Re: [PATCH] efi_loader: Measure the loaded DTB

[Heinrich Schuchardt <xypron.glpk@gmx.de>]

On 12/7/22 16:11, Etienne Carriere wrote:
> Measures the DTB passed to the EFI application upon new boolean config
> switch CONFIG_EFI_TCG2_PROTOCOL_MEASURE_DTB. For platforms where the
> content of the DTB passed to the OS can change across reboots, there is
> not point measuring it hence the config switch to allow platform to not
> embed this feature.
>
> Co-developed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
> ---
>   cmd/bootefi.c             |  9 +++++++++
>   include/efi_loader.h      |  2 ++
>   include/efi_tcg2.h        | 10 ++++++++++
>   include/tpm-v2.h          |  2 ++
>   lib/efi_loader/Kconfig    | 12 ++++++++++++
>   lib/efi_loader/efi_tcg2.c | 36 ++++++++++++++++++++++++++++++++++++
>   6 files changed, 71 insertions(+)
>
> diff --git a/cmd/bootefi.c b/cmd/bootefi.c
> index 2a7d42925d..56e4a1909f 100644
> --- a/cmd/bootefi.c
> +++ b/cmd/bootefi.c
> @@ -315,6 +315,15 @@ efi_status_t efi_install_fdt(void *fdt)
>   		return EFI_LOAD_ERROR;
>   	}
>
> +	/* Measure the installed DTB */
> +	if (CONFIG_IS_ENABLED(EFI_TCG2_PROTOCOL_MEASURE_DTB)) {
> +		ret = efi_tcg2_measure_dtb(fdt);

Why measure here and not in efi_tcg2_measure_efi_app_invocation() where
we measure the SMBIOS table every time when an app is started with
StartImage()? Which may mean measuring it multiple times like when
starting iPXE, when starting GRUB, and again when starting Linux.

What is the value of measuring the device-tree here when GRUB or
systemd-boot afterwards load a different device-tree or modify the
provided device-tree?

> +		if (ret == EFI_SECURITY_VIOLATION) {
> +			log_err("ERROR: failed to measure DTB\n");
> +			return ret;
> +		}
> +	}
> +
>   	/* Prepare device tree for payload */
>   	ret = copy_fdt(&fdt);
>   	if (ret) {
> diff --git a/include/efi_loader.h b/include/efi_loader.h
> index 0899e293e5..7538b6b828 100644
> --- a/include/efi_loader.h
> +++ b/include/efi_loader.h
> @@ -530,6 +530,8 @@ efi_status_t efi_tcg2_notify_exit_boot_services_failed(void);
>   efi_status_t efi_tcg2_measure_efi_app_invocation(struct efi_loaded_image_obj *handle);
>   /* Measure efi application exit */
>   efi_status_t efi_tcg2_measure_efi_app_exit(void);
> +/* Measure DTB */
> +efi_status_t efi_tcg2_measure_dtb(void *fdt);
>   /* Called by bootefi to initialize root node */
>   efi_status_t efi_root_node_register(void);
>   /* Called by bootefi to initialize runtime */
> diff --git a/include/efi_tcg2.h b/include/efi_tcg2.h
> index 874306dc11..b1c3abd097 100644
> --- a/include/efi_tcg2.h
> +++ b/include/efi_tcg2.h
> @@ -233,6 +233,16 @@ struct efi_gpt_data {
>   	gpt_entry partitions[];
>   } __packed;
>
> +/**
> + * struct tdUEFI_PLATFORM_FIRMWARE_BLOB2
> + * @blob_description_size:	Byte size of @data
> + * @data:			Description data
> + */
> +struct uefi_platform_firmware_blob2 {
> +	u8 blob_description_size;
> +	u8 data[];
> +} __packed;
> +
>   struct efi_tcg2_protocol {
>   	efi_status_t (EFIAPI * get_capability)(struct efi_tcg2_protocol *this,
>   					       struct efi_tcg2_boot_service_capability *capability);
> diff --git a/include/tpm-v2.h b/include/tpm-v2.h
> index 737e57551d..2df3dad553 100644
> --- a/include/tpm-v2.h
> +++ b/include/tpm-v2.h
> @@ -105,6 +105,8 @@ struct udevice;
>   	"Exit Boot Services Returned with Failure"
>   #define EFI_EXIT_BOOT_SERVICES_SUCCEEDED    \
>   	"Exit Boot Services Returned with Success"
> +#define EFI_DTB_EVENT_STRING \
> +	"DTB DATA"
>
>   /* TPMS_TAGGED_PROPERTY Structure */
>   struct tpms_tagged_property {
> diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
> index e2b643871b..e490236d14 100644
> --- a/lib/efi_loader/Kconfig
> +++ b/lib/efi_loader/Kconfig
> @@ -337,6 +337,18 @@ config EFI_TCG2_PROTOCOL_EVENTLOG_SIZE
>   		this is going to be allocated twice. One for the eventlog it self
>   		and one for the configuration table that is required from the spec
>
> +config EFI_TCG2_PROTOCOL_MEASURE_DTB
> +	bool "Measure DTB with EFI_TCG2_PROTOCOL"
> +	depends on EFI_TCG2_PROTOCOL
> +	default n

