linux-kernel - Re: [PATCH] tpm: don't destroy chip device prematurely

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Message-ID: <20161004051946.GA10572@intel.com>
Date:   Tue, 4 Oct 2016 08:19:46 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     "Winkler, Tomas" <tomas.winkler@...el.com>
Cc:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        "tpmdd-devel@...ts.sourceforge.net" 
        <tpmdd-devel@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: don't destroy chip device prematurely

On Mon, Oct 03, 2016 at 03:48:36PM +0300, Jarkko Sakkinen wrote:
> On Mon, Oct 03, 2016 at 07:05:48AM +0000, Winkler, Tomas wrote:
> > 
> > > On Sun, Oct 02, 2016 at 01:24:55PM +0300, Jarkko Sakkinen wrote:
> > > > On Sun, Oct 02, 2016 at 01:17:55PM +0300, Jarkko Sakkinen wrote:
> > > > > On Sun, Oct 02, 2016 at 10:39:31AM +0300, Tomas Winkler wrote:
> > > > > > In tpm_del_char_device device_del is called prior to tpm2_shutdown
> > > > > > where it is still used.
> > > > > >
> > > > > > Fortunately, so far chip->dev was used only for printouts int
> > > > > > tpm2_shutdown flow, hence system didn't crash. But with the
> > > > > > introduction of runtime power management it will result in
> > > > > > shutting down the parent device while it still in use.
> > > > > >
> > > > > > Fixes: 20e0152393b41 ("tpm: fix crash in tpm_tis
> > > > > > deinitialization")
> > > > > > Signed-off-by: Tomas Winkler <tomas.winkler@...el.com>
> > > > >
> > > > > Tested-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> > > > > Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> > > >
> > > > Applied.
> > > 
> > > This patch is wrong, I though the comments were clear. All entry points to find
> > > the device must be deleted before we commit to shutting down the device.
> > > 
> > > You need to figure out some other way to solve your problem.
> > 
> > Please  be more specific regarding flows you think will be wrong  with
> > this patch, you must agree that the current code is broken even w/o
> > runtime pm.
> 
> 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:

/* Make the driver uncallable. */
down_write(&chip->ops_sem);
if (chip->flags & TPM_CHIP_FLAG_TPM2)
	tpm2_shutdown(chip, TPM2_SU_CLEAR);
chip->ops = NULL;
up_write(&chip->ops_sem);

cdev_del(&chip->cdev);
device_del(&chip->dev);

/* Make the chip unavailable. */
mutex_lock(&idr_lock);
idr_replace(&dev_nums_idr, NULL, chip->dev_num);
mutex_unlock(&idr_lock);

The worst thing that can happen is -EPIPE.

/Jarkko