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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Fri, 1 Nov 2019 09:28:21 -0700
Message-ID: <CABCJKueLtOJsq+k-ywyUCOU+QCqxjKN2bO76Te4U43g0Xp9g-A@mail.gmail.com>
Subject: Re: [PATCH v3 05/17] add support for Clang's Shadow Call Stack (SCS)
To:     Kees Cook <keescook@chromium.org>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
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On Thu, Oct 31, 2019 at 8:51 PM Kees Cook <keescook@chromium.org> wrote:
> > +/* A random number to mark the end of the shadow stack. */
> > +#define SCS_END_MAGIC        0xaf0194819b1635f6UL
>
> Is 0xaf.... non-canonical for arm64? While "random", it should also
> likely be an "impossible" value to find on the call stack.

Agreed, and yes, this is non-canonical for arm64 and AFAIK all 64-bit
architectures the kernel supports. I'll add a note about it.

Sami