linux-kernel - Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

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Date:   Fri, 18 Oct 2019 19:42:06 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel@...ts.infradead.org,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)

On Fri, Oct 18, 2019 at 6:14 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> This change adds generic support for Clang's Shadow Call Stack, which
> uses a shadow stack to protect return addresses from being overwritten
> by an attacker. Details are available here:
>
>   https://clang.llvm.org/docs/ShadowCallStack.html

(As I mentioned in the other thread, the security documentation there
doesn't fit the kernel usecase.)

[...]
> +config SHADOW_CALL_STACK_VMAP
> +       def_bool n
> +       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.

Without CONFIG_SHADOW_CALL_STACK_VMAP, after 128 small stack frames,
you overflow into random physmap memory even if the main stack is
vmapped... I guess you can't get around that without making the SCS
instrumentation more verbose. :/

Could you maybe change things so that independent of whether you have
vmapped SCS or slab-allocated SCS, the scs_corrupted() check looks at
offset 1024-8 (where it currently is for the slab-allocated case)?
That way, code won't suddenly stop working when you disable
CONFIG_SHADOW_CALL_STACK_VMAP; and especially if you use
CONFIG_SHADOW_CALL_STACK_VMAP for development and testing but disable
it in production, that would be annoying.