KEYS: trusted: allow use of TEE as backend without TCG_TPM support

With recent rework, trusted keys are no longer limited to TPM as trust
source. The Kconfig symbol is unchanged however leading to a few issues:

  - TCG_TPM is required, even if only TEE is to be used
  - Enabling TCG_TPM, but excluding it from available trusted sources
    is not possible
  - TEE=m && TRUSTED_KEYS=y will lead to TEE support being silently
    dropped, which is not the best user experience

Remedy these issues by introducing two new boolean Kconfig symbols:
TRUSTED_KEYS_TPM and TRUSTED_KEYS_TEE with the appropriate
dependencies.

Any new code depending on the TPM trusted key backend in particular
or symbols exported by it will now need to explicitly state that it

  depends on TRUSTED_KEYS && TRUSTED_KEYS_TPM

The latter to ensure the dependency is built and the former to ensure
it's reachable for module builds. There are no such users yet.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Tested-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Tested-by: Andreas Rammhold <andreas@rammhold.de>
Tested-by: Tim Harvey <tharvey@gateworks.com>
Tested-by: Michael Walle <michael@walle.cc> # on ls1028a (non-E and E)
Tested-by: John Ernberg <john.ernberg@actia.se> # iMX8QXP
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

security/keys/Kconfig

@@ -70,23 +70,19 @@ config BIG_KEYS
 
 config TRUSTED_KEYS
 	tristate "TRUSTED KEYS"
-	depends on KEYS && TCG_TPM
-	select CRYPTO
-	select CRYPTO_HMAC
-	select CRYPTO_SHA1
-	select CRYPTO_HASH_INFO
-	select ASN1_ENCODER
-	select OID_REGISTRY
-	select ASN1
+	depends on KEYS
 	help
 	  This option provides support for creating, sealing, and unsealing
 	  keys in the kernel. Trusted keys are random number symmetric keys,
-	  generated and RSA-sealed by the TPM. The TPM only unseals the keys,
-	  if the boot PCRs and other criteria match.  Userspace will only ever
-	  see encrypted blobs.
+	  generated and sealed by a trust source selected at kernel boot-time.
+	  Userspace will only ever see encrypted blobs.
 
 	  If you are unsure as to whether this is required, answer N.
 
+if TRUSTED_KEYS
+source "security/keys/trusted-keys/Kconfig"
+endif
+
 config ENCRYPTED_KEYS
 	tristate "ENCRYPTED KEYS"
 	depends on KEYS

security/keys/trusted-keys/Kconfig

@@ -0,0 +1,29 @@
+config TRUSTED_KEYS_TPM
+	bool "TPM-based trusted keys"
+	depends on TCG_TPM >= TRUSTED_KEYS
+	default y
+	select CRYPTO
+	select CRYPTO_HMAC
+	select CRYPTO_SHA1
+	select CRYPTO_HASH_INFO
+	select ASN1_ENCODER
+	select OID_REGISTRY
+	select ASN1
+	help
+	  Enable use of the Trusted Platform Module (TPM) as trusted key
+	  backend. Trusted keys are random number symmetric keys,
+	  which will be generated and RSA-sealed by the TPM.
+	  The TPM only unseals the keys, if the boot PCRs and other
+	  criteria match.
+
+config TRUSTED_KEYS_TEE
+	bool "TEE-based trusted keys"
+	depends on TEE >= TRUSTED_KEYS
+	default y
+	help
+	  Enable use of the Trusted Execution Environment (TEE) as trusted
+	  key backend.
+
+if !TRUSTED_KEYS_TPM && !TRUSTED_KEYS_TEE
+comment "No trust source selected!"
+endif

security/keys/trusted-keys/Makefile

@@ -5,10 +5,10 @@
 
 obj-$(CONFIG_TRUSTED_KEYS) += trusted.o
 trusted-y += trusted_core.o
-trusted-y += trusted_tpm1.o
+trusted-$(CONFIG_TRUSTED_KEYS_TPM) += trusted_tpm1.o
 
 $(obj)/trusted_tpm2.o: $(obj)/tpm2key.asn1.h
-trusted-y += trusted_tpm2.o
-trusted-y += tpm2key.asn1.o
+trusted-$(CONFIG_TRUSTED_KEYS_TPM) += trusted_tpm2.o
+trusted-$(CONFIG_TRUSTED_KEYS_TPM) += tpm2key.asn1.o
 
-trusted-$(CONFIG_TEE) += trusted_tee.o
+trusted-$(CONFIG_TRUSTED_KEYS_TEE) += trusted_tee.o

security/keys/trusted-keys/trusted_core.c

@@ -27,10 +27,10 @@ module_param_named(source, trusted_key_source, charp, 0);
 MODULE_PARM_DESC(source, "Select trusted keys source (tpm or tee)");
 
 static const struct trusted_key_source trusted_key_sources[] = {
-#if IS_REACHABLE(CONFIG_TCG_TPM)
+#if defined(CONFIG_TRUSTED_KEYS_TPM)
 	{ "tpm", &trusted_key_tpm_ops },
 #endif
-#if IS_REACHABLE(CONFIG_TEE)
+#if defined(CONFIG_TRUSTED_KEYS_TEE)
 	{ "tee", &trusted_key_tee_ops },
 #endif
 };