[PATCH 7/7] tpm: allow the user to select the compiled algorithms

[Heinrich Schuchardt]

On 22.06.24 16:35, Ilias Apalodimas wrote:
> Simon reports that after enabling all algorithms on the TPM some boards
> fail since they don't have enough storage to accommodate the ~5KB growth.
>
> The choice of hash algorithms are determined by the platform and the TPM
> configuration. Failing to cap a PCR in a bank which the platform left
> active is a security vulnerability. It might allow  unsealing of secrets
> if an attacker can replay a good set of measurements into an unused bank.
>
> If MEASURED_BOOT or EFI_TCG2_PROTOCOL is enabled our Kconfig will enable
> all supported hashing algorithms. We still want to allow users to add a
> TPM and not enable measured boot via EFI or bootm though and at the same
> time, control the compiled algorithms for size reasons.
>
> So let's add a function tpm2_allow_extend() which checks the TPM active
> PCRs banks against the one U-Boot was compiled with.
> If all the active PCRs banks are not enabled refuse to extend a PCR but
> otherwise leave the TPM functional.

The paragraph above is bit hard to read. I guess you mean:

We only allow extending PCRs using one of the algorithms selected in the
configuration.

>
> It's worth noting that this is only added on TPM2.0, since TPM1.2 is
> lacking a lot of code at the moment to read the available PCRs.
> We unconditionally enable SHA1 when a TPM is selected, which is the only
> hashing algorithm v1.2 supports.

Why do we need SHA1 if we cannot access PCRs on a TPM1.2?

