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

Kees Cook
2016-06-21T20:18:38Z

On Tue, Jun 21, 2016 at 12:47 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Tue, Jun 21, 2016 at 12:47 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Tuesday, June 21, 2016 10:16:21 AM CEST Kees Cook wrote:
>>> On Tue, Jun 21, 2016 at 2:24 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>>> > On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote:
>>> >>
>>> >> On my laptop, this adds about 1.5µs of overhead to task creation,
>>> >> which seems to be mainly caused by vmalloc inefficiently allocating
>>> >> individual pages even when a higher-order page is available on the
>>> >> freelist.
>>> >
>>> > Would it help to have a fixed virtual address for the stack instead
>>> > and map the current stack to that during a task switch, similar to
>>> > how we handle fixmap pages?
>>> >
>>> > That would of course trade the allocation overhead for a task switch
>>> > overhead, which may be better or worse. It would also give "current"
>>> > a constant address, which may give a small performance advantage
>>> > but may also introduce a new attack vector unless we randomize it
>>> > again.
>>>
>>> Right: we don't want a fixed address. That makes attacks WAY easier.
>>
>> Do we care about making the address more random then? When I look
>> at /proc/vmallocinfo, I see that allocations are all using
>> consecutive addresses, so if you can figure out the virtual
>> address of the stack for one process that would give you a good
>> chance of guessing the address for the next pid.
>
> Quite possibly. We should seriously consider at least randomizing the
> *start* of the vmalloc area, at least on 64-bit architectures.

Yup, this is already under way for x86. Thomas Garnier has a series
that he's been working on:

http://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/log/?h=kaslr/memory

I'd love to see similar for other architectures too.

Thomas just sent me an updated series I'll be putting up for review later today.

-Kees

--
Kees Cook
Chrome OS & Brillo Security