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

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Date:	Fri, 6 Nov 2015 15:41:57 -0800
From:	Laura Abbott <labbott@...hat.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Kees Cook <keescook@...omium.org>,
	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 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?

Thanks,
Laura
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