[PATCH v3 1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add()

[Dmitry Torokhov]

kvm_vfio_group_add() creates kvg instance, links it to kv->group_list,
and calls kvm_vfio_file_set_kvm() with kvg->file as an argument after
dropping kv->lock. If we race group addition and deletion calls, kvg
instance may get freed by the time we get around to calling
kvm_vfio_file_set_kvm().

Previous iterations of the code did not reference kvg->file outside of
the critical section, but used a temporary variable. Still, they had
similar problem of the file reference being owned by kvg structure and
potential for kvm_vfio_group_del() dropping it before
kvm_vfio_group_add() had a chance to complete.

Fix this by moving call to kvm_vfio_file_set_kvm() under the protection
of kv->lock. We already call it while holding the same lock when vfio
group is being deleted, so it should be safe here as well.

Fixes: 2fc1bec15883 ("kvm: set/clear kvm to/from vfio_group when group add/delete")
Reviewed-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
---

v3: added Alex's reviewed-by

v2: updated commit description with the correct "Fixes" tag (per Alex),
    expanded commit description to mention issues with the earlier
    implementation of kvm_vfio_group_add().

 virt/kvm/vfio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index 9584eb57e0ed..cd46d7ef98d6 100644
--- a/virt/kvm/vfio.c
+++ b/virt/kvm/vfio.c
@@ -179,10 +179,10 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	list_add_tail(&kvg->node, &kv->group_list);
 
 	kvm_arch_start_assignment(dev->kvm);
+	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
 
 	mutex_unlock(&kv->lock);
 
-	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
 	kvm_vfio_update_coherency(dev);
 
 	return 0;
-- 
2.41.0.255.g8b1d071c50-goog