Re: [PATCH v2 1/1] tpm: add sysfs exports for all banks of PCR registers

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2020-07-21 at 17:51 -0700, Jerry Snitselaar wrote:
> James Bottomley @ 2020-07-21 17:39 MST:
> 
> > On Tue, 2020-07-21 at 17:02 -0700, Jerry Snitselaar wrote:
> > > James Bottomley @ 2020-07-21 16:37 MST:
> > > 
> > > > On Tue, 2020-07-21 at 16:16 -0700, Jerry Snitselaar wrote:
> > > > > James Bottomley @ 2020-07-21 08:56 MST:
> > > > 
> > > > [...]
> > > > > > +	/*
> > > > > > +	 * This will only trigger if someone has added an
> > > > > > additional
> > > > > > +	 * hash to the tpm_algorithms enum without
> > > > > > incrementing
> > > > > > +	 * TPM_MAX_HASHES.  This has to be a BUG_ON
> > > > > > because
> > > > > > under
> > > > > > this
> > > > > > +	 * condition, the chip->groups array will overflow
> > > > > > corrupting
> > > > > > +	 * the chips structure.
> > > > > > +	 */
> > > > > > +	BUG_ON(chip->groups_cnt > TPM_MAX_HASHES);
> > > > > 
> > > > > Should this check be 3 + TPM_MAX_HASHES like below?
> > > > 
> > > > No, because at this point only a single additional group has
> > > > been addedin addition to the hashes groups.  The first line of
> > > > tpm_sysfs_add_device is
> > > > 
> > > > 	WARN_ON(chip->groups_cnt != 0);
> > > > 
> > > > And then we add the unnamed group.  This loop over the banks
> > > > follows it, so chip->groups_cnt should be nr_banks_allocated by
> > > > the end (it's the index, which is one fewer than the number of
> > > > entries in chip->groups[]).  We have a problem if
> > > > nr_banks_allocated > TPM_MAX_HASHES
> > > > 
> > > > which is what the BUG_ON checks.
> > > > 
> > > > James
> > > 
> > > If the chip supported all 5 listed cases wouldn't groups_cnt be 6
> > > at this point?
> > 
> > Actually, yes, I think it would be because it's pointing at the
> > next free index not the current one.  So it should be BUG_ON (chip-
> > > groups_cnt > TPM_MAX_HASHES + 1)
> > 
> > James
> 
> One other thought, should a note be added above tpm_algorithms to
> note that when that is changed TPM_MAX_HASHES should be changed as
> well?

I certainly can ... it's free.

> With the above change to the BUG_ON you can add to v3:

OK, I also changed the BUG_ON back to a WARN_ON to match the initial
one (if that one ever tripped, we'd get an overflow in the chip-
>groups[] as well, so it seems reasonable to keep them matching).

James


> Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
>