From: Theo Buehler <tb@theobuehler.org>
Subject: Re: rpki-client: fix BGPsec router EKU handling
To: tech@openbsd.org
Date: Mon, 10 Jun 2024 13:17:41 +0200

On Sat, Jun 08, 2024 at 08:05:00PM +0200, Theo Buehler wrote:
> The semantics of EKU is that we need to find the purpose we're looking
> for, and not care about the others. There are legitimate reasons for
> allowing other EKUs. The lot of the 3 BGPsec certs out there are still
> recognized with this diff since all of them only have a single
> keyPurposeId.

Rebased to apply cleanly on top of current.

Index: x509.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/x509.c,v
diff -u -p -r1.98 x509.c
--- x509.c	10 Jun 2024 10:50:13 -0000	1.98
+++ x509.c	10 Jun 2024 11:16:26 -0000
@@ -277,7 +277,7 @@ x509_get_purpose(X509 *x, const char *fn
 	BASIC_CONSTRAINTS		*bc = NULL;
 	EXTENDED_KEY_USAGE		*eku = NULL;
 	const X509_EXTENSION		*ku;
-	int				 crit, ext_flags, is_ca, ku_idx;
+	int				 crit, ext_flags, i, is_ca, ku_idx;
 	enum cert_purpose		 purpose = CERT_PURPOSE_INVALID;
 
 	if (!x509_cache_extensions(x, fn))
@@ -388,20 +388,15 @@ x509_get_purpose(X509 *x, const char *fn
 	}
 
 	/*
-	 * XXX - this isn't quite correct: other EKU OIDs are allowed per
-	 * RFC 8209, section 3.1.3.2, e.g., anyEKU could potentially help
-	 * avoid tripping up validators that don't know about the BGPsec
-	 * router purpose. Drop check or downgrade from error to warning?
+	 * Per RFC 8209, section 3.1.3.2 the id-kp-bgpsec-router OID must be
+	 * present and others are allowed, which we need not recognize. This
+	 * matches RFC 5280, section 4.2.1.12.
 	 */
-	if (sk_ASN1_OBJECT_num(eku) != 1) {
-		warnx("%s: EKU: expected 1 purpose, have %d", fn,
-		    sk_ASN1_OBJECT_num(eku));
-		goto out;
-	}
-
-	if (OBJ_cmp(bgpsec_oid, sk_ASN1_OBJECT_value(eku, 0)) == 0) {
-		purpose = CERT_PURPOSE_BGPSEC_ROUTER;
-		goto out;
+	for (i = 0; i < sk_ASN1_OBJECT_num(eku); i++) {
+		if (OBJ_cmp(bgpsec_oid, sk_ASN1_OBJECT_value(eku, i)) == 0) {
+			purpose = CERT_PURPOSE_BGPSEC_ROUTER;
+			break;
+		}
 	}
 
  out: