linux-kernel - Re: [PATCH v3 3/3] list_debug: Introduce CONFIG_DEBUG_LIST

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Date:   Fri, 11 Aug 2023 15:33:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev@...glegroups.com, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH v3 3/3] list_debug: Introduce CONFIG_DEBUG_LIST_MINIMAL

On Thu, 10 Aug 2023 13:11:58 -0700
Kees Cook <keescook@...omium.org> 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