Re: [PATCH v4 1/1] tpm: add sysfs exports for all banks of PCR registers

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Aug 25, 2020 at 08:33:41AM -0700, James Bottomley wrote:
> On Tue, 2020-08-25 at 18:27 +0300, Jarkko Sakkinen wrote:
> > On Mon, Aug 24, 2020 at 01:20:46PM -0700, James Bottomley wrote:
> > > On Mon, 2020-08-24 at 22:44 +0300, Jarkko Sakkinen wrote:
> > > > On Fri, Aug 21, 2020 at 04:38:47PM -0300, Jason Gunthorpe wrote:
> > > > > On Thu, Aug 20, 2020 at 09:14:44AM -0700, James Bottomley
> > > > > wrote:
> > > > > 
> > > > > > > eg we can't do it because we can't access /dev/tpm for
> > > > > > > permissions or something.
> > > > > > 
> > > > > > I already said that: we can't it's root.root 0600
> > > > > > currently.  All the TSSs seem to change at least /dev/tpmrm
> > > > > > to tpm.tpm 0660 but we can't do that in the kernel because
> > > > > > there's no fixed tpm uid/gid.
> > > > > 
> > > > > Permissions is a pretty good reason to add a sysfs file.
> > > > > 
> > > > > Jason
> > > > 
> > > > I'm not sure why suid/sgid utility to read pcrs would be worse.
> > > 
> > > We don't do root running or suid/sgid binaries any more because
> > > they're exceptional security risks.  That's why both TSSs for TPM
> > > 2.0 change the device ownership.  For Trousers and TPM 1.2 we used
> > > to run the daemon as root until we started getting CVEs about it.
> > > 
> > > James
> > 
> > OK, then a binary blob for pcrs would be sufficient.
> 
> From a sysfs perspective we only do one value per file and we don't
> export binary if a valid and useful ascii representation exists.  On
> both of those kernel principles, the current proposal is canonical.
> 
> James

The event log is also exported as a binary. This patch set pollutes the
sysfs and adds too much overhead for maintaining. Every single algorithm
will needs its own file and needs to be patched to the kernel.

A single 'pcrs' blob could with contents as <alg id, data> pairs would
remain static.

If you speak about principles, please add a reference and/or CC your
patch set also to sysfs maintainers. All I care if what is pragmatically
the best choice.

/Jarkko