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