On Wed, Jan 10, 2018 at 11:54 AM, Linus Torvalds <[email protected]> wrote: > On Wed, Jan 10, 2018 at 11:28 AM, Willy Tarreau <[email protected]> wrote: >> Since we're going to keep running on the same PGD when returning to >> userspace for certain performance-critical tasks, we'll need the user >> pages to be executable. So this code disables the extra protection >> that was added consisting in marking user pages _PAGE_NX so that this >> pgd remains usable for userspace. > > Yeah, no. This is wrong. > > Sure, SMEP gives the same thing in most cases, but not for older CPU's. > > So NX is a really nice way to make sure that PTI really does protect > against user-space gadgets. > > We don't break that, and we definitely don't break that just because > of some broken notion of "let's make page table isolation per-thread". >
If we're going to have a thread without PTI off, that thread needs to run with the same page tables for kernel and user, so it needs NX off on the user part. I don't see any way around it. We could nix the entire concept of fine-grained PTI control, or we could make it require SMEP, I suppose.
