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

Kees Cook
2016-06-21T17:16:21Z

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.

-Kees

--
Kees Cook
Chrome OS & Brillo Security