X.509: Extract signature digest and make self-signed cert checks earlier

Extract the signature digest for an X.509 certificate earlier, at the end
of x509_cert_parse() rather than leaving it to the callers thereof since it
has to be called anyway.

Further, immediately after that, check the signature on self-signed
certificates, also rather in the callers of x509_cert_parse().

We note in the x509_certificate struct the following bits of information:

(1) Whether the signature is self-signed (even if we can't check the
signature due to missing crypto).

(2) Whether the key held in the certificate needs unsupported crypto to be
used. We may get a PKCS#7 message with X.509 certs that we can't make
use of - we just ignore them and give ENOPKG at the end it we couldn't
verify anything if at least one of these unusable certs are in the
chain of trust.

(3) Whether the signature held in the certificate needs unsupported crypto
to be checked. We can still use the key held in this certificate,
even if we can't check the signature on it - if it is held in the
system trusted keyring, for instance. We just can't add it to a ring
of trusted keys or follow it further up the chain of trust.

Making these checks earlier allows x509_check_signature() to be removed and
replaced with direct calls to public_key_verify_signature().

Signed-off-by: David Howells <[e-mail]>

crypto/asymmetric_keys/pkcs7_verify.c

···
 			 x509->subject,
 			 x509->raw_serial_size, x509->raw_serial);
 		x509->seen = true;
-		ret = x509_get_sig_params(x509);
-		if (ret < 0)
-			goto maybe_missing_crypto_in_x509;
+		if (x509->unsupported_key)
+			goto unsupported_crypto_in_x509;
 
 		pr_debug("- issuer %s\n", x509->issuer);
 		sig = x509->sig;
···
 			pr_debug("- authkeyid.skid %*phN\n",
 				 sig->auth_ids[1]->len, sig->auth_ids[1]->data);
 
-		if ((!x509->sig->auth_ids[0] && !x509->sig->auth_ids[1]) ||
-		    strcmp(x509->subject, x509->issuer) == 0) {
+		if (x509->self_signed) {
 			/* If there's no authority certificate specified, then
 			 * the certificate must be self-signed and is the root
 			 * of the chain.  Likewise if the cert is its own
 			 * authority.
 			 */
-			pr_debug("- no auth?\n");
-			if (x509->raw_subject_size != x509->raw_issuer_size ||
-			    memcmp(x509->raw_subject, x509->raw_issuer,
-				   x509->raw_issuer_size) != 0)
-				return 0;
-
-			ret = x509_check_signature(x509->pub, x509);
-			if (ret < 0)
-				goto maybe_missing_crypto_in_x509;
+			if (x509->unsupported_sig)
+				goto unsupported_crypto_in_x509;
 			x509->signer = x509;
 			pr_debug("- self-signed\n");
 			return 0;
···
 				sinfo->index);
 			return 0;
 		}
-		ret = x509_check_signature(p->pub, x509);
+		ret = public_key_verify_signature(p->pub, p->sig);
 		if (ret < 0)
 			return ret;
 		x509->signer = p;
···
 		might_sleep();
 	}
 
-maybe_missing_crypto_in_x509:
+unsupported_crypto_in_x509:
 	/* Just prune the certificate chain at this point if we lack some
 	 * crypto module to go further.  Note, however, we don't want to set
-	 * sinfo->missing_crypto as the signed info block may still be
+	 * sinfo->unsupported_crypto as the signed info block may still be
 	 * validatable against an X.509 cert lower in the chain that we have a
 	 * trusted copy of.
 	 */
-	if (ret == -ENOPKG)
-		return 0;
-	return ret;
+	return 0;
 }
 
 /*
···
 		 enum key_being_used_for usage)
 {
 	struct pkcs7_signed_info *sinfo;
-	struct x509_certificate *x509;
 	int enopkg = -ENOPKG;
-	int ret, n;
+	int ret;
 
 	kenter("");
 
···
 		break;
 	default:
 		return -EINVAL;
-	}
-
-	for (n = 0, x509 = pkcs7->certs; x509; x509 = x509->next, n++) {
-		ret = x509_get_sig_params(x509);
-		if (ret < 0)
-			return ret;
 	}
 
 	for (sinfo = pkcs7->signed_infos; sinfo; sinfo = sinfo->next) {

crypto/asymmetric_keys/x509_cert_parser.c

···
 
 	cert->pub->keylen = ctx->key_size;
 
