Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble memory as ROX

[Nadav Amit]

> On Jun 5, 2023, at 9:10 AM, Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote:
> 
> On Mon, 2023-06-05 at 11:11 +0300, Mike Rapoport wrote:
>> On Sun, Jun 04, 2023 at 10:52:44PM -0400, Steven Rostedt wrote:
>>> On Thu, 1 Jun 2023 16:54:36 -0700
>>> Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>>> 
>>>>> The way text_poke() is used here, it is creating a new writable
>>>>> alias
>>>>> and flushing it for *each* write to the module (like for each
>>>>> write of
>>>>> an individual relocation, etc). I was just thinking it might
>>>>> warrant
>>>>> some batching or something.  
>> 
>>>> I am not advocating to do so, but if you want to have many
>>>> efficient
>>>> writes, perhaps you can just disable CR0.WP. Just saying that if
>>>> you
>>>> are about to write all over the memory, text_poke() does not
>>>> provide
>>>> too much security for the poking thread.
>> 
>> Heh, this is definitely and easier hack to implement :)
> 
> I don't know the details, but previously there was some strong dislike
> of CR0.WP toggling. And now there is also the problem of CET. Setting
> CR0.WP=0 will #GP if CR4.CET is 1 (as it currently is for kernel IBT).
> I guess you might get away with toggling them both in some controlled
> situation, but it might be a lot easier to hack up then to be made
> fully acceptable. It does sound much more efficient though.

Thanks for highlighting this issue. I understand the limitations of
CR0.WP. There is also always the concerns that without CET or other
control flow integrity mechanism, someone would abuse (using ROP/JOP)
functions that clear CR0.WP…