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

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Date:   Thu, 23 Apr 2020 11:09:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.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 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. :) 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().

-- 
Kees Cook