+	/* Grab the signature bits */
+	ret = x509_get_sig_params(cert);
+	if (ret < 0)
+		goto error_decode;
+
 	/* Generate cert issuer + serial number key ID */
 	kid = asymmetric_key_generate_id(cert->raw_serial,
 					 cert->raw_serial_size,
···
 		goto error_decode;
 	}
 	cert->id = kid;
+
+	/* Detect self-signed certificates */
+	ret = x509_check_for_self_signed(cert);
+	if (ret < 0)
+		goto error_decode;
 
 	kfree(ctx);
 	return cert;

crypto/asymmetric_keys/x509_parser.h

···
 	bool		seen;			/* Infinite recursion prevention */
 	bool		verified;
 	bool		trusted;
-	bool		unsupported_crypto;	/* T if can't be verified due to missing crypto */
+	bool		self_signed;		/* T if self-signed (check unsupported_sig too) */
+	bool		unsupported_key;	/* T if key uses unsupported crypto */
+	bool		unsupported_sig;	/* T if signature uses unsupported crypto */
 };
 
 /*
···
  * x509_public_key.c
  */
 extern int x509_get_sig_params(struct x509_certificate *cert);
-extern int x509_check_signature(const struct public_key *pub,
-				struct x509_certificate *cert);
+extern int x509_check_for_self_signed(struct x509_certificate *cert);

crypto/asymmetric_keys/x509_public_key.c

···
 
 	pr_devel("==>%s()\n", __func__);
 
-	if (cert->unsupported_crypto)
-		return -ENOPKG;
-	if (sig->s)
+	if (!cert->pub->pkey_algo)
+		cert->unsupported_key = true;
+
+	if (!sig->pkey_algo)
+		cert->unsupported_sig = true;
+
+	/* We check the hash if we can - even if we can't then verify it */
+	if (!sig->hash_algo) {
+		cert->unsupported_sig = true;
 		return 0;
+	}
 
 	sig->s = kmemdup(cert->raw_sig, cert->raw_sig_size, GFP_KERNEL);
 	if (!sig->s)
···
 	tfm = crypto_alloc_shash(sig->hash_algo, 0, 0);
 	if (IS_ERR(tfm)) {
 		if (PTR_ERR(tfm) == -ENOENT) {
-			cert->unsupported_crypto = true;
-			return -ENOPKG;
+			cert->unsupported_sig = true;
+			return 0;
 		}
 		return PTR_ERR(tfm);
 	}
···
 	pr_devel("<==%s() = %d\n", __func__, ret);
 	return ret;
 }
-EXPORT_SYMBOL_GPL(x509_get_sig_params);
 
 /*
- * Check the signature on a certificate using the provided public key
+ * Check for self-signedness in an X.509 cert and if found, check the signature
+ * immediately if we can.
  */
-int x509_check_signature(const struct public_key *pub,
-			 struct x509_certificate *cert)
+int x509_check_for_self_signed(struct x509_certificate *cert)
 {
-	int ret;
+	int ret = 0;
 
 	pr_devel("==>%s()\n", __func__);
 
-	ret = x509_get_sig_params(cert);
-	if (ret < 0)
-		return ret;
+	if (cert->sig->auth_ids[0] || cert->sig->auth_ids[1]) {
+		/* If the AKID is present it may have one or two parts.  If
+		 * both are supplied, both must match.
+		 */
+		bool a = asymmetric_key_id_same(cert->skid, cert->sig->auth_ids[1]);
+		bool b = asymmetric_key_id_same(cert->id, cert->sig->auth_ids[0]);
 
-	ret = public_key_verify_signature(pub, cert->sig);
-	if (ret == -ENOPKG)
-		cert->unsupported_crypto = true;
-	pr_debug("Cert Verification: %d\n", ret);
+		if (!a && !b)
+			goto not_self_signed;
+
+		ret = -EKEYREJECTED;
+		if (((a && !b) || (b && !a)) &&
+		    cert->sig->auth_ids[0] && cert->sig->auth_ids[1])
+			goto out;
+	}
+
+	ret = public_key_verify_signature(cert->pub, cert->sig);
+	if (ret < 0) {
+		if (ret == -ENOPKG) {
+			cert->unsupported_sig = true;
+			ret = 0;
+		}
+		goto out;
+	}
+
+	pr_devel("Cert Self-signature verified");
+	cert->self_signed = true;
+
+out:
+	pr_devel("<==%s() = %d\n", __func__, ret);
 	return ret;
+
+not_self_signed:
+	pr_devel("<==%s() = 0 [not]\n", __func__);
+	return 0;
 }
-EXPORT_SYMBOL_GPL(x509_check_signature);
 
