Re: [tpmdd-devel] [PATCH RFC v2 3/5] tpm: infrastructure for TPM spaces

From: Jarkko Sakkinen
Date: Mon Jan 16 2017 - 09:48:57 EST


On Mon, Jan 16, 2017 at 06:24:48AM -0800, James Bottomley wrote:
> On Mon, 2017-01-16 at 11:09 +0200, Jarkko Sakkinen wrote:
> > On Thu, Jan 12, 2017 at 05:17:23PM -0800, James Bottomley wrote:
> > > On Thu, 2017-01-12 at 19:46 +0200, Jarkko Sakkinen wrote:
> > > > @@ -189,6 +190,12 @@ struct tpm_chip *tpm_chip_alloc(struct
> > > > device
> > > > *pdev,
> > > > chip->cdev.owner = THIS_MODULE;
> > > > chip->cdev.kobj.parent = &chip->dev.kobj;
> > > >
> > > > + chip->work_space.context_buf = kzalloc(PAGE_SIZE,
> > > > GFP_KERNEL);
> > > > + if (!chip->work_space.context_buf) {
> > > > + rc = -ENOMEM;
> > > > + goto out;
> > > > + }
> > > > +
> > >
> > > I think the work_buf handling can be greatly simplified by making
> > > it a pointer to the space: it's only usable between
> > > tpm2_prepare_space() and tpm2_commit_space() which are protected by
> > > the chip mutex, so there's no need for it to exist outside of these
> > > calls (i.e. it can be NULL).
> > >
> > > Doing it this way also saves the allocation and copying overhead of
> > > work_space.
> > >
> > > The patch below can be folded to effect this.
> >
> > Hey, I have to take my words back. There's a separate buffer for
> > space for a reason. If the transaction fails for example when RM is
> > doing its job, we can revert to the previous set of transient
> > objects.
> >
> > Your change would completely thrawt this. I tried varius ways to heal
> > when RM decorations fail and this is the most fail safe to do it so
> > lets stick with it.
>
> That's why I added the return code check in the other patch: if the
> command fails in the TPM, the space state isn't updated at all, the net
> result being that nothing changes in the space, thus you don't need the
> copy, because there's nothing to revert on a failure.

You are right in what you say but what if you save lets say 5 transient
contexts and ContextSave fails on 2nd. It's not for the command itself
but for falling back to a sane state when tpm2_commit_space fails (to
the previous set of transient objects).

I've never meant it as a fallback for the command itself...

> If you're thinking transaction being a sequence of TPM commands, then
> we might need an ioctl to transfer the space state to/from userspace,
> so it can do rollback for several commands, but that too wouldn't need
> us to have a single prior command saved copy.
>
> James

Here I refer to transaction as a single tpm_transmit.

/Jarkko