This line has no effect.

> +	help
> +	  When enabled, the DTB image passed to the booted EFI image is
> +	  measured using EFI TCG2 protocol. Do not enable this feature if

%s/using/using the/

> +	  the passed DTB contains data that change across platform reboots
> +	  and cannot be used has a predictable measurement. Otherwise

How should the user know this? Shall we create dependencies to other
Kconfig symbols? e.g.

     depends on !NET_RANDOM_ETHADDR

> +	  this feature allows better measurement of the system boot
> +	  sequence.
> +
>   config EFI_LOAD_FILE2_INITRD
>   	bool "EFI_FILE_LOAD2_PROTOCOL for Linux initial ramdisk"
>   	default y
> diff --git a/lib/efi_loader/efi_tcg2.c b/lib/efi_loader/efi_tcg2.c
> index a525ebf75b..51c9d80828 100644
> --- a/lib/efi_loader/efi_tcg2.c
> +++ b/lib/efi_loader/efi_tcg2.c
> @@ -2175,6 +2175,42 @@ out1:
>   	return ret;
>   }
>
> +/**
> + * efi_tcg2_measure_dtb() - measure the dtb used to boot our OS
> + *
> + * @fdt: pointer to the device tree blob
> + *
> + * Return:	status code
> + */
> +efi_status_t efi_tcg2_measure_dtb(void *fdt)
> +{
> +	efi_status_t ret;
> +	struct uefi_platform_firmware_blob2 *blob;
> +	struct udevice *dev;
> +	u32 event_size;
> +
> +	if (!is_tcg2_protocol_installed())
> +		return EFI_SUCCESS;
> +
> +	ret = platform_get_tpm2_device(&dev);
> +	if (ret != EFI_SUCCESS)
> +		return EFI_SECURITY_VIOLATION;
> +
> +	event_size = sizeof(*blob) + sizeof(EFI_DTB_EVENT_STRING) + fdt_totalsize(fdt);
> +	blob = calloc(1, event_size);
> +	if (!blob)
> +		return EFI_OUT_OF_RESOURCES;
> +
> +	blob->blob_description_size = sizeof(EFI_DTB_EVENT_STRING);
> +	memcpy(blob->data, EFI_DTB_EVENT_STRING, blob->blob_description_size);
> +	memcpy(blob->data + blob->blob_description_size, fdt, fdt_totalsize(fdt));
> +
> +	ret = tcg2_measure_event(dev, 0, EV_POST_CODE, event_size, (u8 *)blob);

We should ignore "free space" surrounding blocks. This free space is
expected to contain random data. E.g in copy_fdt() we do not zero out
the memory after the strings block.

On the RISC-V architecture we always write the boot-hart ID into the
device-tree to be backward compatible to old kernels. Which other
device-tree information is random and needs to be ignored when measuring?

Is there a standard defining into which PCR the DTB shall be measured?
Why did you choose pcr_index=0?

Why measure device-trees but not ACPI tables?

Best regards

Heinrich

> +
> +	free(blob);
> +	return ret;
> +}
> +
>   /**
>    * efi_tcg2_measure_efi_app_invocation() - measure efi app invocation
>    *