Re: [PATCH] arm64: make CONFIG_DEBUG_RODATA non-optional

[Kees Cook]

On Thu, Jan 28, 2016 at 6:59 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Thu, Jan 28, 2016 at 06:06:53AM -0800, Kees Cook wrote:
>> On Thu, Jan 28, 2016 at 3:06 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> > One thing I would like to do is to avoid the need for fixup_executable
>> > entirely, by mapping the kernel text RO from the outset. However, that
>> > requires rework of the alternatives patching (to use a temporary RW
>> > alias), and I haven't had the time to look into that yet.
>>
>> This makes perfect sense for the rodata section, but the (future)
>> postinit_rodata section we'll still want to mark RO after init
>> finishes. x86 and ARM cheat y marking both RO after init, and they
>> don't have to pad sections. parisc will need to solve this too.
>
> Depending on how many postinit_rodata variables there are, we might be
> able to drop those in .rodata, have them RO always, and initialise them
> via a temporary RW alias (e.g. something in the vmalloc area).
>
> The only requirement for that is that we use a helper to initialise any
> __postinit_ro variables via a temporary RW alias, e.g.
>
> #define SET_POST_INIT_RO(ptr, v) ({             \\
>         typeof(ptr) __ptr_rw = (ptr)            \\
>         BUG_ON(initcalls_done);                 \\
>         __ptr_rw = create_rw_alias(__ptr);      \\
>         __ptr_rw = v;                           \\
>         destroy_rw_alias(__ptr_rw);             \\
> })
>
> ...
>
> __postinit_ro void *thing;
>
> void __init some_init_func(void) {
>         void *__thing = some_ranodomized_allocator();
>         SET_POSTINIT_RO(thing, thing);
> }

Well, that certainly would make their usage explicit, but I'd really
like to avoid that, especially in the face of trying to const-ify as
much of the kernel as possible to reduce attack surface. I don't want
to have to both mark the variable and its writes, since that would
make the constification gcc plugin a bit more complex.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security