Re: TLB flushes on fixmap changes

[Kees Cook]

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

With that, text_poke() mostly becomes:

rare_write_begin()
memcpy(addr, opcode, len);
rare_write_end()

As for juiciness, if an attacker already has execution control, they
can do more interesting things than text_poke(). But regardless, yes,
it's always made me uncomfortable. :)

-Kees

-- 
Kees Cook
Pixel Security