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

[Andy Lutomirski]

On Tue, Jun 21, 2016 at 12:47 PM, Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxx> wrote:
>> > On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote:
>> >>
>> >> On my laptop, this adds about 1.5Âs 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.

--Andy