Messages in this thread | | | From | Kees Cook <> | Date | Mon, 8 Jan 2018 15:09:00 -0800 | Subject | Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX. |
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On Mon, Jan 8, 2018 at 3:05 PM, Andy Lutomirski <luto@kernel.org> wrote: > On 01/08/2018 09:03 AM, Dave Hansen wrote: >> >> On 01/08/2018 08:12 AM, Willy Tarreau 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. >>> >>> Note: it isn't necessarily the best approach, but one way or another >>> if we want to be able to return to userspace from the kernel, >>> we'll have to have this executable anyway. Another approach >>> might consist in using another pgd for userland+kernel but >>> the current core really looks like an extra careful measure >>> to catch early bugs if any. >> >> >> I don't like this. >> >> I think the prctl() should apply to an entire process, not to a thread. >> If it applies to a process, you can unpoison the PGD. I even had code >> to do this in an earlier version of the (whole system) runtime PTI >> on/off stuff. >> >> Why are you even posting half-baked hacks like this now? Is there >> something super-pressing about this set that we need to lock in a new >> ABI now? >> > > I vote per-thread. > > Anyway, we can easily sync the NX-clearing: just catch the spurious page > fault and clear the bit. Avoiding infinite loops will need a bit of > thought, but it's surely doable. > > Or we set a per-mm flag saying "no NX", then do synchronize_sched() or > similar if we were the first to set it (or take the pagetable lock), then > clear all the NX bits. Again, needs some care, but doable. > > FWIW, the NX trick quite nicely emulates SMEP on non-SMEP hardware, which is > fantastic for Spectre resistance and general hardening. Turning it off > totally defeats that, which hurts a bit. > > Also, Kees should be CC'd here.
Please please keep the NX. As mentioned, this gets us emulated SMEP for "free" with PTI, which Linux has needed for years.
-Kees
-- Kees Cook Pixel Security
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