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

Roberto Sassu
2025-11-27T17:18:02Z

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@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.

Sure, I'm fine with capping. Isn't that enough?

Thanks

Roberto

> > Thanks
> >
> > Roberto
>
> BR, Jarkko