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

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Date:   Wed, 8 Mar 2017 10:17:29 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     "Hon Ching(Vicky) Lo" <honclo@...ux.vnet.ibm.com>
Cc:     tpmdd-devel@...ts.sourceforge.net,
        Ashley Lai <ashley@...leylai.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Peter Huewe <PeterHuewe@....de>
Subject: Re: [tpmdd-devel] [PATCH] vTPM: Fix missing NULL check

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..

Jason