linux-kernel - Re: [RFC 7/7] arm64: map seperately rodata sections for __ro_mostly_after

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Date:   Tue, 21 Feb 2017 12:38:03 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Hoeun Ryu <hoeun.ryu@...il.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Laura Abbott <labbott@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Jeremy Linton <jeremy.linton@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 7/7] arm64: map seperately rodata sections for
 __ro_mostly_after_init section

On Mon, Feb 20, 2017 at 4:45 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Sun, Feb 19, 2017 at 11:35:51AM +0000, Ard Biesheuvel wrote:
>> On 19 February 2017 at 10:04, Hoeun Ryu <hoeun.ryu@...il.com> wrote:
>> > Map rodata sections seperately for the new __ro_mostly_after_init section.
>> > Attribute of memory for __ro_mostly_after_init section can be changed later
>> > so we need a dedicated vmalloced region for set_memory_rw/ro api.
>
>> While it is correct that you are splitting this into three separate
>> segments (otherwise we would not be able to change the permissions
>> later without risking splitting to occur), I think this leads to
>> unnecessary fragmentation.
>>
>> If there is demand for this feature (but you still need to make the
>> argument for that), I wonder if it wouldn't be sufficient, and much
>> more straightforward, to redefine the __ro_after_init semantics to
>> include the kind of subsystem registration and module init context you
>> are targeting, and implement some hooks to temporarily lift the
>> __ro_after_init r/o permission restrictions in a controlled manner.
>
> From a look over the series, I think this is just __write_rarely in
> disguise. I personally think that we should keep __write_rarely and
> __ro_after_init separate, the later being a strictly one-shot affair.

That's my thinking too.

> I had some ideas [1] as to how we could implement __write_rarely without
> carving up the kernel mapping further (and keeping the RW permissions
> local to the thread needing it), but I have not had the time to look
> into that further.

I'm working on a series to do this for x86, but I keep getting
distracted. I hope to get an RFC posted this week.

-Kees

-- 
Kees Cook
Pixel Security