would be a great one to focus on :-)