Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

[Andy Lutomirski <luto@kernel.org>]

On Mon, Apr 10, 2017 at 1:13 PM, Kees Cook <keescook@chromium.org> wrote:
> On Sun, Apr 9, 2017 at 1:24 PM, PaX Team <pageexec@freemail.hu> wrote:
>> On 7 Apr 2017 at 22:07, Andy Lutomirski wrote:
>>> No one has explained how CR0.WP is weaker or slower than my proposal.
>>
>> you misunderstood, Daniel was talking about your use_mm approach.
>>
>>> Here's what I'm proposing:
>>>
>>> At boot, choose a random address A.
>>
>> what is the threat that a random address defends against?
>>
>>>  Create an mm_struct that has a
>>> single VMA starting at A that represents the kernel's rarely-written
>>> section.  Compute O = (A - VA of rarely-written section).  To do a
>>> rare write, use_mm() the mm, write to (VA + O), then unuse_mm().
>>
>> the problem is that the amount of __read_only data extends beyond vmlinux,
>> i.e., this approach won't scale. another problem is that it can't be used
>> inside use_mm and switch_mm themselves (no read-only task structs or percpu
>> pgd for you ;) and probably several other contexts.
>
> These are the limitations that concern me: what will we NOT be able to
> make read-only as a result of the use_mm() design choice? My RFC
> series included a simple case and a constify case, but I did not
> include things like making page tables read-only, etc.

If we make page tables read-only, we may need to have multiple levels
of rareness.  Page table writes aren't all that rare, and I can
imagine distros configuring the kernel so that static structs full of
function pointers are read-only (IMO that should be the default or
even mandatory), but page tables may be a different story.

That being said, CR3-twiddling to write to page tables could actually
work.  Hmm.