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

From: Hon Ching(Vicky) Lo
Date: Tue Mar 14 2017 - 18:42:34 EST


On Wed, 2017-03-08 at 13:52 -0700, Jason Gunthorpe wrote:
> 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
>
I have dug further up along the call stack of
tpm_ibmvtpm_get_desired_dma() and found that there is a locking
mechanism in place at the bus probe level. 'probe' and 'remove'
callbacks are both surrounded by mutex_lock and mutex_unlock on the
device. The code is in the really_probe() and
device_release_driver_internal() accordingly.

Thanks for pointing this out!

Vicky