Re: [PATCH v2] tpm: Disable tpm verifier if tpm is not present

[Michael Chang]

On Tue, Nov 29, 2022 at 04:11:48PM +0100, Daniel Kiper wrote:
> On Fri, Nov 25, 2022 at 03:00:48PM +0800, Michael Chang via Grub-devel wrote:
> > On Thu, Nov 24, 2022 at 05:04:48PM +0100, Daniel Kiper wrote:
> > > On Mon, Oct 17, 2022 at 01:19:08PM +0800, Michael Chang via Grub-devel 
> > > wrote:
> > > > On Fri, Oct 14, 2022 at 11:40:01AM +0200, Daniel Kiper wrote:
> > > > > On Fri, Oct 07, 2022 at 01:37:10PM +0800, Michael Chang via 
> > > > > Grub-devel wrote:
> > > > > > This helps to prevent out of memory error when reading large files 
> > > > > > via disabling
> > > > > > tpm device as verifier has to read all content into memory in one 
> > > > > > chunk to
> > > > > > measure the hash and extend to tpm.
> > > > >
> > > > > How does this patch help when the TPM is present in the system?
> > > >
> > > > If the firmware menu offers option to disable TPM device, then this
> > > > patch can be useful to get around 'out of memory error' through
> > > > disabling TPM device from firmware in order to make tpm verifier won't
> > > > be in the way of reading huge files.
> > > >
> > > > This is essentially a compromised solution as long as tpm module can be
> > > > a built-in module in signed image and at the same time user may come
> > > > across the need to open huge files, for eg, loopback mount in grub for
> > > > the rescue image. In this case they could be opted in to disable tpm
> > > > device from firmware to proceed if they run into out of memory or other
> > > > (slow) reading issues.
> > >
> > > I think I would prefer something similar to this [1] patch. Of course
> > > if [1] is not enough...
> >
> > The tpm verifier attempts to set GRUB_VERIFY_FLAGS_SINGLE_CHUNK for all
> > incoming files, which gets loaded into memory in its entirety as an
> > duplicated copy to disk files. The overhead is too huge to some low
> > profile hardwares with smaller memory or when the boot path has to cover
> > very large files, hence the out of memory error.
> >
> > I think it inevitable to use GRUB_VERIFY_FLAGS_SINGLE_CHUNK as tpm
> > measures and extends file intergrity. But we ought to avoid the overhead
> > when TPM device is not present or disabled by the user.
> >
> > The patch [1] seems to deal with the tpm error which prevents a file
> > from being opened, which is orthogonal to the memory allocation issue in
> > the common verifier before tpm doing measurement.
> 
> This is what I expected. So, that is why I said "Of course if [1] is not
> enough..."... :-) Now I think it would be nice if TPM verifier is
> disabled when the TPM device is broken/disabled/... and/or somebody sets
> a separate environemnt variable in the GRUB. WDYT?

I'm not sure if a separate environment a good idea, because it would
expose the funtion to disable verifier in a way much readily accessible
through one of grub command line interface, grub.cfg and grubenv file so
that the intention have to be very carefully reviewed here.

Thanks,
Michael

> 
> Daniel
> 
> [1] 
> http://git.savannah.gnu.org/gitweb/?p=grub.git;a=commit;h=a4356538d03c5a5350790b6453b523fb9214c2e9
> 
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