linux-kernel - Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm

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Date:   Fri, 5 Feb 2021 11:58:08 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lino Sanfilippo <l.sanfilippo@...bus.com>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jarkko@...nel.org, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, stable@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

On Fri, Feb 05, 2021 at 04:50:13PM +0100, Lino Sanfilippo wrote:
> 
> On 05.02.21 16:15, Jason Gunthorpe wrote:
> > 
> > No, the cdev layer holds the refcount on the device while open is
> > being called.
> > 
> Yes, but the reference that is responsible for the chip deallocation is chip->dev
> which is linked to chip->cdev and represents /dev/tpm, not /dev/tpmrm.
> You are right, we dont have the issue with /dev/tpm for the reason you mentioned.
> But /dev/tpmrm is represented by chip->cdevs and keeping this ref held by the cdev
> layer wont protect us from the chip being freed (which is the reason why we need
> the chip->dev reference in the first place).

No, they are all chained together because they are all in the same
struct:

struct tpm_chip {
	struct device dev;
	struct device devs;
	struct cdev cdev;
	struct cdev cdevs;

dev holds the refcount on memory, when it goes 0 the whole thing is
kfreed.

The rule is dev's refcount can't go to zero while any other refcount
is != 0.

For instance devs holds a get on dev that is put back only when devs
goes to 0:

static void tpm_devs_release(struct device *dev)
{
	struct tpm_chip *chip = container_of(dev, struct tpm_chip, devs);

	/* release the master device reference */
	put_device(&chip->dev);
}

Both cdev elements do something similar inside the cdev layer.

The net result is during any open() the tpm_chip is guarenteed to have
a positive refcount.

Jason