Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

[Kees Cook <keescook@chromium.org>]

On Fri, Nov 27, 2015 at 10:00 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Nov 26, 2015 at 11:59 PM, Ingo Molnar <mingo@kernel.org> wrote:
>>
>> * Andy Lutomirski <luto@amacapital.net> wrote:
>>
>>> > Can you see any fragility in such a technique?
>>>
>>> After Linus shot down my rdmsr/rwmsr decoding patch, good luck...
>>
>> I think that case was entirely different, but I've Cc:-ed Linus to shoot my idea
>> down if it's crap.
>
> Yeah, no, I hate it. I'm with the PaX team on this one - I think there
> are three valid responses, and I think we might want to have a dynamic
> config option (kernel command line or proc or whatever) to pick
> between the two:
>
>  - just oops and kill the machine, like for any other unhandled kernel
> page fault. This is probably what you should have on a server

This is how the v2 series works now.

>  - print a warning and a backtrace, and just mark the page read-write
> so that the machine survives, but we get notified and can fix whatever
> broken code

This seems very easy to add. Should I basically reverse the effects of
mark_rodata_ro(), or should I only make the new ro-after-init section
as RW? (I think the former would be easier.)

>  - have an option to disable the RO data logic.

I added this as "rodata=off" in the v2 series.

> I think that second option is good for debugging. In some places,
> oopses that kill things are just too hard to debug (ie it might be the
> modesetting or early boot or whatever).
>
> In fact, I think we should _start_ with the second option - perhaps
> just during the rc's - and then when we're pretty sure all the silly
> bugs it finds (maybe none, who knows) are handled, we should go to the
> first one.
>
> The third option would be purely for "user that cannot fix things
> directly and has reported the problem can now turn off the distracting
> warning". We should never default to it.
>
> Trying to actually *recover* any other way thanm by turning the area
> read-write is just too damn fragile. You can't just skip over the
> instruction that does the write - there are flags values etc that get
> updated by read-modify-write instructions, but as PaX says, there nmay
> also be subsequent logic that gets confused and actually introduces
> even *more* problems downstream if the write is just discarded.
>
> So maybe we could have some kind of "mark it read-only again later"
> thing that tries to make sure it doesn't stay writable for a long
> time, but quite frankly, I don't think it's worth it. Once the write
> has been done, and the warning has been emitted, there's likely very
> little upside to then trying to close the barn doors after that horse
> has bolted.
>
>                  Linus

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security