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Date: Wed, 2 May 2018 16:37:58 -0700
From: Kees Cook <keescook@...omium.org>
To: Laura Abbott <labbott@...hat.com>
Cc: Alexander Popov <alex.popov@...ux.com>, Mark Rutland <mark.rutland@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] arm64: Clear the stack
On Wed, May 2, 2018 at 4:07 PM, Laura Abbott <labbott@...hat.com> wrote:
> On 05/02/2018 02:31 PM, Kees Cook wrote:
>> struct stackleak {
>> #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> unsigned long lowest;
>> #ifdef CONFIG_STACKLEAK_METRICS
>> unsigned long prev_lowest;
>> #endif
>> #endif
>> };
>>
>
> Is this well defined across all compilers if the plugin is off?
> This seems to compile with gcc at least but 0 sized structs
> make me a little uneasy.
Yup! Or at least, there have been no problems with this and the
seccomp struct, which is empty when !CONFIG_SECCOMP.
>> This is the only difference between x86 and arm64 in this code. What
>> do you think about implementing on_thread_stack() to match x86:
>>
>> if (on_thread_stack())
>> boundary = current_stack_pointer;
>> else
>> boundary = current_top_of_stack();
>>
>> then we could make this common code too instead of having two copies in
>> arch/?
>>
>
> The issue isn't on_thread_stack, it's current_top_of_stack which isn't
> defined on arm64. I agree it would be good if the code would be common
> but I'm not sure how much we want to start trying to force APIs.
Ah, gotcha. Well, I'd rather we had an #ifdef here that two copies of
the code. ;)
>>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>> +void __used check_alloca(unsigned long size)
>>> +{
>>> + unsigned long sp, stack_left;
>>> +
>>> + sp = current_stack_pointer;
>>> +
>>> + stack_left = sp & (THREAD_SIZE - 1);
>>> + BUG_ON(stack_left < 256 || size >= stack_left - 256);
>>> +}
>>> +EXPORT_SYMBOL(check_alloca);
>>
>>
>> This is pretty different from x86. Is this just an artifact of ORC, or
>> something else?
>>
>
> This was based on the earlier version of x86. I'll confess to
> not seeing how the current x86 version ended up with get_stack_info
> but I suspect it's either related to ORC unwinding or it's best
> practice.
Alexander, what was the history here?
-Kees
--
Kees Cook
Pixel Security
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