linux-kernel - Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find

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Message-ID: <20160214065008.GC9551@obsidianresearch.com>
Date:	Sat, 13 Feb 2016 23:50:08 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:	tpmdd-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	Stefan Berger <stefanb@...ibm.com>,
	Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH 1/3] tpm: Hold the kref during tpm_chip_find_get

On Sun, Feb 14, 2016 at 06:55:12AM +0200, Jarkko Sakkinen wrote:
> On Fri, Feb 12, 2016 at 05:04:29PM -0700, Jason Gunthorpe wrote:
> > This was missed during the struct device conversion, we
> > need to hold a kref on the chip to make sure it isn't freed.
> > 
> > Signed-off-by: Jason Gunthorpe <jgunthorpe@...idianresearch.com>
> 
> I'm bit confused about this patch. What is the regression if this
> needs

The patch is simply totally broken, the placement of the get_device is
wrong:

> > @@ -53,6 +53,8 @@ struct tpm_chip *tpm_chip_find_get(int chip_num)
> >  			chip = pos;
> >  			break;
> >  		}
> > +
> > +		get_device(&chip->dev);

It needs to be moved up two lines before the break, into the if
statement.

As for the urgency - today the tpm core relies on module locking to
try and prevent tpm_chip_unregister from racing with stuff like the
above. That is totally broken in modern kernels, but it is what the
core tries to do. Within that framework the get/put are not needed
because of the module locking.

The only time these additional get/put do anything is when we are
racing with tpm_unregister, but if we are racing with unregister then
there are much bigger problems and things will crash anyhow.

So, this patch is just a tiny step.

The revised version of this patch with the rw_sem attempts to address
the complete race.

Jason