Re: [PATCH v8 01/12] add support for Clang's Shadow Call Stack (SCS)

Randy Dunlap
2020-02-19T04:20:23Z

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