[RFC PATCH] seccomp: Add protection keys into seccomp_data

[Jann Horn]

On Mon, Oct 29, 2018 at 5:37 PM Dave Hansen <dave.hansen at intel.com> wrote:
> On 10/29/18 9:25 AM, Kees Cook wrote:
> > On Mon, Oct 29, 2018 at 4:23 AM, Michael Sammler <msammler at mpi-sws.org> wrote:
> >> Add the current value of an architecture specific protection keys
> >> register (currently PKRU on x86) to data available for seccomp-bpf
> >> programs to work on. This allows filters based on the currently
> >> enabled protection keys.
>
> How does the current "assignment" of protection keys to the various uses
> get communicated to the filter?

I assume that you first allocate your protection keys, then install the filter?

> I'm not sure this is a great use for PKRU.  I *think* the basic problem
> is that you want to communicate some rights information down into a
> filter, and you want to communicate it with PKRU.  While it's handy to
> have an extra register that nobody (generally) mucks with, I'm not quite
> convinced that we want to repurpose it this way.

That's not how I understand it; I believe that the context is probably
https://arxiv.org/pdf/1801.06822.pdf ?
My understanding is that PKRU is used for lightweight in-process
sandboxing, and to extend this sandbox protection to the syscall
interface, it is necessary to expose PKRU state to seccomp filters.
In other words, this isn't using PKRU exclusively for passing rights
into a filter, but it has to use PKRU anyway.

> Also, I'm not sure the kernel provides the PKRU guarantees you want at
> the moment.  Our implementation *probably* works, but it's mostly by
> accident.