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

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Message-ID: <CAKwvOd=dO2QjiRWegjCtnMmVguaJ2YHacJRP3SbVVy9jhx-BWw@mail.gmail.com>
Date:   Thu, 31 Oct 2019 10:34:53 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...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 10:27 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>
-- 
Thanks,
~Nick Desaulniers