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 ? > 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. --Andy
