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

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Date:	Fri, 6 Nov 2015 15:49:16 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laura Abbott <labbott@...oraproject.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] arm: Use kernel mm when updating section permissions

On Fri, Nov 6, 2015 at 3:41 PM, Laura Abbott <labbott@...hat.com> 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

-- 
Kees Cook
Chrome OS Security
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