Messages in this thread | | | Date | Mon, 8 Jan 2018 18:21:02 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX. |
| |
* Dave Hansen <dave.hansen@intel.com> 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?
Arguably it was posted as an RFC patch-set, to get feedback early on.
The motivation is clear enough from the announcement I think: to speed up the haproxy performance almost two-fold, without sacrificing the overall security given by PTI against the Meltdown attack. haproxy does not require PTI, as it never executes untrusted code.
Thanks,
Ingo
| |