Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)
[Posted June 21, 2016 by corbet]
| From: |
| Andy Lutomirski <luto-AT-amacapital.net> |
| To: |
| Linus Torvalds <torvalds-AT-linux-foundation.org> |
| Subject: |
| Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core) |
| Date: |
| Tue, 21 Jun 2016 10:27:18 -0700 |
| Message-ID: |
| <CALCETrWEokFnfjx_LjG_S4TizjVo=YcJNpeoSwxHn0-d2+Aw+A@mail.gmail.com> |
| Cc: |
| Andy Lutomirski <luto-AT-kernel.org>, "the arch/x86 maintainers" <x86-AT-kernel.org>, Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>, "linux-arch-AT-vger.kernel.org" <linux-arch-AT-vger.kernel.org>, Borislav Petkov <bp-AT-alien8.de>, Nadav Amit <nadav.amit-AT-gmail.com>, Kees Cook <keescook-AT-chromium.org>, Brian Gerst <brgerst-AT-gmail.com>, "kernel-hardening-AT-lists.openwall.com" <kernel-hardening-AT-lists.openwall.com>, Josh Poimboeuf <jpoimboe-AT-redhat.com>, Jann Horn <jann-AT-thejh.net>, Heiko Carstens <heiko.carstens-AT-de.ibm.com> |
| Archive‑link: | |
Article |
On Tue, Jun 21, 2016 at 10:16 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Jun 21, 2016 at 9:45 AM, Andy Lutomirski <luto@amacapital.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
