linux-kernel - Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

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Message-ID: <CALCETrWEokFnfjx_LjG_S4TizjVo=YcJNpeoSwxHn0-d2+Aw+A@mail.gmail.com>
Date:	Tue, 21 Jun 2016 10:27:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

On Tue, Jun 21, 2016 at 10:16 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Jun 21, 2016 at 9:45 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> So I'm leaning toward fewer cache entries per cpu, maybe just one.
>> I'm all for making it a bit faster, but I think we should weigh that
>> against increasing memory usage too much and thus scaring away the
>> embedded folks.
>
> I don't think the embedded folks will be scared by a per-cpu cache, if
> it's just one or two entries.  And I really do think that even just
> one or two entries will indeed catch a lot of the cases.
>
> And yes, fork+execve() is too damn expensive in page table build-up
> and tear-down. I'm not sure why bash doesn't do vfork+exec for when it
> has to wait for the process anyway, but it doesn't seem to do that.
>

I don't know about bash, but glibc very recently fixed a long-standing
but in posix_spawn and started using clone() in a sensible manner for
this.

FWIW, it may be a while before this can be enabled in distro kernels.
There are some code paths (*cough* crypto users *cough*) that think
that calling sg_init_one with a stack address is a reasonable thing to
do, and it doesn't work with a vmalloced stack.  grsecurity works
around this by using a real lowmem higher-order stack, aliasing it
into vmalloc space, and arranging for virt_to_phys to backtrack the
alias, but eww.  I think I'd rather find and fix the bugs, assuming
they're straightforward.

--Andy