> -----Original Message-----
> From: Rafael J. Wysocki <rafael@xxxxxxxxxx>
> Sent: Thursday, October 29, 2020 7:06 AM
> To: Mark Asselstine <mark.asselstine@xxxxxxxxxxxxx>
> Cc: ACPI Devel Maling List <linux-acpi@xxxxxxxxxxxxxxx>; Kaneda, Erik
> <erik.kaneda@xxxxxxxxx>; Moore, Robert <robert.moore@xxxxxxxxx>
> Subject: Re: [PATCH] ACPICA: avoid double free when object already has a
> zero reference count
>
> +Erik and Bob
>
> On Thu, Oct 29, 2020 at 3:05 AM Mark Asselstine
> <mark.asselstine@xxxxxxxxxxxxx> 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().
Mark, Do you have an example of AML/ASL that you used to determine this double free?
Thanks,
Erik
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> > 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
> >
