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Date: Tue, 26 Jun 2018 17:13:58 -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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	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>, 
	Andrey Ryabinin <aryabinin@...tuozzo.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Thomas Garnier <thgarnie@...gle.com>, 
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	Dave Hansen <dave.hansen@...ux.intel.com>, Mathias Krause <minipli@...glemail.com>, 
	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 Fri, Jun 22, 2018 at 11:58 AM, Laura Abbott <labbott@...hat.com> wrote:
> On 06/22/2018 09:58 AM, Alexander Popov wrote:
>>
>> +       /*
>> +        * Now write the poison value to the kernel stack. Start from
>> +        * 'kstack_ptr' and move up till the new 'boundary'. We assume
>> that
>> +        * the stack pointer doesn't change when we write poison.
>> +        */
>> +       if (on_thread_stack())
>> +               boundary = current_stack_pointer;
>> +       else
>> +               boundary = current_top_of_stack();
>> +
>> +       BUG_ON(boundary - kstack_ptr >= THREAD_SIZE);
>> +
>> +       while (kstack_ptr < boundary) {
>> +               *(unsigned long *)kstack_ptr = STACKLEAK_POISON;
>> +               kstack_ptr += sizeof(unsigned long);
>> +       }
>> +
>> +       /* Reset the 'lowest_stack' value for the next syscall */
>> +       current->lowest_stack = current_top_of_stack() - THREAD_SIZE / 64;
>
>
> on_thread_stack() and current_top_of_stack() are x86 only
> functions currently defined in asm/processor.h. There are
> similar functions in arch/arm64/include/asm/stacktrace.h
> which I think I can use to build those functions. Should
> I just throw the arm64 versions in processor.h or do
> we want to consider abstracting these into something like
> asm/stackleak.h? I'd like to know a direction before I
> start ripping apart stacktrace.h.

static inline unsigned long current_top_of_stack(void)
{
        /*
         *  We can't read directly from tss.sp0: sp0 on x86_32 is special in
         *  and around vm86 mode and sp0 on x86_64 is special because of the
         *  entry trampoline.
         */
        return this_cpu_read_stable(cpu_current_top_of_stack);
}

static inline bool on_thread_stack(void)
{
        return (unsigned long)(current_top_of_stack() -
                               current_stack_pointer) < THREAD_SIZE;
}

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?

FWIW, I don't think a separate asm/stackleak.h is needed.

-Kees

-- 
Kees Cook
Pixel Security

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