linux-kernel - Re: [PATCH] tpm: fix unused-value issues in tpm_try

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Date:   Mon, 15 Oct 2018 11:18:43 +0000
From:   "Winkler, Tomas" <tomas.winkler@...el.com>
To:     "jarkko.sakkinen@...ux.intel.com" <jarkko.sakkinen@...ux.intel.com>
CC:     "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jgg@...pe.ca" <jgg@...pe.ca>,
        "gustavo@...eddedor.com" <gustavo@...eddedor.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "peterhuewe@....de" <peterhuewe@....de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] tpm: fix unused-value issues in tpm_try_transmit

On Mon, 2018-10-15 at 13:41 +0300, Jarkko Sakkinen wrote:
> On Thu, Oct 11, 2018 at 01:27:58PM +0000, Winkler, Tomas wrote:
> > > 
> > > On Wed, Oct 10, 2018 at 08:06:38AM -0600, Jason Gunthorpe wrote:
> > > > On Wed, Oct 10, 2018 at 03:38:17PM +0200, Gustavo A. R. Silva wrote:
> > > > > Currently, there are some values assigned to variable *rc*, which
> > > > > are never actually used in any computation, because such variable is
> > > > > updated at line 550, before they can be used:
> > > > > 
> > > > > 549out:
> > > > > 550        rc = tpm_go_idle(chip, flags);
> > > > > 551        if (rc)
> > > > > 552                goto out;
> > > > > 
> > > > > Fix this by removing such assignments.
> > > > 
> > > > Should this be done by not quashing rc during the error unwind rather
> > > > than dropping the errors?
> > > 
> > > Yeah.`
> > > 
> > > Wondering if tpm_go_idle() should simply be a void-function i.e. issue just a
> > > warning inside (disclaimer: did not revisit its code when writing this).
> > 
> > We did have rather a long discussion about it when it was merged.
> > There are two flows that may crash 
> > rc = tpm2_commit_space()
> > 
> > but you still can need to 
> > 
> > rc  = go_idle()  
> > 
> > which also may crash which may override the previous value. 
> > 
> > Frankly the second one is fatal, the stack will go out of sync.
> > We may do void here as the stack will crash in a subsequent command. 
> > 
> > The 'goto out' is quite a  bug, probably caused by code movement.
> 
> I just looked at the code properly and noticed that there is a regression
> caused by 627448e85c76 ("tpm: separate cmd_ready/go_idle from
> runtime_pm") i.e. when tpm_go_idle() fails it loops back and retries
> tpm_go_idle().
Yes, that's what I said, this part code was moved forth but no the
label.
Tomas