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Date: Tue, 26 Jun 2018 17:21:47 -0700
From: Kees Cook <keescook@...omium.org>
To: Laura Abbott <labbott@...hat.com>
Cc: Alexander Popov <alex.popov@...ux.com>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>, 
	Brad Spengler <spender@...ecurity.net>, Ingo Molnar <mingo@...nel.org>, 
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	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Borislav Petkov <bp@...en8.de>, 
	Richard Sandiford <richard.sandiford@....com>, Thomas Gleixner <tglx@...utronix.de>, 
	"H . Peter Anvin" <hpa@...or.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, 
	"Dmitry V . Levin" <ldv@...linux.org>, Emese Revfy <re.emese@...il.com>, Jonathan Corbet <corbet@....net>, 
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	Vikas Shivappa <vikas.shivappa@...ux.intel.com>, Kyle Huey <me@...ehuey.com>, 
	Dmitry Safonov <dsafonov@...tuozzo.com>, Will Deacon <will.deacon@....com>, 
	Arnd Bergmann <arnd@...db.de>, Florian Weimer <fweimer@...hat.com>, 
	Boris Lukashev <blukashev@...pervictus.com>, Andrey Konovalov <andreyknvl@...gle.com>, 
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v13 (resend) 2/6] x86/entry: Add STACKLEAK erasing the
 kernel stack at the end of syscalls

On Tue, Jun 26, 2018 at 5:13 PM, Kees Cook <keescook@...omium.org> wrote:
> The only arch-specific part is current_top_of_stack(), so it seems
> like just solving current_top_of_stack() and copying on_thread_stack()
> directly should work, yes? I would think sticking those directly into
> asm/processor.h on arm64 would be okay?

The difference in the earlier x86 and arm64 erase_kstack was:

x86:
        if (on_thread_stack())
                boundary = current_stack_pointer;
        else
                boundary = current_top_of_stack();

arm64:
        boundary = current_stack_pointer;

If a sane current_top_of_stack() isn't possible on arm64, I'm not
opposed to a small bit of arch-specific ifdef in there:

        boundary = get_stack_boundary();

static inline unsigned long get_stack_boundary(void)
{
#ifdef CONFIG_X86
        if (!on_thread_stack())
                return current_top_of_stack();
#else
         return current_stack_pointer;
}

-Kees

-- 
Kees Cook
Pixel Security

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