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

[Andy Lutomirski]

On Thu, May 5, 2016 at 11:40 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> 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
>>
>> ?
>
> Right, it cannot - but I think the latest patch was pretty close and pretty
> simple.

No objections from me for that patch.

>
> Thanks,
>
>         Ingo



-- 
Andy Lutomirski
AMA Capital Management, LLC