Re: [PATCH] [RFC] x86: work around MPX Erratum

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Tue, May 3, 2016 at 11:44 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> On Tue, May 3, 2016 at 2:43 PM, Dave Hansen <dave.hansen@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