lkml.org 
[lkml]   [2018]   [Aug]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: TLB flushes on fixmap changes
On Sun, Aug 26, 2018 at 7:20 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>> On Aug 25, 2018, at 9:43 PM, Kees Cook <keescook@chromium.org> wrote:
>>
>>> On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski <luto@kernel.org> wrote:
>>>> On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
>>>> On Fri, 24 Aug 2018 21:23:26 -0700
>>>> Andy Lutomirski <luto@kernel.org> 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

\
 
 \ /
  Last update: 2018-08-26 18:48    [W:0.869 / U:5.860 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site