Re: [PATCH 3/4] firmware: Add support for Qualcomm UEFI Secure Application

[Maximilian Luz]

On 1/17/23 09:24, Johan Hovold wrote:
> On Sun, Jul 24, 2022 at 12:49:48AM +0200, Maximilian Luz wrote:
> > On platforms using the Qualcomm UEFI Secure Application (uefisecapp),
> > EFI variables cannot be accessed via the standard interface in EFI
> > runtime mode. The respective functions return EFI_UNSUPPORTED. On these
> > platforms, we instead need to talk to uefisecapp. This commit provides
> > support for this and registers the respective efivars operations to
> > access EFI variables from the kernel.
> >
> > Communication with uefisecapp follows the standard Qualcomm Trusted
> > Environment (TEE or TrEE) / Secure OS conventions via the respective SCM
> > call interface. This is also the reason why variable access works
> > normally while boot services are active. During this time, said SCM
> > interface is managed by the boot services. When calling
> > ExitBootServices(), the ownership is transferred to the kernel.
> > Therefore, UEFI must not use that interface itself (as multiple parties
> > accessing this interface at the same time may lead to complications) and
> > cannot access variables for us.
> >
> > Signed-off-by: Maximilian Luz <luzmaximilian@xxxxxxxxx>
> > ---
>
> > +static struct platform_driver qcom_uefisecapp_driver = {
> > +	.probe = qcom_uefisecapp_probe,
> > +	.remove = qcom_uefisecapp_remove,
> > +	.driver = {
> > +		.name = "qcom_tee_uefisecapp",
> > +		.of_match_table = qcom_uefisecapp_dt_match,
> > +		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
> > +	},
> > +};
> > +module_platform_driver(qcom_uefisecapp_driver);
>
> I noticed that for efivarfs to work, you're currently relying on having
> the firmware still claim that the variable services are supported in the
> RT_PROP table so that efi core registers the default ops at subsys init
> time (which are later overridden by this driver).
>
> Otherwise efivarfs may fail to initialise when built in:
>
> 	static __init int efivarfs_init(void)
> 	{
> 		if (!efivars_kobject())
> 			return -ENODEV;
>
> 		return register_filesystem(&efivarfs_type);
> 	}
>
> 	module_init(efivarfs_init);
>
> With recent X13s firmware the corresponding bit in the RT_PROP table has
> been cleared so that efivarfs would fail to initialise. Similar problem
> when booting with 'efi=noruntime'.
>
> One way to handle this is to register also the qcom_uefisecapp_driver at
> subsys init time and prevent it from being built as a module (e.g. as is
> done for the SCM driver). I'm using the below patch for this currently.

So I've had another look and I'm not sure this will work reliably:

First, you are correct in case the RT_PROP table is cleared. In that
case, using subsys_initcall() will move the efivar registration before
the efivarfs_init() call.

However, in case EFI indicates support for variables, we will then have
generic_ops_register() and the uefisecapp's driver call running both in
subsys_initcall(). So if I'm not mistaken, this could cause the generic
ops to be registered after the uefisecapp ones, which we want to avoid.

One solution is bumping uefisecapp to fs_initcall(). Or do you have any
other suggestions?

Regards,
Max


>  From 8fecce12d215bd8cab1b8c8f9f0d1e1fe20fe6e7 Mon Sep 17 00:00:00 2001
> From: Johan Hovold <johan+linaro@xxxxxxxxxx>
> Date: Sun, 15 Jan 2023 15:32:34 +0100
> Subject: [PATCH] firmware: qcom_tee_uefisecapp: register at subsys init
>
> Register efivars at subsys init time so that it is available when
> efivarfs probes. For the same reason, also prevent building the driver
> as a module.
>
> This is specifically needed on platforms such as the Lenovo Thinkpad
> X13s where the firmware has cleared the variable services in the RT_PROP
> table so that efi core does not register any efivar callbacks at subsys
> init time (which are later overridden).
>
> Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>
> ---
>   drivers/firmware/Kconfig               | 2 +-
>   drivers/firmware/qcom_tee_uefisecapp.c | 7 ++++++-
>   2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
> index 4e9e2c227899..48e712e363da 100644
> --- a/drivers/firmware/Kconfig
> +++ b/drivers/firmware/Kconfig
> @@ -231,7 +231,7 @@ config QCOM_TEE
>   	select QCOM_SCM
>   
>   config QCOM_TEE_UEFISECAPP
> -	tristate "Qualcomm TrEE UEFI Secure App client driver"
> +	bool "Qualcomm TrEE UEFI Secure App client driver"
>   	select QCOM_TEE
>   	depends on EFI
>   	help
> diff --git a/drivers/firmware/qcom_tee_uefisecapp.c b/drivers/firmware/qcom_tee_uefisecapp.c
> index 65573e4b815a..e83bce4da70a 100644
> --- a/drivers/firmware/qcom_tee_uefisecapp.c
> +++ b/drivers/firmware/qcom_tee_uefisecapp.c
> @@ -754,7 +754,12 @@ static struct platform_driver qcom_uefisecapp_driver = {
>   		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
>   	},
>   };
> -module_platform_driver(qcom_uefisecapp_driver);
> +
> +static int __init qcom_uefisecapp_init(void)
> +{
> +	return platform_driver_register(&qcom_uefisecapp_driver);
> +}
> +subsys_initcall(qcom_uefisecapp_init);
>   
>   MODULE_AUTHOR("Maximilian Luz <luzmaximilian@xxxxxxxxx>");
>   MODULE_DESCRIPTION("Client driver for Qualcomm TrEE/TZ UEFI Secure App");