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

Kees Cook
2015-11-25T23:33:00Z

On Wed, Nov 25, 2015 at 3:05 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> On Wed, 2015-11-25 at 07:03 -0800, Kees Cook wrote:
>> 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.
>
> Yeah I saw that too, but I couldn't see anything in the commit history that
> explained why it was per-arch.

Best I was able to see was that architectures weren't (aren't?) using
the common RODATA section macros in their linker scripts. From a quick
inspection, I think these are all okay now.

-Kees

>
>> > 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. :)
>
> That'd be great, I can test on powerpc, and build test other arches too.
>
> cheers
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/

--
Kees Cook
Chrome OS & Brillo Security