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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