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

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Message-ID: <aSnkdn86LW5mfjey@kernel.org>
Date: Fri, 28 Nov 2025 20:05:42 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
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 Fri, Nov 28, 2025 at 10:21:57AM +0100, Roberto Sassu wrote:
> 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>

https://lore.kernel.org/linux-integrity/aSnQZ4pRWqJai6FW@kernel.org/T/#u

I renewed the PR as I had to drop one patch so it's there in place :-)

Thanks again for reviewed-by.

BR, Jarkko