linux-kernel - Re: [kernel-hardening] Re: [PATCH] fork: make whole stack

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Date:   Mon, 31 Oct 2016 23:11:47 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Daniel Micay <danielmicay@...il.com>
Cc:     Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random

* Daniel Micay:

> On Mon, 2016-10-31 at 22:38 +0100, Florian Weimer wrote:
>> * Daniel Micay:
>> 
>> > -fstack-stack is supposed to handle a single guard by default, and
>> > that's all there is for thread stacks by default.
>> 
>> Okay, then I'll really have to look at the probing offsets again.
>> It's been on my to-do list since about 2012, and arguably, it *is* a
>> user-space thing.
>
> This is concerning too:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66479

Thanks.  This also shows the large stack pointer decrement:

	subq	$4144, %rsp
	orq	$0, (%rsp)

I really don't see how this can be safe with just a single guard page.

> It might be prevented for VLAs by using -fsanitize=vla-bound -fsanitize-
> trap=vla-bound but probably not alloca (or the older -fsanitize-
> undefined-trap-on-error for GCC,  since for some reason it doesn't seem
> to have the new way).

It's certainly reasonable to expect that this was covered by
-fstack-check.