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

Jarkko Sakkinen
2025-11-27T17:14:26Z

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@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?

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.

>
> Thanks
>
> Roberto

BR, Jarkko