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

[Steven Rostedt]

On Thu, 10 Aug 2023 13:11:58 -0700
Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> > [...]
> > +		/*
> > +		 * 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.

A totally hardened machine is one that doesn't run ;-)

Yes, hopefully that when the kernel is configured with HARDENED it will
eliminate steps to a prerequisite attack. I'm sure enabling lockdep would
also help harden the system too. But there is a balance between security
and performance. The more that adding security harms performance, the less
people will use that security.

-- Steve