[kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

[Mathias Krause]

On 7 April 2017 at 15:14, Thomas Gleixner <tglx at linutronix.de> wrote:
> On Fri, 7 Apr 2017, Mathias Krause wrote:
>> On 7 April 2017 at 11:46, Thomas Gleixner <tglx at linutronix.de> wrote:
>> > Whether protected by preempt_disable or local_irq_disable, to make that
>> > work it needs CR0 handling in the exception entry/exit at the lowest
>> > level. And that's just a nightmare maintainence wise as it's prone to be
>> > broken over time.
>>
>> It seems to be working fine for more than a decade now in PaX. So it
>> can't be such a big maintenance nightmare ;)
>
> I really do not care whether PaX wants to chase and verify that over and
> over. I certainly don't want to take the chance to leak CR0.WP ever and I
> very much care about extra stuff to check in the entry/exit path.

Fair enough. However, placing a BUG_ON(!(read_cr0() & X86_CR0_WP))
somewhere sensible should make those "leaks" visible fast -- and their
exploitation impossible, i.e. fail hard.

>> The "proper solution" seems to be much slower compared to just
>> toggling CR0.WP (which is costly in itself, already) because of the
>> TLB invalidation / synchronisation involved.
>
> Why the heck should we care about rare writes being performant?

As soon as they stop being rare and people start extending the r/o
protection to critical data structures accessed often. Then
performance matters.

>> > It's valid (at least on x86) to have a shadow map with the same page
>> > attributes but write enabled. That does not require any fixups of CR0 and
>> > just works.
>>
>> "Just works", sure -- but it's not as tightly focused as the PaX
>> solution which is CPU local, while your proposed solution is globally
>> visible.
>
> Making the world and some more writeable hardly qualifies as tightly
> focussed. Making the mapping concept CPU local is not rocket science
> either. The question is whethers it's worth the trouble.

No, the question is if the value of the concept is well understood and
if people can see what could be done with such a strong primitive.
Apparently not...


Cheers,
Mathias