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
