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

From: Andy Lutomirski
Date: Thu May 05 2016 - 13:15:17 EST

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


> Secondly, even if you were right and if this erratum didn't affect us, I'm still
> happy to use pretty much any excuse to further simplify the x86 security state
> space. 'This erratum suggests that the hardware might be borken without SMEP' is
> excuse enough in my book to couple MPX with SMEP.

I'm fine with any variant of this patch. I just can't see any
scenario in which it matters, so I think it would also be okay to add
a comment somewhere and otherwise ignore it.