Messages in this thread | | | Date | Thu, 5 May 2016 20:40:52 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] [RFC] x86: work around MPX Erratum |
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* 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
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