cosmetic: vlr_auth_fsm: clarify decision on UMTS AKA or GSM AKA

The code deciding on whether UMTS AKA is used was cascaded and convoluted. By
flattening the decisions, they become easier to read and possibly catch more
weird corner cases / log information more clearly.

- First decide what AKA the RES length reflects.
- Then decide whether all prerequisites for UMTS AKA are satisfied.
- Finally, on UTRAN, turn down the auth if we don't have UMTS AKA, and neatly
  log all of the potential causes.

One corner case that should never occur is that the UMTS AKA RES length is
actually the same length as the GSM AKA SRES. If this nevertheless occurs, log
this as an error, though not turning down authentication because of it. (The
effect is that we would favor UMTS AKA when it has a res_len == sizeof(sres)
and would not succeed to GSM AKA. At least the log will tell us why, now.)

Adjust an expected test output, trivial logging difference.

Change-Id: I43f7f301ea85e518bac91f707391a53182e54fab

1 files changed, 42 insertions, 19 deletions

diff --git a/src/libvlr/vlr_auth_fsm.c b/src/libvlr/vlr_auth_fsm.c
index 51e22c9dc..0d0df1172 100644
--- a/src/libvlr/vlr_auth_fsm.c
+++ b/src/libvlr/vlr_auth_fsm.c
@@ -136,34 +136,57 @@ static bool check_auth_resp(struct vlr_subscr *vsub, bool is_r99,
 	struct gsm_auth_tuple *at = vsub->last_tuple;
 	struct osmo_auth_vector *vec = &at->vec;
 	bool check_umts;
+	bool res_is_umts_aka;
 	OSMO_ASSERT(at);
 
 	LOGVSUBP(LOGL_DEBUG, vsub, "received res: %s\n",
 		 osmo_hexdump(res, res_len));
 
 	/* RES must be present and at least 32bit */
-	if (!res || res_len < sizeof(vec->sres)) {
-		LOGVSUBP(LOGL_NOTICE, vsub, "AUTH RES missing or too short "
-			 "(%u)\n", res_len);
+	if (!res || !res_len) {
+		LOGVSUBP(LOGL_NOTICE, vsub, "AUTH SRES/RES missing\n");
 		goto out_false;
 	}
 
-	check_umts = false;
-	if (is_r99 && (vec->auth_types & OSMO_AUTH_TYPE_UMTS)) {
-		check_umts = true;
-		/* We have a R99 capable UE and have a UMTS AKA capable USIM.
-		 * However, the ME may still choose to only perform GSM AKA, as
-		 * long as the bearer is GERAN */
-		if (res_len != vec->res_len) {
-			if (is_utran) {
-				LOGVSUBP(LOGL_NOTICE, vsub,
-					 "AUTH via UTRAN but "
-					 "res_len(%u) != vec->res_len(%u)\n",
-					 res_len, vec->res_len);
-				goto out_false;
-			}
-			check_umts = false;
-		}
+	/* We're deciding the UMTS AKA-ness of the response by the RES size. So let's make sure we can't
+	 * mix them up by size. On UTRAN, we expect full length RES always, no way to mix up there. */
+	if (!is_utran && vec->res_len == sizeof(vec->sres))
+		LOGVSUBP(LOGL_ERROR, vsub, "Unforeseen situation: UMTS AKA's RES length"
+			 " equals the size of SRES: %u -- this code wants to differentiate"
+			 " the two by their size, which won't work properly now.\n", vec->res_len);
+
+	/* RES must be either vec->res_len (UMTS AKA) or sizeof(sres) (GSM AKA) */
+	if (res_len == vec->res_len)
+		res_is_umts_aka = true;
+	else if (res_len == sizeof(vec->sres))
+		res_is_umts_aka = false;
+	else {
+		if (is_utran)
+			LOGVSUBP(LOGL_NOTICE, vsub, "AUTH RES has invalid length: %u."
+				 " Expected %u (UMTS AKA)\n",
+				 res_len, vec->res_len);
+		else
+			LOGVSUBP(LOGL_NOTICE, vsub, "AUTH SRES/RES has invalid length: %u."
+				 " Expected either %zu (GSM AKA) or %u (UMTS AKA)\n",
+				 res_len, sizeof(vec->sres), vec->res_len);
+		goto out_false;
+	}
+
+	check_umts = (is_r99
+		      && (vec->auth_types & OSMO_AUTH_TYPE_UMTS)
+		      && res_is_umts_aka);
+
+	/* Even on an R99 capable MS with a UMTS AKA capable USIM,
+	 * the MS may still choose to only perform GSM AKA, as
+	 * long as the bearer is GERAN -- never on UTRAN: */
+	if (is_utran && !check_umts) {
+		LOGVSUBP(LOGL_ERROR, vsub,
+			 "AUTH via UTRAN, cannot allow GSM AKA"
+			 " (MS is %sR99 capable, vec has %sUMTS AKA tokens, res_len=%u is %s)\n",
+			 is_r99 ? "" : "NOT ",
+			 (vec->auth_types & OSMO_AUTH_TYPE_UMTS) ? "" : "NO ",
+			 res_len, (res_len == vec->res_len)? "valid" : "INVALID on UTRAN");
+		goto out_false;
 	}
 
 	if (check_umts) {