Re: [PATCH 4/5] keys: define build time generated ephemeral kernel CA key

[Stefan Berger <stefanb@linux.ibm.com>]

On 2/11/21 2:54 PM, Nayna Jain wrote:
> Certificates being loaded onto the IMA trusted keyring must be signed by
> a key on either the builtin and secondary trusted keyring.
>
> This patch creates and includes in the kernel image an ephemeral CA
> key, at build time when IMA_APPRAISE_MODSIG is enabled.
>
> Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
> ---
>   Makefile                    |  2 ++
>   certs/Makefile              | 68 ++++++++++++++++++++++++++++++++++---
>   certs/system_certificates.S | 16 ++++++++-
>   3 files changed, 80 insertions(+), 6 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 9c87fdd600d8..a1d4b0a1745e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1475,6 +1475,8 @@ MRPROPER_FILES += include/config include/generated          \
>   		  certs/signing_key.pem certs/signing_key.x509 \
>   		  certs/x509.genkey certs/signing_key.key \
>   		  certs/signing_key.crt certs/signing_key.csr \
> +		  certs/ca_signing_key.pem certs/ca_signing_key.x509 \
> +		  certs/ca_signing_key.srl \
>   		  vmlinux-gdb.py \
>   		  *.spec
>   
> diff --git a/certs/Makefile b/certs/Makefile
> index b2be7eb413d3..c3592ba63a05 100644
> --- a/certs/Makefile
> +++ b/certs/Makefile
> @@ -32,6 +32,14 @@ endif # CONFIG_SYSTEM_TRUSTED_KEYRING
>   clean-files := x509_certificate_list .x509.list
>   
>   ifeq ($(CONFIG_MODULE_SIG),y)
> +SIGN_KEY = y
> +endif
> +
> +ifeq ($(CONFIG_IMA_APPRAISE_MODSIG),y)
> +SIGN_KEY = y
> +endif
> +
> +ifdef SIGN_KEY
>   ###############################################################################
>   #
>   # If module signing is requested, say by allyesconfig, but a key has not been
> @@ -51,6 +59,16 @@ silent_redirect_openssl = 2>/dev/null
>   # external private key, because 'make randconfig' might enable such a
>   # boolean option and we unfortunately can't make it depend on !RANDCONFIG.
>   ifeq ($(CONFIG_MODULE_SIG_KEY),"certs/signing_key.pem")
> +
> +ifeq ($(CONFIG_IMA_APPRAISE_MODSIG),y)
> +# openssl arguments for CA Signed certificate.
> +CA_KEY = certs/ca_signing_key.pem
> +SIGNER = -CA $(CA_KEY) -CAkey $(CA_KEY) -CAcreateserial
> +else
> +# openssl arguments for Self Signed certificate.
> +SIGNER = -signkey $(obj)/signing_key.key
> +endif # CONFIG_IMA_APPRAISE_MODSIG
> +
>   $(obj)/signing_key.pem: $(obj)/x509.genkey
>   	@$(kecho) "###"
>   	@$(kecho) "### Now generating an X.509 key pair to be used for signing modules."
> @@ -60,14 +78,23 @@ $(obj)/signing_key.pem: $(obj)/x509.genkey
>   	@$(kecho) "### needs to be run as root, and uses a hardware random"
>   	@$(kecho) "### number generator if one is available."
>   	@$(kecho) "###"
> +ifeq ($(CONFIG_IMA_APPRAISE_MODSIG),y)
> +	# Generate kernel build time CA Certificate.
> +	@$(Q)openssl req -new -nodes -utf8 \
> +		-$(CONFIG_MODULE_SIG_HASH) -days 36500 \
> +		-subj "/CN=Build time autogenerated kernel CA key" \
> +		-batch -x509 -config $(obj)/x509.genkey \
> +		-outform PEM -out $(CA_KEY) \
> +		-keyout $(CA_KEY) -extensions ca_ext \
> +		$($(quiet)redirect_openssl)
> +endif # CONFIG_IMA_APPRAISE_MODSIG
>   	$(Q)openssl req -new -nodes -utf8 \
>   		-batch -config $(obj)/x509.genkey \
>   		-outform PEM -out $(obj)/signing_key.csr \
>   		-keyout $(obj)/signing_key.key -extensions myexts \
>   		$($(quiet)redirect_openssl)
>   	$(Q)openssl x509 -req -days 36500 -in $(obj)/signing_key.csr \
> -		-outform PEM -out $(obj)/signing_key.crt \
> -		-signkey $(obj)/signing_key.key \
> +		-outform PEM -out $(obj)/signing_key.crt $(SIGNER) \
>   		-$(CONFIG_MODULE_SIG_HASH) -extensions myexts \
>   		-extfile $(obj)/x509.genkey \
>   		$($(quiet)redirect_openssl)

It may make things easier (also below) if the CA was always created and 
the kernel signing key was always signed by that CA rather than doing 
this only in the IMA_APPRAISE_MODSIG case. Maybe someone else has an 
opinion on that?


