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Message-ID: <CAGXu5jK7-D8vE_1qV+ZMayama3YwR-fs8ESjOM=VrhxUvAHxNA@mail.gmail.com>
Date: Wed, 15 Jun 2016 10:49:18 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
Borislav Petkov <bp@...en8.de>, "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Playing with virtually mapped stacks (with guard pages!)
On Wed, Jun 15, 2016 at 10:23 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Jun 15, 2016 at 10:05 AM, Kees Cook <keescook@...omium.org> wrote:
>> On Tue, Jun 14, 2016 at 11:01 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> Hi all-
>>>
>>> If you want to play with virtually mapped stacks, I have it more or
>>> less working on x86 in a branch here:
>>>
>>> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/vmap_stack
>>>
>>> The core bit (virtually map the stack and fix the accounting) is just
>>> a config option, but it needs the arch to opt-in. I suspect that
>>> every arch will have its own set of silly issues to address to make it
>>> work well. For x86, the silly issues are getting the OOPS to work
>>> right and handling some vmalloc_fault oddities to avoid panicing at
>>> random.
>>
>> Awesome! Some notes/questions:
>>
>> - there are a number of typos in commit messages and comments, just FYI
>
> Not surprising. I'll try to find and fix them.
x86/cpa: In populate_pgd, don't set the pgd entry until it's
populated: "anyther CPU propages"
x86/cpa: Warn if kernel_unmap_pages_in_pgd is used inappropriately:
"kenrnel entries"
There was another repeated word, but I can't find it now. :P
>>
>> - where is the guard page added? I don't see anything leaving a hole at the end?
>
> Magic! The vmap code does this for us.
Heh, can you point me to where? Does it have guards at both ends?
>> - where is thread_info? I understand there to be two benefits from
>> vmalloc stack: 1) thread_info can live elsewhere, 2) guard page can
>> exist easily
>
> I think that thread_info is a separate issue except insofar as it's
> needed for full exploit protection. Moving / eliminating it has
> nothing to do with where the stack lives AFAIK. I'll get to it.
Okay, cool.
> I suspect that the hardest part will be eliminating the (mostly
> pointless) thread_info::task field.
IIUC, grsecurity puts a thread_info pointer in the percpu area, if
that's any help.
>> - this seems like it should Oops not warn:
>> WARN_ON_ONCE(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
>> that being wrong seems like a very bad state to continue from
>
> I'll change that.
>
>>
>> - bikeshed: I think the CONFIG should live in arch/Kconfig (with a
>> description of what an arch needs to support for it) and be called
>> HAVE_ARCH_VMAP_STACK so that archs can select it instead of having
>> multiple definitions of CONFIG_VMAP_STACK in each arch.
>
> I'll change that, too.
Awesome! :)
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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