Re: [PATCH v7 01/11] tpm: Cap the number of PCR banks

Roberto Sassu
2025-11-27T16:09:54Z

On Thu, 2025-11-27 at 15:54 +0200, Jarkko Sakkinen wrote:
> From: Jarkko Sakkinen <jarkko.sakkinen@opinsys.com>
>
> tpm2_get_pcr_allocation() does not cap any upper limit for the number of
> banks. Cap the limit to eight banks so that out of bounds values coming
> from external I/O cause on only limited harm.
>
> Cc: Roberto Sassu <roberto.sassu@huawei.com>

Sorry, I realized that you are expecting me to review.

I have a couple of questions:
- Could you explain better how out of bounds would occur, since one
could check the number of PCR banks?
- Is dynamic allocation that bad? And if yes, why?
- Couldn't you just check that the number of available PCR banks is
below the threshold you like and keep dynamic allocation?
- Is removing tpm1_get_pcr_allocation() improving code readability?

Thanks

Roberto

> Fixes: bcfff8384f6c ("tpm: dynamically allocate the allocated_banks array")
> Reviewed-By: Jonathan McDowell <noodles@meta.com>
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@opinsys.com>
> ---
> v7:
> - In Ryzen desktop there is total three banks so yep, eight is probably
> much safer bet than four banks. Fixed the commit message as per remark
> from Jonathan:
>
> https://lore.kernel.org/linux-integrity/aPYg1N0TvrkG6AJI@earth.li/#t
>
> And with that added also reviewed-by.
> v6
> - No changes.
> v5:
> - No changes.
> v4:
> - Revert spurious changes from include/linux/tpm.h.
> - Increase TPM2_MAX_BANKS to 8.
> - Rename TPM2_MAX_BANKS as TPM2_MAX_PCR_BANKS for the sake of clarity.
> v3:
> - Wrote a more clear commit message.
> - Fixed pr_err() message.
> v2:
> - A new patch.
> ---
> drivers/char/tpm/tpm-chip.c | 13 +++++++++----
> drivers/char/tpm/tpm.h | 1 -
> drivers/char/tpm/tpm1-cmd.c | 25 -------------------------
> drivers/char/tpm/tpm2-cmd.c | 8 +++-----
> include/linux/tpm.h | 8 +++++---
> 5 files changed, 17 insertions(+), 38 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index e25daf2396d3..6cb25862688f 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -559,14 +559,19 @@ static int tpm_add_hwrng(struct tpm_chip *chip)
>
> static int tpm_get_pcr_allocation(struct tpm_chip *chip)
> {
> - int rc;
> + int rc = 0;
>
> if (tpm_is_firmware_upgrade(chip))
> return 0;
>
> - rc = (chip->flags & TPM_CHIP_FLAG_TPM2) ?
> - tpm2_get_pcr_allocation(chip) :
> - tpm1_get_pcr_allocation(chip);
> + if (!(chip->flags & TPM_CHIP_FLAG_TPM2)) {
> + chip->allocated_banks[0].alg_id = TPM_ALG_SHA1;
> + chip->allocated_banks[0].digest_size = hash_digest_size[HASH_ALGO_SHA1];
> + chip->allocated_banks[0].crypto_id = HASH_ALGO_SHA1;
> + chip->nr_allocated_banks = 1;
> + } else {
> + rc = tpm2_get_pcr_allocation(chip);
> + }
>
> if (rc > 0)
> return -ENODEV;
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index 2726bd38e5ac..a37712c02e44 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -252,7 +252,6 @@ int tpm1_pcr_read(struct tpm_chip *chip, u32 pcr_idx, u8 *res_buf);
> ssize_t tpm1_getcap(struct tpm_chip *chip, u32 subcap_id, cap_t *cap,
> const char *desc, size_t min_cap_length);
> int tpm1_get_random(struct tpm_chip *chip, u8 *out, size_t max);
> -int tpm1_get_pcr_allocation(struct tpm_chip *chip);
> unsigned long tpm_calc_ordinal_duration(struct tpm_chip *chip, u32 ordinal);
> int tpm_pm_suspend(struct device *dev);
> int tpm_pm_resume(struct device *dev);
> diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
> index 11088bda4e68..708bc553437b 100644
> --- a/drivers/char/tpm/tpm1-cmd.c
> +++ b/drivers/char/tpm/tpm1-cmd.c
> @@ -786,28 +786,3 @@ int tpm1_pm_suspend(struct tpm_chip *chip, u32 tpm_suspend_pcr)
>
> return rc;
> }
> -
> -/**
> - * tpm1_get_pcr_allocation() - initialize the allocated bank
> - * @chip: TPM chip to use.
> - *
> - * The function initializes the SHA1 allocated bank to extend PCR
> - *
> - * Return:
> - * * 0 on success,
> - * * < 0 on error.
> - */
> -int tpm1_get_pcr_allocation(struct tpm_chip *chip)
> -{
> - chip->allocated_banks = kcalloc(1, sizeof(*chip->allocated_banks),
> - GFP_KERNEL);
> - if (!chip->allocated_banks)
> - return -ENOMEM;
> -
> - chip->allocated_banks[0].alg_id = TPM_ALG_SHA1;
> - chip->allocated_banks[0].digest_size = hash_digest_size[HASH_ALGO_SHA1];
> - chip->allocated_banks[0].crypto_id = HASH_ALGO_SHA1;
> - chip->nr_allocated_banks = 1;
> -
> - return 0;
> -}
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 7d77f6fbc152..97501c567c34 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -538,11 +538,9 @@ ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
>
> nr_possible_banks = be32_to_cpup(
> (__be32 *)&buf.data[TPM_HEADER_SIZE + 5]);
> -
> - chip->allocated_banks = kcalloc(nr_possible_banks,
> - sizeof(*chip->allocated_banks),
> - GFP_KERNEL);
> - if (!chip->allocated_banks) {
> + if (nr_possible_banks > TPM2_MAX_PCR_BANKS) {
> + pr_err("tpm: unexpected number of banks: %u > %u",
> + nr_possible_banks, TPM2_MAX_PCR_BANKS);
> rc = -ENOMEM;
> goto out;
> }
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index dc0338a783f3..eb0ff071bcae 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -26,7 +26,9 @@
> #include <crypto/aes.h>
>
> #define TPM_DIGEST_SIZE 20 /* Max TPM v1.2 PCR size */
> -#define TPM_MAX_DIGEST_SIZE SHA512_DIGEST_SIZE
> +
> +#define TPM2_MAX_DIGEST_SIZE SHA512_DIGEST_SIZE
> +#define TPM2_MAX_PCR_BANKS 8
>
> struct tpm_chip;
> struct trusted_key_payload;
> @@ -68,7 +70,7 @@ enum tpm2_curves {
>
> struct tpm_digest {
> u16 alg_id;
> - u8 digest[TPM_MAX_DIGEST_SIZE];
> + u8 digest[TPM2_MAX_DIGEST_SIZE];
> } __packed;
>
> struct tpm_bank_info {
> @@ -189,7 +191,7 @@ struct tpm_chip {
> unsigned int groups_cnt;
>
> u32 nr_allocated_banks;
> - struct tpm_bank_info *allocated_banks;
> + struct tpm_bank_info allocated_banks[TPM2_MAX_PCR_BANKS];
> #ifdef CONFIG_ACPI
> acpi_handle acpi_dev_handle;
> char ppi_version[TPM_PPI_VERSION_LEN + 1];