Re: [PATCH v11 00/11] x86: PIE support to extend KASLR randomization

H. Peter Anvin
2020-03-04T18:46:25Z

On 2020-03-04 10:21, Kees Cook wrote:
> On Wed, Mar 04, 2020 at 10:21:36AM +0100, Peter Zijlstra wrote:
>> But at what cost; it does unspeakable ugly to the asm. And didn't a
>> kernel compiled with the extended PIE range produce a measurably slower
>> kernel due to all the ugly?
>
> Was that true? I thought the final results were a wash and that earlier
> benchmarks weren't accurate for some reason? I can't find the thread
> now. Thomas, do you have numbers on that?
>
> BTW, I totally agree that fgkaslr is the way to go in the future. I
> am mostly arguing for this under the assumption that it doesn't
> have meaningful performance impact and that it gains the kernel some
> flexibility in the kinds of things it can do in the future. If the former
> is not true, then I'd agree, the benefit needs to be more clear.
>

"Making the assembly really ugly" by itself is a reason not to do it, in my
Not So Humble Opinion[TM]; but the reason the kernel and small memory models
exist in the first place is because there is a nonzero performance impact of
the small-PIC memory model. Having modules in separate regions would further
add the cost of a GOT references all over the place (PLT is optional, useless
and deprecated for eager binding) *plus* might introduce at least one new
vector of attack: overwrite a random GOT slot, and just wait until it gets hit
by whatever code path it happens to be in; the exact code path doesn't matter.
From an kASLR perspective this is *very* bad, since you only need to guess the
general region of a GOT rather than an exact address.

The huge memory model, required for arbitrary placement, has a very
significant performance impact.

The assembly code is *very* different across memory models.

-hpa