Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

[Lino Sanfilippo]

On 05.02.21 at 16:58, Jason Gunthorpe wrote:
eference 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.

Well this chaining is exactly what does not work nowadays and what the patch is supposed
to fix: currently we dont ever take the extra ref (not even in TPM 2 case, note that
TPM_CHIP_FLAG_TMP2 is never set), so

-	if (chip->flags & TPM_CHIP_FLAG_TPM2)
-		get_device(&chip->dev);
+	get_device(&chip->dev);


and tpm_devs_release() is never called, since there is nothing that ever puts devs, so


+	rc = devm_add_action_or_reset(pdev,
+				      (void (*)(void *)) put_device,
+				      &chip->devs);


The race with only get_device()/putdevice() in tpm_common_open()/tpm_common_release() is:

1. tpm chip is allocated with dev refcount = 1, devs refcount = 1
2. /dev/tpmrm is opened but before we get the ref to dev in tpm_common() another thread
rmmmods the chip driver:
3. the chip is unregistered, dev is put with refcount = 0 and the whole chip struct is freed
3. Now open() proceeds, tries to grab the extra ref chip->dev from a chip that has already
been deallocated and the system crashes.

As I already wrote, that approach was my first thought, too, but since the result crashed due to the
race condition, I chose the approach in patch 1.

Regards,
Lino

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