linux-kernel - Re: [PATCH] KEYS: trusted: Fix trusted key backends when building as module

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Message-ID: <0a684d56-66d0-184e-4853-9faafa2d243d@pengutronix.de>
Date:   Mon, 19 Jul 2021 09:10:01 +0200
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Andreas Rammhold <andreas@...mhold.de>,
        James Bottomley <jejb@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Re: [PATCH] KEYS: trusted: Fix trusted key backends when building as
 module

Hello Andreas,

On 16.07.21 10:17, Andreas Rammhold wrote:
> Before this commit the kernel could end up with no trusted key sources
> even thought both of the currently supported backends (tpm & tee) were
> compoiled as modules. This manifested in the trusted key type not being
> registered at all.

I assume (TPM) trusted key module use worked before the TEE rework? If so,

an appropriate Fixes: Tag would then be in order.

> When checking if a CONFIG_… preprocessor variable is defined we only
> test for the builtin (=y) case and not the module (=m) case. By using
> the IS_ENABLE(…) macro we to test for both cases.

It looks to me like you could now provoke a link error if TEE is a module
and built-in trusted key core tries to link against trusted_key_tee_ops.

One solution for that IS_REACHABLE(). Another is to address the root cause,
which is the inflexible trusted keys Kconfig description:

- Trusted keys despite TEE support can still only be built when TCG_TPM is enabled
- There is no support to have TEE or TPM enabled without using those for
  enabled trusted keys as well
- As you noticed, module build of the backend has issues

I addressed these three issues in a patch[1], a month ago, but have yet to
receive feedback.

[1]: https://lore.kernel.org/linux-integrity/f8285eb0135ba30c9d846cf9dd395d1f5f8b1efc.1624364386.git-series.a.fatoum@pengutronix.de/

Cheers,
Ahmad

> Signed-off-by: Andreas Rammhold <andreas@...mhold.de>
> ---
>  security/keys/trusted-keys/trusted_core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/security/keys/trusted-keys/trusted_core.c b/security/keys/trusted-keys/trusted_core.c
> index d5c891d8d353..fd640614b168 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 IS_ENABLED(CONFIG_TCG_TPM)
>  	{ "tpm", &trusted_key_tpm_ops },
>  #endif
> -#if defined(CONFIG_TEE)
> +#if IS_ENABLED(CONFIG_TEE)
>  	{ "tee", &trusted_key_tee_ops },
>  #endif
>  };
> 


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