linux-kernel - Re: [PATCH v4 3/5] tee: generic TEE subsystem

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Date:	Wed, 8 Jul 2015 16:53:21 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Jens Wiklander <jens.wiklander@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Rob Herring <robh+dt@...nel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	valentin.manea@...wei.com, jean-michel.delorme@...com,
	emmanuel.michel@...com, javier@...igon.com,
	Mark Rutland <mark.rutland@....com>,
	Michal Simek <michal.simek@...inx.com>
Subject: Re: [PATCH v4 3/5] tee: generic TEE subsystem

On Wed, Jul 08, 2015 at 05:16:12PM -0600, Jason Gunthorpe wrote:
> On Wed, Jul 08, 2015 at 03:33:25PM -0700, Greg Kroah-Hartman wrote:
> > > The basic issue is that cdev_del doesn't seem to be synchronizing.
> > > 
> > > The use after free race is then something like:
> > > 
> > >    struct tpm_chip {
> > >  	struct device dev;
> > > 	struct cdev cdev;
> > 
> > Oops, right there's your problem.  You can't have two reference counted
> > objects trying to manage the memory of a single structure.  No matter
> > what you do, it's going to be a pain to deal with this, so don't :)
> 
> Sure, generally, yes, but that isn't done for no reason, it is to make
> open straightforward:
> 
> static int tpm_open(struct inode *inode, struct file *file)
> {
> 	struct tpm_chip *chip =
> 		container_of(inode->i_cdev, struct tpm_chip, cdev);
> 
> We need to recover the tpm_chip associated with the char device
> node, in a way that is holding a kref on it, without racing with
> cdev_del/etc
> 
> This scheme does mean that if we have a struct file we have a kref on
> the cdev, and if we have cdev then we have a kref on the tpm_chip,
> which is really easy to use properly.
> 
> > > Ie we need cdev to hold a ref on tpm_chip->dev until cdev_put is
> > > called.
> > 
> > No, separate them, make the cdev a pointer and all should be fine.
> 
> Okay, cdev_alloc takes care of the cdev lifetime.
> 
> Do you have a simple solution to replace container_of as well?
> 
> What would you think about something like:
> 
>  cdev_alloc(&chip->dev.kref)

Just pick either the cdev to handle the lifetime rules, or the struct
device, you'll still need a container_of().  Just don't do both as odds
are the lifetime rules is going to get really hard to debug and ensure
that everything is correct on the shutdown/release path.

thanks,

greg k-h
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