linux-kernel - Re: [RFC v2 0/6] Introduce TEE based Trusted Keys support

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Message-ID: <CAFA6WYOwcO5-cyaJf3tMMAdyVHJo=BzmCWtsjA3S8aj5g-GZxQ@mail.gmail.com>
Date:   Thu, 1 Aug 2019 15:30:07 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Janne Karhunen <janne.karhunen@...il.com>
Cc:     keyrings@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Jonathan Corbet <corbet@....net>, dhowells@...hat.com,
        jejb@...ux.ibm.com,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "tee-dev @ lists . linaro . org" <tee-dev@...ts.linaro.org>
Subject: Re: [RFC v2 0/6] Introduce TEE based Trusted Keys support

On Thu, 1 Aug 2019 at 13:30, Janne Karhunen <janne.karhunen@...il.com> wrote:
>
> On Thu, Aug 1, 2019 at 10:40 AM Sumit Garg <sumit.garg@...aro.org> wrote:
>
> > > I chose the userspace plugin due to this, you can use userspace aids
> > > to provide any type of service. Use the crypto library you desire to
> > > do the magic you want.
> >
> > Here TEE isn't similar to a user-space crypto library. In our case TEE
> > is based on ARM TrustZone which only allows TEE communications to be
> > initiated from privileged mode. So why would you like to route
> > communications via user-mode (which is less secure) when we have
> > standardised TEE interface available in kernel?
>
> The physical access guards for reading/writing the involved critical
> memory are identical as far as I know? Layered security is generally a
> good thing, and the userspace pass actually adds a layer, so not sure
> which is really safer?
>

AFAIK, layered security is better in case we move from lower privilege
level to higher privilege level rather than in reverse order.

-Sumit

> In my case the rerouting was to done generalize it. Any type of trust
> source, anywhere.
>
>
> > > > Isn't actual purpose to have trusted keys is to protect user-space
> > > > from access to kernel keys in plain format? Doesn't user mode helper
> > > > defeat that purpose in one way or another?
> > >
> > > Not really. CPU is in the user mode while running the code, but the
> > > code or the secure keydata being is not available to the 'normal'
> > > userspace. It's like microkernel service/driver this way. The usermode
> > > driver is part of the kernel image and it runs on top of a invisible
> > > rootfs.
> >
> > Can you elaborate here with an example regarding how this user-mode
> > helper will securely communicate with a hardware based trust source
> > with other user-space processes denied access to that trust source?
>
> The other user mode processes will never see the device node to open.
> There is none in existence for them; it only exists in the ramfs based
> root for the user mode helper.
>
>
> --
> Janne