Date: Wed, 8 Jul 2015 17:16:12 -0600
From: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        devicetree@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Rob Herring <robh+dt@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>, valentin.manea@huawei.com,
        jean-michel.delorme@st.com, emmanuel.michel@st.com, javier@javigon.com,
        Mark Rutland <mark.rutland@arm.com>,
        Michal Simek <michal.simek@xilinx.com>
Subject: Re: [PATCH v4 3/5] tee: generic TEE subsystem
Message-ID: <20150708231612.GB20068@obsidianresearch.com>
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 <20150708171026.GA11740@obsidianresearch.com>
 <20150708211129.GA29824@kroah.com>
 <20150708222649.GA20068@obsidianresearch.com>
 <20150708223325.GA5843@kroah.com>
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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)

?

Jason
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