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

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Message-ID: <CABCJKuf7KsgkzqHGMAVjyS5Zo_ix8a2d6vsRT6OGSUgVLuJ70g@mail.gmail.com>
Date:   Fri, 18 Oct 2019 10:56:04 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Jann Horn <jannh@...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 <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 10:42 AM Jann Horn <jannh@...gle.com> wrote:
> (As I mentioned in the other thread, the security documentation there
> doesn't fit the kernel usecase.)

True. I'll add a note about it here too.

> 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. :/

That's correct. In our testing, 128 stack frames is nearly twice the
maximum amount that's been used (on an arm64 device), and for many use
cases, allocating a full page is simply too costly despite the
advantages.

> 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.

Yes, that's a great idea. I'll change this in v2.

Sami