Re: [PATCH v4 3/6] bootm: Support boot measurement

Ilias Apalodimas
2023-01-26T06:54:26Z

Hi Eddie,

On Wed, Jan 25, 2023 at 11:18:07AM -0600, Eddie James wrote:
> Add a configuration option to measure the boot through the bootm
> function. Add the measurement state to the booti and bootz paths
> as well.
>
> Signed-off-by: Eddie James <eajames@linux.ibm.com>
> ---
> boot/Kconfig | 23 ++++++++++++++++
> boot/bootm.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++
> cmd/booti.c | 1 +
> cmd/bootm.c | 2 ++
> cmd/bootz.c | 1 +
> include/bootm.h | 2 ++
> include/image.h | 1 +
> 7 files changed, 100 insertions(+)
>
> diff --git a/boot/Kconfig b/boot/Kconfig
> index fdcfbae7b2..831b9e954c 100644
> --- a/boot/Kconfig
> +++ b/boot/Kconfig
> @@ -601,6 +601,29 @@ config LEGACY_IMAGE_FORMAT
> loaded. If a board needs the legacy image format support in this
> case, enable it here.
>
> +config MEASURED_BOOT
> + bool "Measure boot images and configuration to TPM and event log"
> + depends on HASH && TPM_V2
> + help
> + This option enables measurement of the boot process. Measurement
> + involves creating cryptographic hashes of the binary images that
> + are booting and storing them in the TPM. In addition, a log of
> + these hashes is stored in memory for the OS to verify the booted
> + images and configuration. Enable this if the OS has configured
> + some memory area for the event log and you intend to use some
> + attestation tools on your system.
> +
> +if MEASURED_BOOT
> + config MEASURE_DEVICETREE
> + bool "Measure the devicetree image"
> + default y if MEASURED_BOOT
> + help
> + On some platforms, the devicetree is not static as it may contain
> + random MAC addresses or other such data that changes each boot.
> + Therefore, it should not be measured into the TPM. In that case,
> + disable the measurement here.
> +endif # MEASURED_BOOT
> +
> config SUPPORT_RAW_INITRD
> bool "Enable raw initrd images"
> help
> diff --git a/boot/bootm.c b/boot/bootm.c
> index 15fce8ad95..c8423f2c60 100644
> --- a/boot/bootm.c
> +++ b/boot/bootm.c
> @@ -22,6 +22,7 @@
> #include <asm/global_data.h>
> #include <asm/io.h>
> #include <linux/sizes.h>
> +#include <tpm-v2.h>
> #if defined(CONFIG_CMD_USB)
> #include <usb.h>
> #endif
> @@ -659,6 +660,72 @@ int bootm_process_cmdline_env(int flags)
> return 0;
> }
>
> +int bootm_measure(struct bootm_headers *images)
> +{
> + int ret = 0;
> +
> + /* Skip measurement if EFI is going to do it */
> + if (images->os.os == IH_OS_EFI &&
> + IS_ENABLED(CONFIG_EFI_TCG2_PROTOCOL) &&
> + IS_ENABLED(CONFIG_BOOTM_EFI))
> + return ret;
> +
> + if (IS_ENABLED(CONFIG_MEASURED_BOOT)) {
> + struct tcg2_event_log elog;
> + struct udevice *dev;
> + void *initrd_buf;
> + void *image_buf;
> + const char *s;
> + u32 rd_len;
> +
> + ret = tcg2_measurement_init(&dev, &elog);
> + if (ret)
> + return ret;
> +
> + image_buf = map_sysmem(images->os.image_start,
> + images->os.image_len);
> + ret = tcg2_measure_data(dev, &elog, 8, images->os.image_len,
> + image_buf, EV_COMPACT_HASH,
> + strlen("linux") + 1, (u8 *)"linux");
> + if (ret)
> + goto unmap_image;
> +
> + rd_len = images->rd_end - images->rd_start;
> + initrd_buf = map_sysmem(images->rd_start, rd_len);
> + ret = tcg2_measure_data(dev, &elog, 8, rd_len, initrd_buf,
> + EV_COMPACT_HASH, strlen("initrd") + 1,
> + (u8 *)"initrd");

The kernel, if loaded with EFI, measures the initrd data into PCR9.
Although it's not important can we please use the same PCR here?

> + if (ret)
> + goto unmap_initrd;
> +
> + if (IS_ENABLED(CONFIG_MEASURE_DEVICETREE)) {
> + ret = tcg2_measure_data(dev, &elog, 9, images->ft_len,
> + (u8 *)images->ft_addr,
> + EV_TABLE_OF_DEVICES,
> + strlen("dts") + 1,
> + (u8 *)"dts");

The PC client spec describes how ACPI data should be measured. Although it
has no wording on the DTB, I think it should be treated in a similar
fashion. This [0] was trying to do the same thing for EFI. Any reason we
can't use PCR0 here?

> + if (ret)
> + goto unmap_initrd;
> + }
> +
> + s = env_get("bootargs");
> + if (!s)
> + s = "";
> + ret = tcg2_measure_data(dev, &elog, 1, strlen(s) + 1, (u8 *)s,
> + EV_PLATFORM_CONFIG_FLAGS,
> + strlen(s) + 1, (u8 *)s);
> +
> +unmap_initrd:
> + unmap_sysmem(initrd_buf);

[0] https://lore.kernel.org/u-boot/20221207151110.529106-1-etienne.carriere@linaro.org/

Regards
/Ilias