On Thu, May 5, 2016 at 11:40 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * 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.
No objections from me for that patch. > > Thanks, > > Ingo -- Andy Lutomirski AMA Capital Management, LLC
