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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Wed, 19 Feb 2020 09:25:43 -0800
Message-ID: <CABCJKudVbSMEXWTPw+bzzMuEf_kNsrfYiY53S5ZhWqGB9ynFEA@mail.gmail.com>
Subject: Re: [PATCH v8 01/12] add support for Clang's Shadow Call Stack (SCS)
To:     Randy Dunlap <rdunlap@infradead.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>,
        Mark Rutland <mark.rutland@arm.com>,
        James Morse <james.morse@arm.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        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 Tue, Feb 18, 2020 at 8:20 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> Hi Sami,
>
> a couple of minor tweaks:
>
> On 2/18/20 4:08 PM, Sami Tolvanen wrote:
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 98de654b79b3..66b34fd0df54 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -526,6 +526,40 @@ config STACKPROTECTOR_STRONG
> >         about 20% of all kernel functions, which increases the kernel code
> >         size by about 2%.
> >
> > +config ARCH_SUPPORTS_SHADOW_CALL_STACK
> > +     bool
> > +     help
> > +       An architecture should select this if it supports Clang's Shadow
> > +       Call Stack, has asm/scs.h, and implements runtime support for shadow
> > +       stack switching.
> > +
> > +config SHADOW_CALL_STACK
> > +     bool "Clang Shadow Call Stack"
> > +     depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
> > +     help
> > +       This option enables Clang's Shadow Call Stack, which uses a
> > +       shadow stack to protect function return addresses from being
> > +       overwritten by an attacker. More information can be found from
>
>                                                               found in
>
> > +       Clang's documentation:
> > +
> > +         https://clang.llvm.org/docs/ShadowCallStack.html
> > +
> > +       Note that security guarantees in the kernel differ from the ones
> > +       documented for user space. The kernel must store addresses of shadow
> > +       stacks used by other tasks and interrupt handlers in memory, which
> > +       means an attacker capable reading and writing arbitrary memory may
>
>                             capable of

Thanks, Randy! I'll fix these in the next version.

Sami