Currently it is possible that the final put of a KVM reference comes from
vfio during its device close operation. This occurs while the vfio group
lock is held; however, if the vfio device is still in the kvm device list,
then the following call chain could result in a deadlock:
kvm_put_kvm
-> kvm_destroy_vm
-> kvm_destroy_devices
-> kvm_vfio_destroy
-> kvm_vfio_file_set_kvm
-> vfio_file_set_kvm
-> group->group_lock/group_rwsem
Avoid this scenario by adding kvm_put_kvm_async which will perform the
kvm_destroy_vm asynchronously if the refcount reaches 0.
Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/gvt/kvmgt.c | 6 +++++-
drivers/s390/crypto/vfio_ap_ops.c | 7 ++++++-
include/linux/kvm_host.h | 3 +++
virt/kvm/kvm_main.c | 22 ++++++++++++++++++++++
4 files changed, 36 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index 8ae7039b3683..24511c877572 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -703,7 +703,11 @@ static void intel_vgpu_close_device(struct vfio_device *vfio_dev)
kvm_page_track_unregister_notifier(vgpu->vfio_device.kvm,
&vgpu->track_node);
- kvm_put_kvm(vgpu->vfio_device.kvm);
+ /*
+ * Avoid possible deadlock on any currently-held vfio lock by
+ * ensuring the potential kvm_destroy_vm call is done asynchronously
+ */
+ kvm_put_kvm_async(vgpu->vfio_device.kvm);
kvmgt_protect_table_destroy(vgpu);
gvt_cache_destroy(vgpu);
diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
index e93bb9c468ce..a37b2baefb36 100644
--- a/drivers/s390/crypto/vfio_ap_ops.c
+++ b/drivers/s390/crypto/vfio_ap_ops.c
@@ -1574,7 +1574,12 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
kvm_arch_crypto_clear_masks(kvm);
vfio_ap_mdev_reset_queues(&matrix_mdev->qtable);
- kvm_put_kvm(kvm);
+ /*
+ * Avoid possible deadlock on any currently-held vfio lock by
+ * ensuring the potential kvm_destroy_vm call is done
+ * asynchronously
+ */
+ kvm_put_kvm_async(kvm);
matrix_mdev->kvm = NULL;
release_update_locks_for_kvm(kvm);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4f26b244f6d0..2ef6a5102265 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -34,6 +34,7 @@
#include <linux/instrumentation.h>
#include <linux/interval_tree.h>
#include <linux/rbtree.h>
+#include <linux/workqueue.h>
#include <linux/xarray.h>
#include <asm/signal.h>
@@ -793,6 +794,7 @@ struct kvm {
struct kvm_stat_data **debugfs_stat_data;
struct srcu_struct srcu;
struct srcu_struct irq_srcu;
+ struct work_struct async_work;
pid_t userspace_pid;
bool override_halt_poll_ns;
unsigned int max_halt_poll_ns;
@@ -963,6 +965,7 @@ void kvm_exit(void);
void kvm_get_kvm(struct kvm *kvm);
bool kvm_get_kvm_safe(struct kvm *kvm);
void kvm_put_kvm(struct kvm *kvm);
+void kvm_put_kvm_async(struct kvm *kvm);
bool file_is_kvm(struct file *file);
void kvm_put_kvm_no_destroy(struct kvm *kvm);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 13e88297f999..fbe8d127028b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1353,6 +1353,28 @@ void kvm_put_kvm(struct kvm *kvm)
}
EXPORT_SYMBOL_GPL(kvm_put_kvm);
+static void kvm_put_async_fn(struct work_struct *work)
+{
+ struct kvm *kvm = container_of(work, struct kvm,
+ async_work);
+
+ kvm_destroy_vm(kvm);
+}
+
+/*
+ * Put a reference but only destroy the vm asynchronously. Can be used in
+ * cases where the caller holds a mutex that could cause deadlock if
+ * kvm_destroy_vm is triggered
+ */
+void kvm_put_kvm_async(struct kvm *kvm)
+{
+ if (refcount_dec_and_test(&kvm->users_count)) {
+ INIT_WORK(&kvm->async_work, kvm_put_async_fn);
+ schedule_work(&kvm->async_work);
+ }
+}
+EXPORT_SYMBOL_GPL(kvm_put_kvm_async);
+
/*
* Used to put a reference that was taken on behalf of an object associated
* with a user-visible file descriptor, e.g. a vcpu or device, if installation
--
2.39.0
