Re: [PATCH 26/26] x86, pkeys: Documentation

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Oct 1, 2015 at 1:39 PM, Kees Cook <keescook@google.com> wrote:
> On Thu, Oct 1, 2015 at 4:17 AM, Ingo Molnar <mingo@kernel.org> wrote:
>>
>> * Dave Hansen <dave@sr71.net> wrote:
>>
>>> > If yes then this could be a significant security feature / usecase for pkeys:
>
> Which CPUs (will) have pkeys?
>
>>> > executable sections of shared libraries and binaries could be mapped with pkey
>>> > access disabled. If I read the Intel documentation correctly then that should
>>> > be possible.
>>>
>>> Agreed.  I've even heard from some researchers who are interested in this:
>>>
>>> https://www.infsec.cs.uni-saarland.de/wp-content/uploads/sites/2/2014/10/nuernberger2014ccs_disclosure.pdf
>>
>> So could we try to add an (opt-in) kernel option that enables this transparently
>> and automatically for all PROT_EXEC && !PROT_WRITE mappings, without any
>> user-space changes and syscalls necessary?
>
> I would like this very much. :)
>
>> Beyond the security improvement, this would enable this hardware feature on most
>> x86 Linux distros automatically, on supported hardware, which is good for testing.
>>
>> Assuming it boots up fine on a typical distro, i.e. assuming that there are no
>> surprises where PROT_READ && PROT_EXEC sections are accessed as data.
>
> I can't wait to find out what implicitly expects PROT_READ from
> PROT_EXEC mappings. :)

There's one annoying issue at least:

mprotect_pkey(..., PROT_READ | PROT_EXEC, 0) sets protection key 0.
mprotect_pkey(..., PROT_EXEC, 0) maybe sets protection key 15 or
whatever we use for this.  What does mprotect_pkey(..., PROT_EXEC, 0)
do?  What if the caller actually wants key 0?  What if some CPU vendor
some day implements --x for real?


Also, how do we do mprotect_pkey and say "don't change the key"?

>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC

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