Messages in this thread | | | Subject | Re: [PATCH 1/1] s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks | From | Tony Krowiak <> | Date | Wed, 10 Feb 2021 15:34:24 -0500 |
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On 2/10/21 5:53 AM, Cornelia Huck wrote: > On Tue, 9 Feb 2021 14:48:30 -0500 > Tony Krowiak <akrowiak@linux.ibm.com> wrote: > >> This patch fixes a circular locking dependency in the CI introduced by >> commit f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM >> pointer invalidated"). The lockdep only occurs when starting a Secure >> Execution guest. Crypto virtualization (vfio_ap) is not yet supported for >> SE guests; however, in order to avoid CI errors, this fix is being >> provided. >> >> The circular lockdep was introduced when the masks in the guest's APCB >> were taken under the matrix_dev->lock. While the lock is definitely >> needed to protect the setting/unsetting of the KVM pointer, it is not >> necessarily critical for setting the masks, so this will not be done under >> protection of the matrix_dev->lock. >> >> Fixes: f21916ec4826 ("s390/vfio-ap: clean up vfio_ap resources when KVM pointer invalidated") >> Cc: stable@vger.kernel.org >> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com> >> --- >> drivers/s390/crypto/vfio_ap_ops.c | 75 ++++++++++++++++++------------- >> 1 file changed, 45 insertions(+), 30 deletions(-) >> >> static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev) >> { >> - kvm_arch_crypto_clear_masks(matrix_mdev->kvm); >> - matrix_mdev->kvm->arch.crypto.pqap_hook = NULL; >> - vfio_ap_mdev_reset_queues(matrix_mdev->mdev); >> - kvm_put_kvm(matrix_mdev->kvm); >> - matrix_mdev->kvm = NULL; >> + if (matrix_mdev->kvm) { > If you're doing setting/unsetting under matrix_dev->lock, is it > possible that matrix_mdev->kvm gets unset between here and the next > line, as you don't hold the lock?
That is highly unlikely because the only place the matrix_mdev->kvm pointer is cleared is in this function which is called from only two places: the notifier that handles the VFIO_GROUP_NOTIFY_SET_KVM notification when the KVM pointer is cleared; the vfio_ap_mdev_release() function which is called when the mdev fd is closed (i.e., when the guest is shut down). The fact is, with the only end-to-end implementation currently available, the notifier callback is never invoked to clear the KVM pointer because the vfio_ap_mdev_release callback is invoked first and it unregisters the notifier callback.
Having said that, I suppose there is no guarantee that there will not be different userspace clients in the future that do things in a different order. At the very least, it wouldn't hurt to protect against that as you suggest below.
> > Maybe you could > - grab a reference to kvm while holding the lock > - call the mask handling functions with that kvm reference > - lock again, drop the reference, and do the rest of the processing? > >> + kvm_arch_crypto_clear_masks(matrix_mdev->kvm); >> + mutex_lock(&matrix_dev->lock); >> + matrix_mdev->kvm->arch.crypto.pqap_hook = NULL; >> + vfio_ap_mdev_reset_queues(matrix_mdev->mdev); >> + kvm_put_kvm(matrix_mdev->kvm); >> + matrix_mdev->kvm = NULL; >> + mutex_unlock(&matrix_dev->lock); >> + } >> }
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