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

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Date:   Mon, 19 Jul 2021 13:36:09 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc:     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 <linux-integrity@...r.kernel.org>,
        "open list:ASYMMETRIC KEYS" <keyrings@...r.kernel.org>,
        "open list:SECURITY SUBSYSTEM" 
        <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Re: [PATCH] KEYS: trusted: Fix trusted key backends when building as module

On Mon, 19 Jul 2021 at 12:40, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>
> 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.
>

That's true.

> 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.

That's an oversight on my part since this patch was part of the new
CAAM trust source patch-set. Although I do admit that it was on my
TODO list. So I have provided some feedback on that patch. Can you
post the next version as an independent fix patch?

-Sumit

>
> [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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