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

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Message-ID: <20170116144847.p7nh3vvmqdoqlq7y@intel.com>
Date:   Mon, 16 Jan 2017 16:48:47 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <jejb@...ux.vnet.ibm.com>
Cc:     tpmdd-devel@...ts.sourceforge.net,
        open list <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org
Subject: Re: [tpmdd-devel] [PATCH RFC v2 3/5] tpm: infrastructure for TPM
 spaces

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