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

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Date:   Mon, 20 Feb 2017 12:45:32 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Hoeun Ryu <hoeun.ryu@...il.com>, Kees Cook <keescook@...omium.org>,
        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 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.

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.

Thanks,
Mark.

[1] http://www.openwall.com/lists/kernel-hardening/2016/11/18/3