Hi Ilias,
On Sun, 23 Jun 2024 at 05:49, Ilias Apalodimas
<ilias.apalodi...@linaro.org> 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 is 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. We only allow
> extending PCRs if the algorithms selected during build match the TPM
> configuration.
>
> It's worth noting that this is only added for TPM2.0, since TPM1.2 is
> lacking a lot of code at the moment to read the available PCR banks.
> We unconditionally enable SHA1 when a TPM is selected, which is the only
> hashing algorithm v1.2 supports.
>
> Signed-off-by: Ilias Apalodimas <ilias.apalodi...@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(-)
Reviewed-by: Simon Glass <s...@chromium.org>
Tested-by: Simon Glass <s...@chromium.org> # chromebook-link
nits / thoughts for the future below
>
> 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 aedf2c0f4f5c..4fd19c52fd70 100644
> --- a/include/tpm-v2.h
> +++ b/include/tpm-v2.h
> @@ -278,48 +278,40 @@ 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",
This is a duplicate of common/hash.c (we can worry about all this later)
> TPM2_ALG_SHA512,
> TCG2_BOOT_HASH_ALG_SHA512,
> TPM2_SHA512_DIGEST_SIZE,
so is this
> },
> +#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,
> @@ -712,6 +704,41 @@ 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
U-Boot's
> + * 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
might
> + * since it doesn't have a TPM driver, which U-Boot needs to replace. The
> algorit h
algorithm
> + * 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)
nit: return uint? There is no need to mask the register here
> +{
> + 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;
> +}
At some point we could have a HASH_ALGOT variable in hash.h which
encodes the hash algorithm, then some of this will be common
> +
> +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;
> +}
> --
> 2.45.2
>
REgards,
Simon
