On Fri, Jul 12, 2019 at 10:45:06AM -0600, Andy Lutomirski wrote: > > > > On Jul 12, 2019, at 10:37 AM, Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> wrote: > > > > > > > >> On 7/12/19 5:16 PM, Thomas Gleixner wrote: > >>> On Fri, 12 Jul 2019, Peter Zijlstra wrote: > >>>> On Fri, Jul 12, 2019 at 01:56:44PM +0200, Alexandre Chartre wrote: > >>>> > >>>> I think that's precisely what makes ASI and PTI different and independent. > >>>> PTI is just about switching between userland and kernel page-tables, while > >>>> ASI is about switching page-table inside the kernel. You can have ASI without > >>>> having PTI. You can also use ASI for kernel threads so for code that won't > >>>> be triggered from userland and so which won't involve PTI. > >>> > >>> PTI is not mapping kernel space to avoid speculation crap (meltdown). > >>> ASI is not mapping part of kernel space to avoid (different) speculation crap (MDS). > >>> > >>> See how very similar they are? > >>> > >>> Furthermore, to recover SMT for userspace (under MDS) we not only need > >>> core-scheduling but core-scheduling per address space. And ASI was > >>> specifically designed to help mitigate the trainwreck just described. > >>> > >>> By explicitly exposing (hopefully harmless) part of the kernel to MDS, > >>> we reduce the part that needs core-scheduling and thus reduce the rate > >>> the SMT siblngs need to sync up/schedule. > >>> > >>> But looking at it that way, it makes no sense to retain 3 address > >>> spaces, namely: > >>> > >>> user / kernel exposed / kernel private. > >>> > >>> Specifically, it makes no sense to expose part of the kernel through MDS > >>> but not through Meltdow. Therefore we can merge the user and kernel > >>> exposed address spaces. > >>> > >>> And then we've fully replaced PTI. > >>> > >>> So no, they're not orthogonal. > >> Right. If we decide to expose more parts of the kernel mappings then that's > >> just adding more stuff to the existing user (PTI) map mechanics. > > > > If we expose more parts of the kernel mapping by adding them to the existing > > user (PTI) map, then we only control the mapping of kernel sensitive data but > > we don't control user mapping (with ASI, we exclude all user mappings). > > > > How would you control the mapping of userland sensitive data and exclude them > > from the user map? > > As I see it, if we think part of the kernel is okay to leak to VM guests, > then it should think it’s okay to leak to userspace and versa. At the end > of the day, this may just have to come down to an administrator’s choice > of how careful the mitigations need to be. > > > Would you have the application explicitly identify sensitive > > data (like Andy suggested with a /dev/xpfo device)? > > That’s not really the intent of my suggestion. I was suggesting that > maybe we don’t need ASI at all if we allow VMs to exclude their memory > from the kernel mapping entirely. Heck, in a setup like this, we can > maybe even get away with turning PTI off under very, very controlled > circumstances. I’m not quite sure what to do about the kernel random > pools, though. I think KVM already allows excluding VMs memory from the kernel mapping with the "new guest mapping interface" [1]. The memory managed by the host can be restricted with "mem=" and KVM maps/unmaps the guest memory pages only when needed. It would be interesting to see if /dev/xpfo or even madvise(MAKE_MY_MEMORY_PRIVATE) can be made useful for multi-tenant container hosts. [1] https://lore.kernel.org/lkml/1548966284-28642-1-git-send-email-karahmed@xxxxxxxxx/ -- Sincerely yours, Mike.
