Re: [PATCH v7 1/4] KEYS: trusted: Add generic trusted keys framework

[Mimi Zohar]

On Wed, 2020-10-21 at 11:16 +0530, Sumit Garg wrote:
> Thanks Mimi for your comments.
> 
> On Wed, 21 Oct 2020 at 08:51, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, 2020-10-07 at 15:37 +0530, Sumit Garg wrote:
> >
> > > +/*
> > > + * trusted_destroy - clear and free the key's payload
> > > + */
> > > +static void trusted_destroy(struct key *key)
> > > +{
> > > +     kfree_sensitive(key->payload.data[0]);
> > > +}
> > > +
> > > +struct key_type key_type_trusted = {
> > > +     .name = "trusted",
> > > +     .instantiate = trusted_instantiate,
> > > +     .update = trusted_update,
> > > +     .destroy = trusted_destroy,
> > > +     .describe = user_describe,
> > > +     .read = trusted_read,
> > > +};
> > > +EXPORT_SYMBOL_GPL(key_type_trusted);
> > > +
> > > +static int __init init_trusted(void)
> > > +{
> > > +     int i, ret = 0;
> > > +
> > > +     for (i = 0; i < ARRAY_SIZE(trusted_key_sources); i++) {
> > > +             if (trusted_key_source &&
> > > +                 strncmp(trusted_key_source, trusted_key_sources[i].name,
> > > +                         strlen(trusted_key_sources[i].name)))
> > > +                     continue;
> > > +
> > > +             trusted_key_ops = trusted_key_sources[i].ops;
> > > +
> > > +             ret = trusted_key_ops->init();
> > > +             if (!ret)
> > > +                     break;
> > > +     }
> >
> > In the case when the module paramater isn't specified and both TPM and
> > TEE are enabled, trusted_key_ops is set to the last source initialized.
> 
> I guess there is some misunderstanding. Here it's only a single trust
> source (TPM *or* TEE) is initialized and only that trust source would
> be active at runtime. And trusted_key_ops would be initialized to the
> first trust source whose initialization is successful (see check: "if
> (!ret)").

My mistake.

> 
> > After patch 2/4, the last trusted source initialized is TEE.  If the
> > intention is to limit it to either TPM or TEE, then trusted_key_ops
> > should have a default value, which could be overwritten at runtime.
> > That would address Luke Hind's concerns of making the decision at
> > compile time.
> 
> I think traversing the trust source list with the initial value being
> TPM would be default value.

Agreed
> 
> >
> > trusted_key_ops should be defined as __ro_after_init, like is currently
> > done for other LSM structures.
> 
> Sure, will do.

Thanks
> 
> >
> > > +
> > > +     /*
> > > +      * encrypted_keys.ko depends on successful load of this module even if
> > > +      * trusted key implementation is not found.
> > > +      */
> > > +     if (ret == -ENODEV)
> > > +             return 0;
> > > +
> > > +     return ret;
> > > +}
> > > +
> > > +static void __exit cleanup_trusted(void)
> > > +{
> > > +     trusted_key_ops->exit();
> >
> > If the intention is really to support both TPM and TEE trusted keys at
> > the same time, as James suggested, then the same "for" loop as in
> > init_trusted() is needed here and probably elsewhere.
> 
> Current intention is to only support a single trust source (TPM or
> TEE) at runtime. But in future if there are use-cases then framework
> can be extended to support multiple trust sources at runtime as well.

Ok, the last sentence of the patch description, "Also, add a module
parameter in order to select a particular trust source in case a
platform support multiple trust sources.", needs to be expanded to:
- indicate only one trust source at a time is supported
- indicate the default, if the module_param is not specified

I would also change the word from "add" to "define".   The new "source"
module parameter needs to be added to the admin-guide/kernel-parameters 
documentation.

thanks,

Mimi