Re: [PATCH v2 1/6] KEYS: trusted: allow use of TEE as backend without TCG_TPM support

[Sumit Garg]

Hi Ahmad,

On Tue, 22 Jun 2021 at 18:08, Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> wrote:
>
> 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 Kconfig symbols:
> TRUSTED_KEYS_TPM and TRUSTED_KEYS_TEE with the appropriate
> dependencies.
>

This should include a fixes tag to the rework commit.

> Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx>
> ---
> To: James Bottomley <jejb@xxxxxxxxxxxxx>
> To: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> To: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> To: David Howells <dhowells@xxxxxxxxxx>
> Cc: James Morris <jmorris@xxxxxxxxx>
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> Cc: "Horia Geantă" <horia.geanta@xxxxxxx>
> Cc: Aymen Sghaier <aymen.sghaier@xxxxxxx>
> Cc: Udit Agarwal <udit.agarwal@xxxxxxx>
> Cc: Jan Luebbe <j.luebbe@xxxxxxxxxxxxxx>
> Cc: David Gstir <david@xxxxxxxxxxxxx>
> Cc: Richard Weinberger <richard@xxxxxx>
> Cc: Franck LENORMAND <franck.lenormand@xxxxxxx>
> Cc: Sumit Garg <sumit.garg@xxxxxxxxxx>
> Cc: keyrings@xxxxxxxxxxxxxxx
> Cc: linux-crypto@xxxxxxxxxxxxxxx
> Cc: linux-integrity@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-security-module@xxxxxxxxxxxxxxx
> ---
>  security/keys/Kconfig                     | 14 ++++++-------
>  security/keys/trusted-keys/Kconfig        | 25 ++++++++++++++++++++++++-
>  security/keys/trusted-keys/Makefile       |  8 +++++---
>  security/keys/trusted-keys/trusted_core.c |  4 ++--
>  4 files changed, 39 insertions(+), 12 deletions(-)
>  create mode 100644 security/keys/trusted-keys/Kconfig
>
> diff --git a/security/keys/Kconfig b/security/keys/Kconfig
> index 64b81abd087e..6fdb953b319f 100644
> --- a/security/keys/Kconfig
> +++ b/security/keys/Kconfig
> @@ -70,23 +70,23 @@ config BIG_KEYS
>
>  config TRUSTED_KEYS
>         tristate "TRUSTED KEYS"
> -       depends on KEYS && TCG_TPM
> +       depends on KEYS

>         select CRYPTO
>         select CRYPTO_HMAC
>         select CRYPTO_SHA1
>         select CRYPTO_HASH_INFO

Should move these as well to TRUSTED_KEYS_TPM as the core code doesn't
mandate their need.

> -       select ASN1_ENCODER
> -       select OID_REGISTRY
> -       select ASN1
>         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
> diff --git a/security/keys/trusted-keys/Kconfig b/security/keys/trusted-keys/Kconfig
> new file mode 100644
> index 000000000000..24af4aaceebf
> --- /dev/null
> +++ b/security/keys/trusted-keys/Kconfig
> @@ -0,0 +1,25 @@
> +config TRUSTED_KEYS_TPM
> +       bool "TPM-based trusted keys"
> +       depends on TCG_TPM >= TRUSTED_KEYS
> +       default y
> +       select ASN1_ENCODER
> +       select OID_REGISTRY
> +       select ASN1
> +       help
> +         Enable use of the Trusted Platform Module (TPM) as trusted key
> +         backend. Trusted keys are 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
> diff --git a/security/keys/trusted-keys/Makefile b/security/keys/trusted-keys/Makefile
> index feb8b6c3cc79..96fc6c377398 100644
> --- a/security/keys/trusted-keys/Makefile
> +++ b/security/keys/trusted-keys/Makefile
> @@ -5,10 +5,12 @@
>
>  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_TRUSTED_KEYS_TEE) += trusted_tee.o
>
>  trusted-$(CONFIG_TEE) += trusted_tee.o

This should be dropped.

-Sumit

> diff --git a/security/keys/trusted-keys/trusted_core.c b/security/keys/trusted-keys/trusted_core.c
> index d5c891d8d353..8cab69e5d0da 100644
> --- a/security/keys/trusted-keys/trusted_core.c
> +++ b/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 defined(CONFIG_TCG_TPM)
> +#if defined(CONFIG_TRUSTED_KEYS_TPM)
>         { "tpm", &trusted_key_tpm_ops },
>  #endif
> -#if defined(CONFIG_TEE)
> +#if defined(CONFIG_TRUSTED_KEYS_TEE)
>         { "tee", &trusted_key_tee_ops },
>  #endif
>  };
> --
> git-series 0.9.1