Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

Andrew Lutomirski
2014-04-24T22:32:16Z

On Thu, Apr 24, 2014 at 3:24 PM, H. Peter Anvin <hpa@linux.intel.com> wrote:
> On 04/23/2014 09:53 PM, Andrew Lutomirski wrote:
>>>
>>> - The user can put arbitrary data in registers before returning to the
>>> LDT in order to get it saved at a known address accessible from the
>>> kernel. With SMAP and KASLR this might otherwise be difficult.
>>
>> For one thing, this only matters on Haswell. Otherwise the user can
>> put arbitrary data in userspace.
>>
>> On Haswell, the HPET fixmap is currently a much simpler vector that
>> can do much the same thing, as long as you're willing to wait for the
>> HPET counter to contain some particular value. I have patches that
>> will fix that as a side effect.
>>
>> Would it pay to randomize the location of the espfix area? Another
>> somewhat silly idea is to add some random offset to the CPU number mod
>> NR_CPUS so that at attacker won't know which ministack is which.
>
> Since we store the espfix stack location explicitly, as long as the
> scrambling happens in the initialization code that's fine. However, we
> don't want to reduce locality lest we massively blow up the memory
> requirements.

I was imagining just randomizing a couple of high bits so the whole
espfix area moves as a unit.

>
> We could XOR with a random constant with no penalty at all. Only
> problem is that this happens early, so the entropy system is not yet
> available. Fine if we have RDRAND, but...

How many people have SMAP and not RDRAND? I think this is a complete
nonissue for non-SMAP systems.

>> Peter, is this idea completely nuts? The only exceptions that can
>> happen there are NMI, MCE, #DB, #SS, and #GP. The first four use IST,
>> so they won't double-fault.
>
> It is completely nuts, but sometimes completely nuts is actually useful.
> It is more complexity, to be sure, but it doesn't seem completely out
> of the realm of reason, and avoids having to unwind the ministack except
> in the normally-fatal #DF handler. #DFs are documented as not
> recoverable, but we might be able to do something here.
>
> The only real disadvantage I see is the need for more bookkeeping
> metadata. Basically the bitmask in espfix_64.c now needs to turn into
> an array, plus we need a second percpu variable. Given that if
> CONFIG_NR_CPUS=8192 the array has 128 entries I think we can survive that.

Doing something in #DF needs percpu data? What am I missing?

--Andy