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

[Tian, Kevin]

> From: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> Sent: Saturday, July 15, 2023 6:46 AM
> 
> 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>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>