* Andy Lutomirski <l...@amacapital.net> wrote:
> On Tue, May 3, 2016 at 11:44 PM, Ingo Molnar <mi...@kernel.org> wrote:
> >
> > * Andy Lutomirski <l...@amacapital.net> wrote:
> >
> >> On Tue, May 3, 2016 at 2:43 PM, Dave Hansen <dave.han...@intel.com> wrote:
> >> > On 05/03/2016 02:31 PM, Andy Lutomirski wrote:
> >> >> Having actually read the erratum: how can this affect Linux at all
> >> >> under any scenario where user code hasn't already completely
> >> >> compromised the kernel?
> >> >>
> >> >> I.e. why do we care about this erratum?
> >> >
> >> > First of all, with SMEP, it doesn't affect us. At all.
> >> >
> >> > Without SMEP, there would have to be a page accessible to userspace that
> >> > the
> >> > kernel executes instructions from. The only thing that I can think of
> >> > that's
> >> > normally user-accessible and not _controlled_ by userspace is the VDSO.
> >> > But
> >> > the kernel never actually executes from it, so it doesn't matter here.
> >> >
> >> > I've heard reports of (but no actual cases in the wild of) folks
> >> > remapping
> >> > kernel text to be user-accessible so that userspace can execute it, or of
> >> > having the kernel jump into user-provided libraries. Those are both
> >> > obviously
> >> > bonkers and would only be done with out-of-tree gunk, but even if
> >> > somebody did
> >> > that, they would be safe from the erratum, with this workaround.
> >>
> >> I'm not convinced this is worth adding any code for, though. If someone
> >> adds
> >> out of tree crap that does this and manually turns off SMEP, I think they
> >> should
> >> get to keep both pieces. Frankly, I think I'd *prefer* if the kernel
> >> crashed
> >> when calling user addresses like that just to discourage it.
> >
> > So the thing is, this doesn't have to be any (or much) code per se: my
> > suggestion
> > was to make MPX depend on SMEP on the Kconfig level, so that it's not
> > possible to
> > build MPX without having SMEP.
>
> I don't think I understand that suggestion. How can Kconfig protect against:
>
> qemu -cpu host,-smep
>
> ?
Right, it cannot - but I think the latest patch was pretty close and pretty
simple.
Thanks,
Ingo
