Re: [PATCH v3 3/3] list_debug: Introduce CONFIG_DEBUG_LIST_MINIMAL

[Marco Elver]

On Thu, 10 Aug 2023 at 22:12, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Wed, Aug 09, 2023 at 06:32:37PM +0200, Marco Elver wrote:
> > On Wed, Aug 09, 2023 at 11:30AM -0400, Steven Rostedt wrote:
> > [...]
> > >
> > > I would actually prefer DEBUG_LIST to select HARDEN_LIST and not the other
> > > way around. It logically doesn't make sense that HARDEN_LIST would select
> > > DEBUG_LIST. That is, I could by default want HARDEN_LIST always on, but not
> > > DEBUG_LIST (because who knows, it may add other features I don't want). But
> > > then, I may have stumbled over something and want more info, and enable
> > > DEBUG_LIST (while still having HARDEN_LIST) enabled.
> > >
> > > I think you are looking at this from an implementation perspective and not
> > > the normal developer one.
> > >
> > [...]
> > >
> > > That is, if DEBUG_LIST is enabled, we always call the
> > > __list_add_valid_or_report(), but if only HARDEN_LIST is enabled, then we
> > > do the shortcut.
> >
> > Good point - I think this is better. See below tentative v4.
> >
> > Kees: Does that also look more like what you had in mind?
>
> Yeah, this looks good. My only nit would be a naming one. All the
> other hardening features are named "HARDENED", but perhaps the "ED"
> is redundant in the others. Still, consistency seems nicer. What do you
> think of CONFIG_LIST_HARDENED ? (The modern trend for Kconfig naming tends
> to keep the subsystem name first and then apply optional elements after.)

Naming is a bit all over. :-/
I agree with the <subsystem>_<suboption> scheme, generally. I think
initially I tried to keep the name shorter, and also find a good
counter-part to DEBUG_<suboption>, therefore HARDEN_LIST.

Let's just change it to CONFIG_LIST_HARDENED, given the existing
"HARDENED" options.

I don't have a strong preference.

> One note: do the LKDTM list hardening tests still pass? i.e.
> CORRUPT_LIST_ADD
> CORRUPT_LIST_DEL

Yes, they do. Though I need to also adjust BUG_ON_DATA_CORRUPTION to
select LIST_HARDENED, and the test should check for the new option
(which is implied by DEBUG_LIST now). There will be an additional
patch to adjust that.

> > [...]
> > +             /*
> > +              * With the hardening version, elide checking if next and prev
> > +              * are NULL, LIST_POISON1 or LIST_POISON2, since the immediate
> > +              * dereference of them below would result in a fault.
> > +              */
> > +             if (likely(prev->next == entry && next->prev == entry))
> > +                     return true;
>
> I'm not super excited about skipping those checks, since they are
> values that can be reached through kernel list management confusion. If
> an attacker is using a system where the zero-page has been mapped
> and is accessible (i.e. lacking SMAP etc), then attacks could still
> be constructed. However, I do recognize this chain of exploitation
> prerequisites is getting rather long, so probably this is a reasonable
> trade off on modern systems.

Sure, it's a trade-off for systems which do have the bare minimum of
modern hardware security features.