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

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Message-ID: <20181015104156.GA13408@linux.intel.com>
Date:   Mon, 15 Oct 2018 13:41:56 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     "Winkler, Tomas" <tomas.winkler@...el.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Peter Huewe <peterhuewe@....de>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: fix unused-value issues in tpm_try_transmit

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().

/Jarkko