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Date: Thu, 26 Nov 2015 18:17:06 +0100
From: Quentin Casasnovas <quentin.casasnovas@...cle.com>
To: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Cc: Kees Cook <keescook@...omium.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: status: GRKERNSEC_KSTACKOVERFLOW
On Thu, Nov 26, 2015 at 12:45:42AM +0100, Quentin Casasnovas wrote:
> On Tue, Nov 24, 2015 at 11:10:09AM -0800, Kees Cook wrote:
> > Hi,
> >
>
> Hi Kees,
>
> > I just wanted to check in and see how progress was going on the stack
> > overflow feature. Anything we can help with?
> >
>
> Sorry for not following up on this, I've been busy and haven't had the time
> to finish it properly. I've pushed an initial WIP break up of the
> KSTACK_OVERFLOW feature on my github:
>
> https://github.com/casasnovas/linux/tree/quentin-split-kstackoverflow
>
> This is far from being complete though, and hasn't been cleaned at all. I
> didn't share it earlier because I don't think I fully understand it and
> haven't tested it yet. In "short", there's mention of guard pages in the
> Kconfig help:
>
> If you say Y here, the kernel's process stacks will be allocated with
> vmalloc instead of the kernel's default allocator. This introduces guard
> ^^^^^
> pages that in combination with the alloca checking of the STACKLEAK
> ^^^^^
> feature prevents all forms of kernel process stack overflow abuse. Note
> that this is different from kernel stack buffer overflows. """
>
> And I couldn't find anything about it in the code. Maybe it's just coming
> from a mis-interpretation of the above text, but I was expecting this to
> mean there would be a PROT_NONE guard page after the end of the stack, so
> that read/writes below it could be trapped. It could also be that I missed
> some parts in my initial break-up or simply am missing something.
>
Alright, there's a guard page by default when using vmap() unless
VM_NO_GUARD is in the flags. I had a feeling I was missing some bits.. ;)
Quentin
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