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

[Sumit Garg]

Hi Maximilian,

On 02/08/22 18:52, Maximilian Luz 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.
>
> A similar argumentation might apply to the TPM app.

See below, there is already an existing TPM app driver [2] in kernel 
although the app is based on OP-TEE.

> > 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?

The Linux TEE framework already provides an in-kernel interface to TEE 
as well via TEE bus [1]. There are already multiple kernel drivers [2] 
[3] [4] [5] [6] [7] using it. So an EFI driver can be an addition to that.

Now coming on to TEE implementations, the drivers I mentioned are based 
on OP-TEE where devices are queried/enumerated during OP-TEE probe here 
[8]. So in similar manner QTEE smcinvoke driver should be able to 
register devices on the TEE bus.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/staging/tee.rst#n56

[2] drivers/char/tpm/tpm_ftpm_tee.c

[3] drivers/char/hw_random/optee-rng.c

[4] drivers/firmware/arm_scmi/optee.c

[5] security/keys/trusted-keys/trusted_tee.c

[6] drivers/firmware/broadcom/tee_bnxt_fw.c

[7] drivers/rtc/rtc-optee.c

[8] drivers/tee/optee/device.c

-Sumit

PS. TBH, I haven't looked into detail workings for the QTEE smcinvoke 
driver.

> 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