[PATCH v3 2/2] certs: Add support for using elliptic curve keys for signing modules

[Stefan Berger]

Add support for using elliptic curve keys for signing modules. It uses
a NIST P384 (secp384r1) key if the user chooses an elliptic curve key
and will have ECDSA support built into the kernel.

Note: A developer choosing an ECDSA key for signing modules should still
delete the signing key (rm certs/signing_key.*) when building an older
version of a kernel that only supports RSA keys. Unless kbuild automati-
cally detects and generates a new kernel module key, ECDSA-signed kernel
modules will fail signature verification.

Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>

---
v3:
  - added missing OIDs for ECDSA signed hashes to pkcs7_sig_note_pkey_algo
  - added recommendation to use string hash to Kconfig help text

v2:
  - check for ECDSA key by id-ecPublicKey from output line
    'Public Key Algorithm: id-ecPublicKey'.
---
 certs/Kconfig                         | 26 ++++++++++++++++++++++++++
 certs/Makefile                        |  9 +++++++++
 crypto/asymmetric_keys/pkcs7_parser.c |  8 ++++++++
 3 files changed, 43 insertions(+)

diff --git a/certs/Kconfig b/certs/Kconfig
index 48675ad319db..d58f16c9f2d9 100644
--- a/certs/Kconfig
+++ b/certs/Kconfig
@@ -15,6 +15,32 @@ config MODULE_SIG_KEY
          then the kernel will automatically generate the private key and
          certificate as described in Documentation/admin-guide/module-signing.rst
 
+choice
+	prompt "Type of module signing key to be generated"
+	default MODULE_SIG_KEY_TYPE_RSA
+	help
+	 The type of module signing key type to generate. This option
+	 does not apply if a #PKCS11 URI is used.
+
+config MODULE_SIG_KEY_TYPE_RSA
+	bool "RSA"
+	depends on MODULE_SIG || IMA_APPRAISE_MODSIG
+	help
+	 Use an RSA key for module signing.
+
+config MODULE_SIG_KEY_TYPE_ECDSA
+	bool "ECDSA"
+	select CRYPTO_ECDSA
+	depends on MODULE_SIG || IMA_APPRAISE_MODSIG
+	help
+	 Use an elliptic curve key (NIST P384) for module signing. Consider
+	 using a strong hash like sha256 or sha384 for hashing modules.
+
+	 Note: Remove all ECDSA signing keys, e.g. certs/signing_key.pem,
+	 when falling back to building Linux 5.11 and older kernels.
+
+endchoice
+
 config SYSTEM_TRUSTED_KEYRING
 	bool "Provide system-wide ring of trusted keys"
 	depends on KEYS
diff --git a/certs/Makefile b/certs/Makefile
index f64bc89ccbf1..c2fabc288550 100644
--- a/certs/Makefile
+++ b/certs/Makefile
@@ -62,7 +62,15 @@ ifeq ($(CONFIG_MODULE_SIG_KEY),"certs/signing_key.pem")
 
 X509TEXT=$(shell openssl x509 -in $(CONFIG_MODULE_SIG_KEY) -text)
 
+# Support user changing key type
+ifdef CONFIG_MODULE_SIG_KEY_TYPE_ECDSA
+keytype_openssl = -newkey ec -pkeyopt ec_paramgen_curve:secp384r1
+$(if $(findstring id-ecPublicKey,$(X509TEXT)),,$(shell rm -f $(CONFIG_MODULE_SIG_KEY)))
+endif
+
+ifdef CONFIG_MODULE_SIG_KEY_TYPE_RSA
 $(if $(findstring rsaEncryption,$(X509TEXT)),,$(shell rm -f $(CONFIG_MODULE_SIG_KEY)))
+endif
 
 $(obj)/signing_key.pem: $(obj)/x509.genkey
 	@$(kecho) "###"
@@ -77,6 +85,7 @@ $(obj)/signing_key.pem: $(obj)/x509.genkey
 		-batch -x509 -config $(obj)/x509.genkey \
 		-outform PEM -out $(obj)/signing_key.pem \
 		-keyout $(obj)/signing_key.pem \
+		$(keytype_openssl) \
 		$($(quiet)redirect_openssl)
 	@$(kecho) "###"
 	@$(kecho) "### Key pair generated."
diff --git a/crypto/asymmetric_keys/pkcs7_parser.c b/crypto/asymmetric_keys/pkcs7_parser.c
index 967329e0a07b..6592279d839a 100644
--- a/crypto/asymmetric_keys/pkcs7_parser.c
+++ b/crypto/asymmetric_keys/pkcs7_parser.c
@@ -269,6 +269,14 @@ int pkcs7_sig_note_pkey_algo(void *context, size_t hdrlen,
 		ctx->sinfo->sig->pkey_algo = "rsa";
 		ctx->sinfo->sig->encoding = "pkcs1";
 		break;
+	case OID_id_ecdsa_with_sha1:
+	case OID_id_ecdsa_with_sha224:
+	case OID_id_ecdsa_with_sha256:
+	case OID_id_ecdsa_with_sha384:
+	case OID_id_ecdsa_with_sha512:
+		ctx->sinfo->sig->pkey_algo = "ecdsa";
+		ctx->sinfo->sig->encoding = "x962";
+		break;
 	default:
 		printk("Unsupported pkey algo: %u\n", ctx->last_oid);
 		return -ENOPKG;
-- 
2.29.2