Re: TLB flushes on fixmap changes

[Kees Cook]

On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>
>> On Aug 25, 2018, at 9:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>
>>> On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>> On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>>>> On Fri, 24 Aug 2018 21:23:26 -0700
>>>> Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>>> Couldn't text_poke() use kmap_atomic()?  Or, even better, just change CR3?
>>>>
>>>> No, since kmap_atomic() is only for x86_32 and highmem support kernel.
>>>> In x86-64, it seems that returns just a page address. That is not
>>>> good for text_poke, since it needs to make a writable alias for RO
>>>> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache?
>>>>
>>>
>>> I just re-read text_poke().  It's, um, horrible.  Not only is the
>>> implementation overcomplicated and probably buggy, but it's SLOOOOOW.
>>> It's totally the wrong API -- poking one instruction at a time
>>> basically can't be efficient on x86.  The API should either poke lots
>>> of instructions at once or should be text_poke_begin(); ...;
>>> text_poke_end();.
>>>
>>> Anyway, the attached patch seems to boot.  Linus, Kees, etc: is this
>>> too scary of an approach?  With the patch applied, text_poke() is a
>>> fantastic exploit target.  On the other hand, even without the patch
>>> applied, text_poke() is every bit as juicy.
>>
>> 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).

-Kees

-- 
Kees Cook
Pixel Security