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

[Jason Gunthorpe <jgunthorpe@obsidianresearch.com>]

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