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

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Message-ID: <CABCJKudb2_OH5CRFm64rxv-VVnuOrO-ZOrXRHg8hR98Vj+BzVw@mail.gmail.com>
Date:   Thu, 31 Oct 2019 10:27:26 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     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>,
        Kees Cook <keescook@...omium.org>,
        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 10:18 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.

Sami