Re: [PATCH 0/4] firmware: Add support for Qualcomm UEFI Secure Application

Ard Biesheuvel
2022-08-02T14:03:03Z

On Tue, 2 Aug 2022 at 15:22, Maximilian Luz <luzmaximilian@gmail.com> wrote:
>
>
>
> On 8/2/22 13:51, Srinivas Kandagatla wrote:
> > Hi Maximilian,
> >
> > On 23/07/2022 23:49, Maximilian Luz wrote:
> >> On modern Qualcomm platforms, access to EFI variables is restricted to
> >> the secure world / TrustZone, i.e. the Trusted Execution Environment
> >> (TrEE or TEE) as Qualcomm seems to call it. To access EFI variables, we
> >> therefore need to talk to the UEFI Secure Application (uefisecapp),
> >> residing in the TrEE.
> >>
> >> This series adds support for accessing EFI variables on those platforms.
> >>
> >> To do this, we first need to add some SCM call functions used to manage
> >> and talk to Secure Applications. A very small subset of this interface
> >> is added in the second patch (whereas the first one exports the required
> >> functions for that). Interface specifications are extracted from [1].
> >> While this does not (yet) support re-entrant SCM calls (including
> >> callbacks and listeners), this is enough to talk to the aforementioned
> >> uefisecapp on a couple of platforms (I've tested this on a Surface Pro X
> >> and heard reports from Lenovo Flex 5G, Lenovo Thinkpad x13s, and Lenovo
> >> Yoga C630 devices).
> >>
> >> The third patch adds a client driver for uefisecapp, installing the
> >> respective efivar operations. The application interface has been reverse
> >> engineered from the Windows QcTrEE8180.sys driver.
> >>
> >> Apart from uefisecapp, there are more Secure Applications running that
> >> we might want to support in the future. For example, on the Surface Pro
> >> X (sc8180x-based), the TPM is also managed via one.
> >>
> >> I'm not sure whether this should go to drivers/firmware or to
> >> drivers/soc/qcom. I've put this into firmware as all of this is
> >> essentially an interface to the secure firmware running in the TrustZone
> >> (and SCM stuff is handled here already), but please let me know if I
> >> should move this.
> >
> > From what I see so far is that this is adapted from downstream qseecom driver, this approach could work for a limited usecases but not scalable, as we cannot add drivers for each Qualcomm specific TA in kernel.
> > This has to be handled in much generic way using Linux TEE framework, and let the userspace side deal with TA specific bits.
>
> I generally agree with the sentiment, however UEFI variables should IMHO be
> handled by the kernel. Moving handling of those to userspace breaks things like
> EFI-based pstore and efivarfs. The latter will in turn break some user-space
> tools (most notably efibootmgr used by e.g. GRUB and I think fwupdmgr which
> needs to set some capsule variables). Ideally, we would find a way to not break
> these, i.e. have them work out-of-the-box.
>

Only capsule-on-disk requires SetVariable() at runtime, and I doubt
whether these platforms implement any of that.

> A similar argumentation might apply to the TPM app.
>

There is a difference, though - the TPM is modeled as a device and
runtime access to it is implemented as a device driver, which is only
accessed from user space.

> > AFAIU, Qualcomm is moving away from qseecom interface to new smc-invoke interface, most of Qualcomm SoCs starting from SDM660 already have support to this.
> >
> > This interface provides a better abstracted IPC mechanism to talk to TA. Most of these TA specific interfaces are packed in closed userspace source.
> > Having said that QTEE smcinvoke driver can be modeled as a proper TEE driver with Userspace driving the TA specific bits using existing tee uapis.
> > This also brings in other features like loading, Listeners aka callbacks, secure memory allocations..etc.
> >
> > In the past, I have tried to do a prototype of this smcinvoke driver as a proper tee driver, incase you are interested patches are at https://git.linaro.org/landing-teams/working/qualcomm/kernel.git/log/?h=tracking-qcomlt-qcomtee
> > Plan is to discuss with Qualcomm and send it for upstream review.
>
> Thanks for this information! So as far as I understand it, this is currently an
> interface to user-space only, i.e. does not allow in-kernel drivers for apps?
> It would be great if this could then be extended to handle (the bare minimum
> of) in-kernel drivers (i.e. only things that the kernel itself needs, like EFI
> variables). Alternatively, I'm happy to hear suggestions on how we not break
> the aforementioned things while moving handling off to userspace.
>
> > I think its worth exploring if uefisecapp can talk smcinvoke.
> > I can ping Qualcomm engineers to see if that is doable.
>
> I think that would be great! Thanks!
>
> Regards,
> Max