On Fri, 14 Jul 2023 15:45:32 -0700
Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> wrote:
> 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>
> ---
Applied series to vfio next branch for v6.6. There's a minor rebase
involved, so please double check the results:
https://github.com/awilliam/linux-vfio/commits/next
Thanks,
Alex
>
> 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;