> @@ -95,19 +122,50 @@ $(obj)/x509.genkey:
>   	@echo >>$@ "keyUsage=digitalSignature"
>   	@echo >>$@ "subjectKeyIdentifier=hash"
>   	@echo >>$@ "authorityKeyIdentifier=keyid"
> +	@echo >>$@
> +	@echo >>$@ "[ ca_ext ]"
> +	@echo >>$@ "keyUsage=critical,keyCertSign"
> +	@echo >>$@ "basicConstraints=critical,CA:TRUE,pathlen:0"
> +	@echo >>$@ "subjectKeyIdentifier=hash"
> +	@echo >>$@ "authorityKeyIdentifier=keyid"
>   endif # CONFIG_MODULE_SIG_KEY
>   
>   $(eval $(call config_filename,MODULE_SIG_KEY))
> +SUBJECT=CN = Build time autogenerated kernel key
> +ISSUER=$(shell openssl x509 -in certs/signing_key.crt -noout -issuer)
>   
>   # If CONFIG_MODULE_SIG_KEY isn't a PKCS#11 URI, depend on it
> +
> +# GCC PR#66871 again.
> +ifeq ($(CONFIG_IMA_APPRAISE_MODSIG),y)
> +
> +# Remove existing keys if it is self-signed.
> +$(if $(findstring $(SUBJECT),$(ISSUER)),$(shell rm -f certs/signing_key.* certs/x509.genkey))
> +CA_KEY = certs/ca_signing_key.pem
> +
> +$(obj)/system_certificates.o: $(obj)/ca_signing_key.x509 $(obj)/signing_key.x509
> +
> +targets += ca_signing_key.x509
> +$(obj)/ca_signing_key.x509: $(obj)/signing_key.x509 scripts/extract-cert FORCE
> +	$(call if_changed,extract_certs,$(CA_KEY))
> +
> +targets += signing_key.x509
> +$(obj)/signing_key.x509: $(obj)/signing_key.pem scripts/extract-cert FORCE
> +	$(call if_changed,extract_certs,$(MODULE_SIG_KEY_SRCPREFIX)$(CONFIG_MODULE_SIG_KEY))
> +else
> +
> +# Remove existing keys if it is CA signed.
> +$(if $(findstring $(SUBJECT),$(ISSUER)),,$(shell rm -f certs/ca_signing_key.* certs/signing_key.* certs/x509.genkey))
> +
>   ifeq ($(patsubst pkcs11:%,%,$(firstword $(MODULE_SIG_KEY_FILENAME))),$(firstword $(MODULE_SIG_KEY_FILENAME)))
>   X509_DEP := $(MODULE_SIG_KEY_SRCPREFIX)$(MODULE_SIG_KEY_FILENAME)
>   endif
>   
> -# GCC PR#66871 again.
>   $(obj)/system_certificates.o: $(obj)/signing_key.x509
>   
>   targets += signing_key.x509
> -$(obj)/signing_key.x509: scripts/extract-cert $(X509_DEP) FORCE
> +$(obj)/signing_key.x509: certs/signing_key.pem scripts/extract-cert $(X509_DEP) FORCE
>   	$(call if_changed,extract_certs,$(MODULE_SIG_KEY_SRCPREFIX)$(CONFIG_MODULE_SIG_KEY))
> -endif # CONFIG_MODULE_SIG
> +
> +endif # CONFIG_IMA_APPRAISE_MODSIG
> +endif # SIGN_KEY
> diff --git a/certs/system_certificates.S b/certs/system_certificates.S
> index 8f29058adf93..e10043800a7e 100644
> --- a/certs/system_certificates.S
> +++ b/certs/system_certificates.S
> @@ -8,8 +8,13 @@
>   	.globl system_certificate_list
>   system_certificate_list:
>   __cert_list_start:
> -#ifdef CONFIG_MODULE_SIG
> +__module_cert_start:
> +#if defined(CONFIG_MODULE_SIG) || defined(CONFIG_IMA_APPRAISE_MODSIG)
>   	.incbin "certs/signing_key.x509"
> +#endif
> +__module_cert_end:
> +#ifdef CONFIG_IMA_APPRAISE_MODSIG
> +	.incbin "certs/ca_signing_key.x509"
>   #endif
>   	.incbin "certs/x509_certificate_list"
>   __cert_list_end:
> @@ -35,3 +40,12 @@ system_certificate_list_size:
>   #else
>   	.long __cert_list_end - __cert_list_start
>   #endif
> +
> +	.align 8
> +	.globl module_cert_size
> +	module_cert_size:
> +#ifdef CONFIG_64BIT
> +	.quad __module_cert_end - __module_cert_start
> +#else
> +	.long __module_cert_end - __module_cert_start
> +#endif