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:

quoted

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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.

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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