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

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Message-ID: <a6e73690e73b7a3e190719d179dbc73b93d1c1f1.camel@huaweicloud.com>
Date: Thu, 27 Nov 2025 18:17:42 +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 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?

Thanks

Roberto

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