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

[Andrew Lutomirski]

On Thu, Apr 24, 2014 at 3:24 PM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> 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
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