Messages in this thread | | | Date | Tue, 9 Jan 2018 05:22:01 +0100 | From | Willy Tarreau <> | Subject | Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX. |
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Hi Andy,
On Mon, Jan 08, 2018 at 03:05:48PM -0800, Andy Lutomirski wrote: > On 01/08/2018 09:03 AM, Dave Hansen wrote: > > On 01/08/2018 08:12 AM, Willy Tarreau wrote: > I vote per-thread.
The per-mm approach that Linus suggested doesn't look bad either and makes quite some sense.
> 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.
That's an excellent idea, eventhough I have no idea how to implement it :-)
> 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.
Yes I figured exactly this when I faced this protection!
> Turning it off totally defeats that, which hurts a bit.
I agree, that's why I'd like it to be conditional. Probably that with your idea of catching the page fault and the per-mm flag it would work quite well, but before being able to do this I still have a lot to explore :-/
> Also, Kees should be CC'd here.
Yes I've added him and you (and a few others) in CC of all forthcoming patches. Sorry for not adding you initially, I simply wanted to share a quick experiment and initiate a discussion.
Willy
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