Re: [RFC PATCH] seccomp: Add protection keys into seccomp_data

[Dave Hansen <dave.hansen@intel.com>]

On 10/29/18 9:48 AM, Jann Horn wrote:
> On Mon, Oct 29, 2018 at 5:37 PM Dave Hansen <dave.hansen@intel.com> wrote:
>> 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.

PKRU gives information about rights to various bits of application data.
 From that, a seccomp filter can infer the context, and thus the ability
for the code to call a given syscall at a certain point in time.

This makes PKRU an opt-in part of the syscall ABI, which is pretty
interesting.  We _could_ do the same kind of thing with any callee-saved
general purpose register, but PKRU is particularly attractive because
there is only one instruction that writes to it (well, outside of
XSAVE*), and random library code is very unlikely at this point to be
using it.

PKRU getting reset on signals, and the requirement now that it *can't*
be changed if you make syscalls probably needs to get thought about very
carefully before we do this, though.