linux-kernel - Re: [PATCH 4.4 40/57] tpm: Provide strong locking for device removal

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Date:   Wed, 26 Jul 2017 14:03:06 -0600
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Ben Hutchings <ben.hutchings@...ethink.co.uk>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Subject: Re: [PATCH 4.4 40/57] tpm: Provide strong locking for device removal

On Wed, Jul 26, 2017 at 12:56:37PM -0700, Greg Kroah-Hartman wrote:
> On Tue, Jul 25, 2017 at 11:56:01PM +0100, Ben Hutchings wrote:
> > On Wed, 2017-07-19 at 13:12 +0200, Greg Kroah-Hartman wrote:
> > > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > 
> > > From: Jason Gunthorpe <jgunthorpe@...idianresearch.com>
> > > 
> > > commit 4e26195f240d73150e8308ae42874702e3df8d2c upstream.
> > > 
> > > Add a read/write semaphore around the ops function pointers so
> > > ops can be set to null when the driver un-registers.
> > [...]
> > > @@ -49,10 +99,10 @@ struct tpm_chip *tpm_chip_find_get(int c
> > >  		if (chip_num != TPM_ANY_NUM && chip_num != pos->dev_num)
> > >  			continue;
> > >  
> > > -		if (try_module_get(pos->dev.parent->driver->owner)) {
> > > +		/* rcu prevents chip from being free'd */
> > > +		if (!tpm_try_get_ops(pos))
> > [...]
> > 
> > But an RCU read-side critical section is an atomic context, and
> > semaphore operations can block!  Fixed upstream by:
> > 
> > commit 15516788e581eb32ec1c50e5f00aba3faf95d817
> > Author: Stefan Berger <stefanb@...ux.vnet.ibm.com>
> > Date:   Mon Feb 29 08:53:02 2016 -0500
> > 
> >     tpm: Replace device number bitmap with IDR
> 
> Ugh, that's a big patch.
> 
> Jason, Stefan, and Jarkko, what do you think?  Should I also take this
> for 4.4-stable?

15516 is part of the series that included 4e26, I wouldn't take that
series piecemeal, as Ben observes..

I think it would be safer to avoid all these backport patches and
instead restructure the important TPM shutdown patch so that it is
'less safe'. This would mean there is a chance that the another TPM
user could send a command after shutdown, but realistically that is
not likely to happen.

Jason