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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Aug 04, 2017 at 12:59:56PM -0700, Greg Kroah-Hartman wrote:
> On Tue, Aug 01, 2017 at 01:22:54AM +0300, Jarkko Sakkinen wrote:
> > On Fri, Jul 28, 2017 at 03:42:18PM -0700, Greg Kroah-Hartman wrote:
> > > On Wed, Jul 26, 2017 at 02:03:06PM -0600, Jason Gunthorpe wrote:
> > > > 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@obsidianresearch.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@linux.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.
> > > 
> > > Ok, so what do you want me to do here?
> > > 
> > > thanks,
> > > 
> > > greg k-h
> > 
> > Sorry for late response. I just came from four week leave (have been
> > watching kernel mails only 1-2 times a week and missed this thread).
> > 
> > I would actually think that taking this patch would make sense as the
> > changes are trivial and also because this code has reminded almost
> > unchanged after it was added.
> 
> Ok, now queued up, thanks.

But it's messy, if someone could verify that this is all working
properly on the 4.4-stable kernel with the next -rc release, I would
really appreciate it.

thanks,

greg k-h