linux-kernel - Re: [PATCH v3 13/17] arm64: preserve x18 when CPU is suspended

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Message-ID: <201910312059.59F983B@keescook>
Date:   Thu, 31 Oct 2019 20:59:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 13/17] arm64: preserve x18 when CPU is suspended

On Thu, Oct 31, 2019 at 10:34:53AM -0700, Nick Desaulniers wrote:
> On Thu, Oct 31, 2019 at 10:27 AM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> >
> > On Thu, Oct 31, 2019 at 10:18 AM Nick Desaulniers
> > <ndesaulniers@...gle.com> wrote:
> > > > +#ifdef CONFIG_SHADOW_CALL_STACK
> > > > +       ldr     x18, [x0, #96]
> > > > +       str     xzr, [x0, #96]
> > >
> > > How come we zero out x0+#96, but not for other offsets? Is this str necessary?
> >
> > It clears the shadow stack pointer from the sleep state buffer, which
> > is not strictly speaking necessary, but leaves one fewer place to find
> > it.
> 
> That sounds like a good idea.  Consider adding comments or to the
> commit message so that the str doesn't get removed accidentally in the
> future.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Yeah, with the comment added:

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook