Re: [PATCH] arm: Use kernel mm when updating section permissions

From: Kees Cook
Date: Fri Nov 06 2015 - 18:49:21 EST

On Fri, Nov 6, 2015 at 3:41 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 11/06/2015 12:46 PM, Russell King - ARM Linux wrote:
>> On Fri, Nov 06, 2015 at 10:44:32AM -0800, Laura Abbott wrote:
>>> with my test patch. I think setting both current->active_mm and &init_mm
>>> is sufficient. Maybe explicitly setting swapper_pg_dir would be cleaner?
>> Please, stop thinking like this. If you're trying to change the kernel
>> section mappings after threads have been spawned, you need to change
>> them for _all_ threads, which means you need to change them for every
>> page table that's in existence at that time - you can't do just one
>> table and hope everyone updates, it doesn't work like that.
> That's a bad assumption assumption on my part based on what I was
> observing. At the time of mark_rodata_ro, the only threads present
> are kernel threads which aren't going to have task->mm. Only the
> running thread is going to have active_mm. None of those are init_mm.
> To be complete we need:
> - Update every task->mm for every thread in every process
> - Update current->active_mm
> - Update &init_mm explicitly
> All this would need to be done under stop_machine as well. Does that cover
> everything or am I still off?

I still think we need to find an earlier place to do this. :(


Kees Cook
Chrome OS Security
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