Re: [tpmdd-devel] [PATCH] vTPM: Fix missing NULL check

From: Jason Gunthorpe
Date: Wed Mar 08 2017 - 13:09:38 EST

On Tue, Mar 07, 2017 at 11:12:43PM -0500, Hon Ching(Vicky) Lo wrote:
> On Mon, 2017-03-06 at 16:19 -0700, Jason Gunthorpe wrote:

> > Also, how does locking work here? Does the vio core prevent
> > tpm_ibmvtpm_get_desired_dma and tpm_ibmvtpm_remove from running
> > concurrently?
> No, vio core doesn't prevent tpm_ibmvtpm_get_desired_dma and tpm_ibmvtpm_remove
> from running concurrently.
> vio_bus_probe calls vio_cmo_bus_probe which calls tpm_ibmvtpm_get_desired_dma.
> tpm_ibmvtpm_get_desired_dma is called before the code enters critical section.
> There is no locking mechanism around tpm_ibmvtpm_remove in vio_bus_remove.
> What's the concern here?

tpm_ibmvtpm_remove makes the pointer that tpm_ibmvtpm_get_desired_dma
is accessing invalid, so some kind of locking is technically required
so that the two things do not create a use after free race:

> > + /* For tpm_ibmvtpm_get_desired_dma */
> > + dev_set_drvdata(&vdev->dev, NULL);
> > kfree(ibmvtpm);

Eg with the kfree above.

It may be that the driver core prevents probe/remove from running
concurrently and things are fine, but this is something to confirm..