linux-kernel - [PATCH 3/7] KEYS: X.509: Parse Key Usage

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Date:   Tue,  5 Apr 2022 21:53:33 -0400
From:   Eric Snowberg <eric.snowberg@...cle.com>
To:     dhowells@...hat.com, dwmw2@...radead.org, jarkko@...nel.org,
        zohar@...ux.ibm.com, linux-integrity@...r.kernel.org
Cc:     herbert@...dor.apana.org.au, davem@...emloft.net,
        dmitry.kasatkin@...il.com, jmorris@...ei.org, serge@...lyn.com,
        roberto.sassu@...wei.com, nramas@...ux.microsoft.com,
        eric.snowberg@...cle.com, pvorel@...e.cz, tiwai@...e.de,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: [PATCH 3/7] KEYS: X.509: Parse Key Usage

Parse the X.509 Key Usage.  The key usage extension defines the purpose of
the key contained in the certificate.

   id-ce-keyUsage OBJECT IDENTIFIER ::=  { id-ce 15 }

      KeyUsage ::= BIT STRING {
           digitalSignature        (0),
           contentCommitment       (1),
           keyEncipherment         (2),
           dataEncipherment        (3),
           keyAgreement            (4),
           keyCertSign             (5),
           cRLSign                 (6),
           encipherOnly            (7),
           decipherOnly            (8) }

If the keyCertSign is set, store it in the x509_certificate structure.
This will be used in a follow on patch that requires knowing the
certificate key usage type.

Signed-off-by: Eric Snowberg <eric.snowberg@...cle.com>
---
 crypto/asymmetric_keys/x509_cert_parser.c | 20 ++++++++++++++++++++
 crypto/asymmetric_keys/x509_parser.h      |  1 +
 2 files changed, 21 insertions(+)

diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 30f7374ea9c0..a89f1e0c8a0f 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -576,6 +576,26 @@ int x509_process_extension(void *context, size_t hdrlen,
 		return 0;
 	}
 
+	if (ctx->last_oid == OID_keyUsage) {
+		/*
+		 * Get hold of the keyUsage bit string to validate keyCertSign
+		 * v[1] is the encoding size
+		 *       (Expect either 0x02 or 0x03, making it 1 or 2 bytes)
+		 * v[2] is the number of unused bits in the bit string
+		 *       (If >= 3 keyCertSign is missing)
+		 * v[3] and possibly v[4] contain the bit string
+		 * 0x04 is where KeyCertSign lands in this bit string (from
+		 *      RFC 5280 4.2.1.3)
+		 */
+		if (v[0] != ASN1_BTS || vlen < 4)
+			return -EBADMSG;
+		if (v[1] == 0x02 && v[2] <= 2 && (v[3] & 0x04))
+			ctx->cert->is_kcs_set = true;
+		else if (vlen > 4 && v[1] == 0x03 && (v[3] & 0x04))
+			ctx->cert->is_kcs_set = true;
+		return 0;
+	}
+
 	if (ctx->last_oid == OID_authorityKeyIdentifier) {
 		/* Get hold of the CA key fingerprint */
 		ctx->raw_akid = v;
diff --git a/crypto/asymmetric_keys/x509_parser.h b/crypto/asymmetric_keys/x509_parser.h
index dc45df9f6594..d6ac0985d8a5 100644
--- a/crypto/asymmetric_keys/x509_parser.h
+++ b/crypto/asymmetric_keys/x509_parser.h
@@ -39,6 +39,7 @@ struct x509_certificate {
 	bool		unsupported_sig;	/* T if signature uses unsupported crypto */
 	bool		blacklisted;
 	bool		is_root_ca;		/* T if basic constraints CA is set */
+	bool		is_kcs_set;		/* T if keyCertSign is set */
 };
 
 /*
-- 
2.27.0