Re: TLB flushes on fixmap changes

[Kees Cook]

On Sun, Aug 26, 2018 at 1:15 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Sun, 26 Aug 2018, Andy Lutomirski wrote:
>> > On Aug 26, 2018, at 9:47 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> >> On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> >>> I tried to convince Ingo to use this method for doing "write rarely"
>> >>> and he soundly rejected it. :) I've always liked this because AFAICT,
>> >>> it's local to the CPU. I had proposed it in
>> >>> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d
>> >>
>> >> Ingo, can you clarify why you hate it?  I personally would rather use CR3, but CR0 seems like a fine first step, at least for text_poke.
>> >
>> > Sorry, it looks like it was tglx, not Ingo:
>> >
>> > https://lkml.kernel.org/r/alpine.DEB.2.20.1704071048360.1716@nanos
>> >
>> > This thread is long, and one thing that I think went unanswered was
>> > "why do we want this to be fast?" the answer is: for doing page table
>> > updates. Page tables are becoming a bigger target for attacks now, and
>> > it's be nice if they could stay read-only unless they're getting
>> > updated (with something like this).
>> >
>> >
>> It kind of sounds like tglx would prefer the CR3 approach. And indeed my
>> patch has a serious problem wrt the NMI code.
>
> That's exactly the problem I have with CR0. It leaves everything and some
> more writeable for any code which can interrupt that section.

I thought the point was that the implementation I suggested was
NMI-proof? (And in reading Documentation/preempt-locking.txt it sounds
like disabling interrupts is redundant to preempt_disable()? But I
don't understand how; it looks like the preempt stuff is advisory?)

-Kees

-- 
Kees Cook
Pixel Security