 /*
  * Check the new certificate against the ones in the trust keyring.  If one of
···
 	struct key *key;
 	int ret = 1;
 
+	if (!sig->auth_ids[0] && !sig->auth_ids[1])
+		return 1;
+
 	if (!trust_keyring)
 		return -EOPNOTSUPP;
-
 	if (ca_keyid && !asymmetric_key_id_partial(sig->auth_ids[1], ca_keyid))
 		return -EPERM;
+	if (cert->unsupported_sig)
+		return -ENOPKG;
 
 	key = x509_request_asymmetric_key(trust_keyring,
 					  sig->auth_ids[0], sig->auth_ids[1],
 					  false);
-	if (!IS_ERR(key))  {
-		if (!use_builtin_keys
-		    || test_bit(KEY_FLAG_BUILTIN, &key->flags))
-			ret = x509_check_signature(key->payload.data[asym_crypto],
-						   cert);
-		key_put(key);
+	if (IS_ERR(key))
+		return PTR_ERR(key);
+
+	if (!use_builtin_keys ||
+	    test_bit(KEY_FLAG_BUILTIN, &key->flags)) {
+		ret = public_key_verify_signature(
+			key->payload.data[asym_crypto], cert->sig);
+		if (ret == -ENOPKG)
+			cert->unsupported_sig = true;
 	}
+	key_put(key);
 	return ret;
 }
 
···
 	pr_devel("Cert Issuer: %s\n", cert->issuer);
 	pr_devel("Cert Subject: %s\n", cert->subject);
 
-	if (!cert->pub->pkey_algo ||
-	    !cert->sig->pkey_algo ||
-	    !cert->sig->hash_algo) {
+	if (cert->unsupported_key) {
 		ret = -ENOPKG;
 		goto error_free_cert;
 	}
 
 	pr_devel("Cert Key Algo: %s\n", cert->pub->pkey_algo);
 	pr_devel("Cert Valid period: %lld-%lld\n", cert->valid_from, cert->valid_to);
-	pr_devel("Cert Signature: %s + %s\n",
-		 cert->sig->pkey_algo,
-		 cert->sig->hash_algo);
 
 	cert->pub->id_type = "X509";
 
-	/* Check the signature on the key if it appears to be self-signed */
-	if ((!cert->sig->auth_ids[0] && !cert->sig->auth_ids[1]) ||
-	    asymmetric_key_id_same(cert->skid, cert->sig->auth_ids[1]) ||
-	    asymmetric_key_id_same(cert->id, cert->sig->auth_ids[0])) {
-		ret = x509_check_signature(cert->pub, cert); /* self-signed */
-		if (ret < 0)
-			goto error_free_cert;
-	} else if (!prep->trusted) {
+	/* See if we can derive the trustability of this certificate.
+	 *
+	 * When it comes to self-signed certificates, we cannot evaluate
+	 * trustedness except by the fact that we obtained it from a trusted
+	 * location.  So we just rely on x509_validate_trust() failing in this
+	 * case.
+	 *
+	 * Note that there's a possibility of a self-signed cert matching a
+	 * cert that we have (most likely a duplicate that we already trust) -
+	 * in which case it will be marked trusted.
+	 */
+	if (cert->unsupported_sig || cert->self_signed) {
+		public_key_signature_free(cert->sig);
+		cert->sig = NULL;
+	} else {
+		pr_devel("Cert Signature: %s + %s\n",
+			 cert->sig->pkey_algo, cert->sig->hash_algo);
+
 		ret = x509_validate_trust(cert, get_system_trusted_keyring());
 		if (ret)
 			ret = x509_validate_trust(cert, get_ima_mok_keyring());
+		if (ret == -EKEYREJECTED)
+			goto error_free_cert;
 		if (!ret)
-			prep->trusted = 1;
+			prep->trusted = true;
 	}
 
 	/* Propose a description */