linux-kernel - Re: [PATCH v12 01/12] add support for Clang's Shadow Call Stack (SCS)

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Message-ID: <20200424101230.GB21141@willie-the-truck>
Date:   Fri, 24 Apr 2020 11:12:31 +0100
From:   Will Deacon <will@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 01/12] add support for Clang's Shadow Call Stack (SCS)

On Thu, Apr 23, 2020 at 11:09:24AM -0700, Kees Cook wrote:
> On Wed, Apr 22, 2020 at 07:00:40PM +0100, Will Deacon wrote:
> > On Wed, Apr 22, 2020 at 10:54:45AM -0700, Kees Cook wrote:
> > > On Mon, Apr 20, 2020 at 07:14:42PM -0700, Sami Tolvanen wrote:
> > > > +void scs_release(struct task_struct *tsk)
> > > > +{
> > > > +	void *s;
> > > > +
> > > > +	s = __scs_base(tsk);
> > > > +	if (!s)
> > > > +		return;
> > > > +
> > > > +	WARN_ON(scs_corrupted(tsk));
> > > > +
> > > 
> > > I'd like to have task_set_scs(tsk, NULL) retained here, to avoid need to
> > > depend on the released task memory getting scrubbed at a later time.
> > 
> > Hmm, doesn't it get zeroed almost immediately by kmem_cache_free() if
> > INIT_ON_FREE_DEFAULT_ON is set? That seems much better than special-casing
> > SCS, as there's a tonne of other useful stuff kicking around in the
> > task_struct and treating this specially feels odd to me.
> 
> That's going to be an uncommon config except for the most paranoid of
> system builders. :)

Sounds like a perfect fit, then ;)

> Having this get wiped particular thing wiped is just
> a decent best practice for what is otherwise treated as a "secret", just
> like crypto routines wipe their secrets before free().

Sorry, but I don't buy that analogy. The SCS pointer is stored in memory
all over the place and if it needs to treated in the same way as crypto
secrets then this whole thing needs rethinking. On top of that, where
crypto routines may wipe their secrets, we don't do what is being proposed
for the SCS pointer to other similar pieces of data, such as pointer
authentication keys.

Will