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

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Message-ID: <CAGXu5jK69kytOxp1xaojsGO2JXcZ+u0iHuKWD9sis_LFaCUC9Q@mail.gmail.com>
Date:	Wed, 4 Nov 2015 17:13:59 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Laura Abbott <labbott@...oraproject.org>
Cc:	Russell King <linux@....linux.org.uk>,
	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 Wed, Nov 4, 2015 at 5:06 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Nov 4, 2015 at 5:00 PM, Laura Abbott <labbott@...oraproject.org> 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.
>>
>> Signed-off-by: Laura Abbott <labbott@...oraproject.org>
>> ---
>> I found this while trying to convince myself of something.
>> Dumping the page table via debugfs and writing to kernel text were both
>> showing the lack of mappings. This was observed on QEMU. Maybe it's just a
>> QEMUism but if not it probably should go to stable.
>
> Well that's weird! debugfs showed the actual permissions that lacked
> RO? I wonder what changed. I tested this both with debugfs and lkdtm's
> KERN_WRITE test when the patches originally landed.

The comment will need adjusting too. I have a memory of needing to use
current->mm to deal with some crazy errata and handling TLB flushes...

-Kees

-- 
Kees Cook
Chrome OS Security
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