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Date: Fri, 18 Oct 2019 10:56:04 -0700
From: Sami Tolvanen <>
To: Jann Horn <>
Cc: Will Deacon <>, Catalin Marinas <>, 
	Steven Rostedt <>, Ard Biesheuvel <>, 
	Dave Martin <>, Kees Cook <>, 
	Laura Abbott <>, Mark Rutland <>, 
	Nick Desaulniers <>, 
	clang-built-linux <>, 
	Kernel Hardening <>, 
	linux-arm-kernel <>, 
	kernel list <>
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 <> 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

> 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.


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