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Subject: Re: [PATCH v8 01/12] add support for Clang's Shadow Call Stack (SCS)
To:     Sami Tolvanen <samitolvanen@google.com>,
        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@arm.com
Cc:     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@googlegroups.com,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20191018161033.261971-1-samitolvanen@google.com>
 <20200219000817.195049-1-samitolvanen@google.com>
 <20200219000817.195049-2-samitolvanen@google.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <60ec3a49-7b71-df31-f231-b48ff135b718@infradead.org>
Date:   Tue, 18 Feb 2020 20:19:56 -0800
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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

> +	  be able to locate them and hijack control flow by modifying shadow
> +	  stacks that are not currently in use.
> +
> +config SHADOW_CALL_STACK_VMAP
> +	bool "Use virtually mapped shadow call stacks"
> +	depends on SHADOW_CALL_STACK
> +	help
> +	  Use virtually mapped shadow call stacks. Selecting this option
> +	  provides better stack exhaustion protection, but increases per-thread
> +	  memory consumption as a full page is allocated for each shadow stack.
> +
> +
>  config HAVE_ARCH_WITHIN_STACK_FRAMES
>  	bool
>  	help


thanks.
-- 
~Randy