linux-kernel - Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

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Date:	Fri, 27 Nov 2015 12:09:23 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	PaX Team <pageexec@...email.hu>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Mathias Krause <minipli@...glemail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-arch <linux-arch@...r.kernel.org>,
	Emese Revfy <re.emese@...il.com>
Subject: Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

On Fri, Nov 27, 2015 at 12:03 PM, Kees Cook <keescook@...omium.org> wrote:
> On Fri, Nov 27, 2015 at 10:00 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, Nov 26, 2015 at 11:59 PM, Ingo Molnar <mingo@...nel.org> wrote:
>>>
>>> * Andy Lutomirski <luto@...capital.net> wrote:
>>>
>>>> > Can you see any fragility in such a technique?
>>>>
>>>> After Linus shot down my rdmsr/rwmsr decoding patch, good luck...
>>>
>>> I think that case was entirely different, but I've Cc:-ed Linus to shoot my idea
>>> down if it's crap.
>>
>> Yeah, no, I hate it. I'm with the PaX team on this one - I think there
>> are three valid responses, and I think we might want to have a dynamic
>> config option (kernel command line or proc or whatever) to pick
>> between the two:
>>
>>  - just oops and kill the machine, like for any other unhandled kernel
>> page fault. This is probably what you should have on a server
>
> This is how the v2 series works now.
>
>>  - print a warning and a backtrace, and just mark the page read-write
>> so that the machine survives, but we get notified and can fix whatever
>> broken code
>
> This seems very easy to add. Should I basically reverse the effects of
> mark_rodata_ro(), or should I only make the new ro-after-init section
> as RW? (I think the former would be easier.)

I'd suggest verifying that the page in question is
.data..ro_after_init and, if so, marking that one page RW.

--Andy
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