Date: Wed, 5 Oct 2016 13:09:15 +0300
From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: "Winkler, Tomas" <tomas.winkler@intel.com>,
        "tpmdd-devel@lists.sourceforge.net" 
        <tpmdd-devel@lists.sourceforge.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] tpm: don't destroy chip device prematurely
Message-ID: <20161005100915.GB20851@intel.com>
References: <1475393971-12715-1-git-send-email-tomas.winkler@intel.com>
 <20161002101755.GA25844@intel.com>
 <20161002102455.GA27464@intel.com>
 <20161002212126.GA25872@obsidianresearch.com>
 <5B8DA87D05A7694D9FA63FD143655C1B542F466B@hasmsx108.ger.corp.intel.com>
 <20161003124836.GE9990@intel.com>
 <20161004051946.GA10572@intel.com>
 <20161004164738.GA17149@obsidianresearch.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161004164738.GA17149@obsidianresearch.com>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2586
Lines: 58

On Tue, Oct 04, 2016 at 10:47:38AM -0600, Jason Gunthorpe wrote:
> On Tue, Oct 04, 2016 at 08:19:46AM +0300, Jarkko Sakkinen wrote:
> 
> > > Make the driver uncallable first. The worst race that can happen is that
> > > open("/dev/tpm0", ...) returns -EPIPE. I do not consider this fatal at
> > > all.
> > 
> > No responses for this reasonable proposal so I'll show what I mean:
> 
> How is this any better than what Thomas proposed? It seems much worse
> to me since now we have even more stuff in the wrong order.
> 
> There are three purposes to the ordering as it stands today
>  1) To guarantee that tpm2_shutdown is the last command delivered to
>     the TPM. When it is issued all other ways to access the device
>     are hard fenced off.
>  2) To hard fence the tpm subsystem for the 'platform' driver. Once
>     tpm_del_char_device completes no callback into the driver
>     is possible *at all*. The driver can destroy everything
>     (iounmap, dereg irq, etc) and the driver module can be unloaded.
>  3) To prevent oopsing with the sysfs code. Recall this comment
> 
>         /* The sysfs routines rely on an implicit tpm_try_get_ops, device_del
>          * is called before ops is null'd and the sysfs core synchronizes this
>          * removal so that no callbacks are running or can run again
>          */
> 
>     device_del is what eliminates the sysfs access path, so
>     ordering device_del after ops = null is just unconditionally
>     wrong.
> 
> I still haven't heard an explanation why Thomas's other patches need
> this, or why trying to change this ordering makes any sense at
> all considering how the subsystem is constructed.
> 
> Further, if tpm_crb now needs a registered device, how on earth do all
> the chip ops we call work *before* registration? Or is that another
> bug?
> 
> Why can't tpm_crb return to the pre-registration operating state
> in the driver remove function before calling unregister?
> 
> None of this makes any sense to me.
> 
> This whole thing was very carefully constructed to work *correctly*
> during unregister. Many other subsystems have races and bugs during
> remove (eg see the securityfs discussion). TPM has a hard requirement
> to support safe unregister due to the vtpm stuff, so we don't get to
> screw it up just to support one driver.

Obviously a device is needed because it's required by the PM runtime
FW. I'm not following what you're saying about tpm2_shutdown(). With
the change I proposed it's the *very last* command delivered to the
device (because it's fenced by write lock).

> Jason

/Jarkko