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

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Date:	Thu, 5 Nov 2015 08:20:42 -0800
From:	Laura Abbott <labbott@...hat.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laura Abbott <labbott@...oraproject.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] arm: Use kernel mm when updating section permissions

On 11/05/2015 01:46 AM, Russell King - ARM Linux wrote:
> On Wed, Nov 04, 2015 at 05:00:39PM -0800, Laura Abbott wrote:
>> Currently, read only permissions are not being applied even
>> when CONFIG_DEBUG_RODATA is set. This is because section_update
>> uses current->mm for adjusting the page tables. current->mm
>> need not be equivalent to the kernel version. Use pgd_offset_k
>> to get the proper page directory for updating.
>
> What are you trying to achieve here?  You can't use these functions
> at run time (after the first thread has been spawned) to change
> permissions, because there will be multiple copies of the kernel
> section mappings, and those copies will not get updated.
>
> In any case, this change will probably break kexec and ftrace, as
> the running thread will no longer see the updated page tables.
>

I think I was hitting that exact problem with multiple copies
not getting updated. The section_update code was being called
and I was seeing the tables get updated but nothing was being
applied when I tried to write to text or check the debugfs
page table. The current flow is:

rest_init -> kernel_thread(kernel_init) and from that thread
mark_rodata_ro. So mark_rodata_ro is always going to happen
in a thread.

Do we need to update for both init_mm and the first running
thread?

Thanks,
Laura
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