Best regards

Heinrich

>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
>   boot/Kconfig     |  4 ++++
>   include/tpm-v2.h | 59 +++++++++++++++++++++++++++++++++++-------------
>   lib/Kconfig      |  6 ++---
>   lib/tpm-v2.c     | 40 +++++++++++++++++++++++++++++---
>   4 files changed, 87 insertions(+), 22 deletions(-)
>
> diff --git a/boot/Kconfig b/boot/Kconfig
> index 6f3096c15a6f..b061891e109c 100644
> --- a/boot/Kconfig
> +++ b/boot/Kconfig
> @@ -734,6 +734,10 @@ config LEGACY_IMAGE_FORMAT
>   config MEASURED_BOOT
>   	bool "Measure boot images and configuration when booting without EFI"
>   	depends on HASH && TPM_V2
> +	select SHA1
> +	select SHA256
> +	select SHA384
> +	select SHA512
>   	help
>   	  This option enables measurement of the boot process when booting
>   	  without UEFI . Measurement involves creating cryptographic hashes
> diff --git a/include/tpm-v2.h b/include/tpm-v2.h
> index eac04d1c6831..fccb07fa4695 100644
> --- a/include/tpm-v2.h
> +++ b/include/tpm-v2.h
> @@ -277,48 +277,40 @@ struct digest_info {
>   #define TCG2_BOOT_HASH_ALG_SM3_256 0x00000010
>
>   static const struct digest_info hash_algo_list[] = {
> +#if IS_ENABLED(CONFIG_SHA1)
>   	{
>   		"sha1",
>   		TPM2_ALG_SHA1,
>   		TCG2_BOOT_HASH_ALG_SHA1,
>   		TPM2_SHA1_DIGEST_SIZE,
>   	},
> +#endif
> +#if IS_ENABLED(CONFIG_SHA256)
>   	{
>   		"sha256",
>   		TPM2_ALG_SHA256,
>   		TCG2_BOOT_HASH_ALG_SHA256,
>   		TPM2_SHA256_DIGEST_SIZE,
>   	},
> +#endif
> +#if IS_ENABLED(CONFIG_SHA384)
>   	{
>   		"sha384",
>   		TPM2_ALG_SHA384,
>   		TCG2_BOOT_HASH_ALG_SHA384,
>   		TPM2_SHA384_DIGEST_SIZE,
>   	},
> +#endif
> +#if IS_ENABLED(CONFIG_SHA512)
>   	{
>   		"sha512",
>   		TPM2_ALG_SHA512,
>   		TCG2_BOOT_HASH_ALG_SHA512,
>   		TPM2_SHA512_DIGEST_SIZE,
>   	},
> +#endif
>   };
>
> -static inline u16 tpm2_algorithm_to_len(enum tpm2_algorithms a)
> -{
> -	switch (a) {
> -	case TPM2_ALG_SHA1:
> -		return TPM2_SHA1_DIGEST_SIZE;
> -	case TPM2_ALG_SHA256:
> -		return TPM2_SHA256_DIGEST_SIZE;
> -	case TPM2_ALG_SHA384:
> -		return TPM2_SHA384_DIGEST_SIZE;
> -	case TPM2_ALG_SHA512:
> -		return TPM2_SHA512_DIGEST_SIZE;
> -	default:
> -		return 0;
> -	}
> -}
> -
>   /* NV index attributes */
>   enum tpm_index_attrs {
>   	TPMA_NV_PPWRITE		= 1UL << 0,
> @@ -711,6 +703,41 @@ enum tpm2_algorithms tpm2_name_to_algorithm(const char *name);
>    */
>   const char *tpm2_algorithm_name(enum tpm2_algorithms);
>
> +/**
> + * tpm2_algorithm_to_len() - Return an algorithm length for supported algorithm id
> + *
> + * @algorithm_id: algorithm defined in enum tpm2_algorithms
> + * Return: len or 0 if not supported
> + */
> +u16 tpm2_algorithm_to_len(enum tpm2_algorithms algo);
> +
> +/*
> + * When measured boot is enabled via EFI or bootX commands all the algorithms
> + * above are selected by our Kconfigs. Due to U-Boots nature of being small there
> + * are cases where we need some functionality from the TPM -- e.g storage or RNG
> + * but we don't want to support measurements.
> + *
> + * The choice of hash algorithms are determined by the platform and the TPM
> + * configuration. Failing to cap a PCR in a bank which the platform left
> + * active is a security vulnerability. It permits the unsealing of secrets
> + * if an attacker can replay a good set of measurements into an unused bank.
> + *
> + * On top of that a previous stage bootloader (e.g TF-A), migh pass an eventlog
> + * since it doesn't have a TPM driver, which U-Boot needs to replace. The algorit h
> + * choice is a compile time option in that case and we need to make sure we conform.
> + *
> + * Add a variable here that sums the supported algorithms U-Boot was compiled
> + * with so we can refuse to do measurements if we don't support all of them
> + */
> +
> +/**
> + * tpm2_allow_extend() - Check if extending PCRs is allowed and safe
> + *
> + * @dev: TPM device
> + * Return: true if allowed
> + */
> +bool tpm2_allow_extend(struct udevice *dev);
> +
>   /**
>    * tpm2_is_active_pcr() - check the pcr_select. If at least one of the PCRs
>    *			  supports the algorithm add it on the active ones
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 189e6eb31aa1..b3baa4b85b07 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -439,9 +439,6 @@ config TPM
>   	depends on DM
>   	imply DM_RNG
>   	select SHA1
> -	select SHA256
> -	select SHA384
> -	select SHA512
>   	help
>   	  This enables support for TPMs which can be used to provide security
>   	  features for your board. The TPM can be connected via LPC or I2C
> @@ -449,6 +446,9 @@ config TPM
>   	  command to interactive the TPM. Driver model support is provided
>   	  for the low-level TPM interface, but only one TPM is supported at
>   	  a time by the TPM library.
> +	  For size reasons only SHA1 is selected which is supported on TPM1.2.
> +	  If you want a fully functional TPM enable all hashing algorithms.
> +	  If you enabled measured boot all hashing algorithms are selected.
>
>   config SPL_TPM
>   	bool "Trusted Platform Module (TPM) Support in SPL"
> diff --git a/lib/tpm-v2.c b/lib/tpm-v2.c
> index 36aace03cf4e..59e6cbafafaa 100644
> --- a/lib/tpm-v2.c
> +++ b/lib/tpm-v2.c
> @@ -196,6 +196,11 @@ u32 tpm2_pcr_extend(struct udevice *dev, u32 index, u32 algorithm,
>
>   	if (!digest)
>   		return -EINVAL;
> +
> +	if (!tpm2_allow_extend(dev)) {
> +		log_err("Cannot extend PCRs if all the TPM enabled algorithms are not supported\n");
> +		return -EINVAL;
> +	}
>   	/*
>   	 * Fill the command structure starting from the first buffer:
>   	 *     - the digest
> @@ -409,11 +414,10 @@ int tpm2_get_pcr_info(struct udevice *dev, struct tpml_pcr_selection *pcrs)
>
>   	pcrs->count = get_unaligned_be32(response);
>   	/*
> -	 * We only support 5 algorithms for now so check against that
> +	 * We only support 4 algorithms for now so check against that
>   	 * instead of TPM2_NUM_PCR_BANKS
>   	 */
> -	if (pcrs->count > ARRAY_SIZE(hash_algo_list) ||
> -	    pcrs->count < 1) {
> +	if (pcrs->count > 4 || pcrs->count < 1) {
>   		printf("%s: too many pcrs: %u\n", __func__, pcrs->count);
>   		return -EMSGSIZE;
>   	}
> @@ -880,3 +884,33 @@ const char *tpm2_algorithm_name(enum tpm2_algorithms algo)
>   	return "";
>   }
>
> +u16 tpm2_algorithm_to_len(enum tpm2_algorithms algo)
> +{
> +	size_t i;
> +
> +	for (i = 0; i < ARRAY_SIZE(hash_algo_list); ++i) {
> +		if (hash_algo_list[i].hash_alg == algo)
> +			return hash_algo_list[i].hash_len;
> +	}
> +
> +	return 0;
> +}
> +
> +bool tpm2_allow_extend(struct udevice *dev)
> +{
> +	struct tpml_pcr_selection pcrs;
> +	size_t i;
> +	int rc;
> +
> +	rc = tpm2_get_pcr_info(dev, &pcrs);
> +	if (rc)
> +		return false;
> +
> +	for (i = 0; i < pcrs.count; i++) {
> +		if (tpm2_is_active_pcr(&pcrs.selection[i]) &&
> +		    !tpm2_algorithm_to_len(pcrs.selection[i].hash))
> +			return false;
> +	}
> +
> +	return true;
> +}