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

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Date:   Wed, 8 Mar 2017 13:52:36 -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 Wed, Mar 08, 2017 at 03:28:11PM -0500, Hon Ching(Vicky) Lo wrote:
> On Wed, 2017-03-08 at 10:17 -0700, Jason Gunthorpe wrote:
> > 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:
> 
> I don't think we need to worry about locking in this specific case. 
> tpm_ibmvtpm_get_desired_dma was designed to return a default value
> in the case when the chip is not available.

You have to worry about it to prevent a use after free race:

          CPU0                                CPU1
tpm_ibmvtpm_remove()                 tpm_ibmvtpm_get_desired_dma()

				     chip = dev_get_drvdata(dev);
dev_set_drvdata(&vdev->dev, NULL);
                                     if (chip)
                 		        ibmvtpm = dev_get_drvdata(&chip->dev);
kfree(ibmvtpm);
                                        // *ibmvtpm is now a use-after-free

Jason