Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init read-only memory

Kees Cook
2015-11-25T15:03:42Z

On Tue, Nov 24, 2015 at 4:54 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> On Tue, 2015-11-24 at 16:44 -0800, Kees Cook wrote:
>> On Tue, Nov 24, 2015 at 4:34 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> > On Nov 24, 2015 1:38 PM, "Kees Cook" <keescook@chromium.org> wrote:
>> > >
>> > > One of the easiest ways to protect the kernel from attack is to reduce
>> > > the internal attack surface exposed when a "write" flaw is available. By
>> > > making as much of the kernel read-only as possible, we reduce the
>> > > attack surface.
>> > >
>> > > Many things are written to only during __init, and never changed
>> > > again. These cannot be made "const" since the compiler will do the wrong
>> > > thing (we do actually need to write to them). Instead, move these items
>> > > into a memory region that will be made read-only during mark_rodata_ro()
>> > > which happens after all kernel __init code has finished.
>> > >
>> > > This introduces __read_only as a way to mark such memory, and adds some
>> > > documentation about the existing __read_mostly marking.
>> >
>> > Obligatory bikeshed: __ro_after_init, please. It's barely longer,
>> > and it directly explains what's going on. __read_only makes me think
>> > that it's really read-only and could, for example, actually be in ROM.
>>
>> I'm fine with that. Anyone else want to chime in before I send a v2?
>
> I'm not clear on why this is x86 only?

I was initially looking at how __read_mostly got implemented, and it
seemed like section names were done on a per-arch basis. But it
doesn't seem like that needs to be true.

> It looks like it would work on any arch, or is there some toolchain
> requirement?

Given that the other sections are in the common linux.lds.h file, it
seems unlikely to me. I'll try it in an arch-agnostic way and see what
happens. :)

-Kees

--
Kees Cook
Chrome OS & Brillo Security