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

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Message-ID: <eff8c944622851683597a4738a2089c4b9a15b71.camel@huaweicloud.com>
Date: Fri, 28 Nov 2025 10:21:57 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: linux-integrity@...r.kernel.org, ross.philipson@...cle.com, Jonathan
 McDowell <noodles@...th.li>, Stefano Garzarella <sgarzare@...hat.com>,
 Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>, Roberto Sassu
 <roberto.sassu@...wei.com>,  Jonathan McDowell <noodles@...a.com>, Peter
 Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 01/11] tpm: Cap the number of PCR banks

On Thu, 2025-11-27 at 20:52 +0200, Jarkko Sakkinen wrote:
> On Thu, Nov 27, 2025 at 06:17:42PM +0100, Roberto Sassu wrote:
> > On Thu, 2025-11-27 at 19:14 +0200, Jarkko Sakkinen wrote:
> > > On Thu, Nov 27, 2025 at 05:09:38PM +0100, Roberto Sassu wrote:
> > > > On Thu, 2025-11-27 at 15:54 +0200, Jarkko Sakkinen wrote:
> > > > > From: Jarkko Sakkinen <jarkko.sakkinen@...nsys.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@...wei.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?
> > > 
> > > nr_possible_banks is read from external source i.e., neither kernel nor
> > > CPU fully control its value. This causes *uncontrolled* dynamic
> > > allocation. Thus, it must be capped to some value.
> > 
> > Sure, I'm fine with capping. Isn't that enough?
> 
> It makes sense to make the whole memory allocation then infallible,
> especially since it does not have much effect on diff. And it has
> not significant effect on memory usage either.

Ok. In that case (even if it does not get in):

Reviewed-by: Roberto Sassu <roberto.sassu@...wei.com>

Roberto

> But I do see one completely spurious and actually unintended change 
> that I spotted: tpm1_get_pcr_allocation. It there's no intention
> doing this it has just carried over the series.
> 
> I reverted that part, which make it look like a proper bug fix:
> 
> diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
> index 11088bda4e68..6849f216ba0b 100644
> --- a/drivers/char/tpm/tpm1-cmd.c
> +++ b/drivers/char/tpm/tpm1-cmd.c
> @@ -799,11 +799,6 @@ int tpm1_pm_suspend(struct tpm_chip *chip, u32 tpm_suspend_pcr)
>   */
>  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;
> 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];
> 
> 
> > 
> > Thanks
> > 
> > Roberto
> > 
> > > > Thanks
> > > > 
> > > > Roberto
> > > 
> > > BR, Jarkko
> > 
> 
> BR, Jarkko