Re: [PATCH v2 2/2] certs: Add support for using elliptic curve keys for signing modules

[Jessica Yu <jeyu@xxxxxxxxxx>]

+++ Stefan Berger [08/04/21 11:24 -0400]:
> 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>
>
> ---
> v2:
>  - check for ECDSA key by id-ecPublicKey from output line
>    'Public Key Algorithm: id-ecPublicKey'.
> ---
> certs/Kconfig                         | 25 +++++++++++++++++++++++++
> certs/Makefile                        |  9 +++++++++
> crypto/asymmetric_keys/pkcs7_parser.c |  4 ++++
> 3 files changed, 38 insertions(+)
>
> diff --git a/certs/Kconfig b/certs/Kconfig
> index 48675ad319db..6f8337874ae0 100644
> --- a/certs/Kconfig
> +++ b/certs/Kconfig
> @@ -15,6 +15,31 @@ 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.
> +
> +	 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..2546ec6a0505 100644
> --- a/crypto/asymmetric_keys/pkcs7_parser.c
> +++ b/crypto/asymmetric_keys/pkcs7_parser.c
> @@ -269,6 +269,10 @@ 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_sha256:
> +		ctx->sinfo->sig->pkey_algo = "ecdsa";
> +		ctx->sinfo->sig->encoding = "x962";
> +		break;

Hi Stefan,

Does CONFIG_MODULE_SIG_KEY_TYPE_ECDSA have a dependency on MODULE_SIG_SHA256?
By default, MODULE_SIG_SHA1 is selected when CONFIG_MODULE_SIG is enabled.
I was doing some quick testing and found that when I enabled
MODULE_SIG_KEY_TYPE_ECDSA I get a "Unsupported pkey algo: 5" error on
module load, which goes away after fixing my config and selecting
MODULE_SIG_SHA256.

Thanks,

Jessica