Re: [PATCH] ACPICA: avoid double free when object already has a zero reference count

["Rafael J. Wysocki" <rafael@kernel.org>]

+Erik and Bob

On Thu, Oct 29, 2020 at 3:05 AM Mark Asselstine
<mark.asselstine@windriver.com> wrote:
>
> The first trip into acpi_ut_update_ref_count() for an object where
> 'object->common.reference_count' is 1 and we are performing a
> REF_DECREMENT will result in 'new_count' being 0 and thus the object
> is deleted via acpi_ut_delete_internal_obj().
>
> If for some reason we make a subsequent trip into
> acpi_ut_update_ref_count() with the same object,
> object->common.reference_count' will be 0 and performing a
> REF_DECREMENT will produce a warning msg "Reference Count is already
> zero, cannot decrement", 'new_count' will again be 0 and the already
> deleted object will be attempted to be deleted again via
> acpi_ut_delete_internal_obj().
>
> Since the object deletion doesn't NULL the object the calls to
> acpi_ut_delete_internal_obj(), acpi_ut_delete_object_desc(),
> acpi_os_release_object(), kmem_cache_free() will operate on the object
> as if it hasn't been deleted. In many cases this can result in no
> issues, but if you are using the slab and a new object has been
> created with the same address this can be the cause slab corruption.
>
> Adding a check if we are decrementing to 0 for the first time and only
> calling acpi_ut_delete_internal_obj() in this case will prevent
> another attempt at deleting the object.
>
> Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
> ---
>  drivers/acpi/acpica/utdelete.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/acpi/acpica/utdelete.c b/drivers/acpi/acpica/utdelete.c
> index 4c0d4e434196..c6b860fd9eb5 100644
> --- a/drivers/acpi/acpica/utdelete.c
> +++ b/drivers/acpi/acpica/utdelete.c
> @@ -421,9 +421,9 @@ acpi_ut_update_ref_count(union acpi_operand_object *object, u32 action)
>                                       ACPI_GET_FUNCTION_NAME, object,
>                                       object->common.type, new_count));
>
> -               /* Actually delete the object on a reference count of zero */
> +               /* If we haven't already, actually delete the object on a reference count of zero */
>
> -               if (new_count == 0) {
> +               if (new_count == 0 && original_count != 0) {
>                         acpi_ut_delete_internal_obj(object);
>                 }
>                 message = "Decrement";
> --
> 2.17.1
>