Messages in this thread | | | Date | Mon, 8 Jan 2018 18:50:28 +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 09:05 AM, Thomas Gleixner wrote: > > On Mon, 8 Jan 2018, 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 surely want to keep that as a safety measure. The entry code is simple to > > get wrong and running with the wrong pagetables by a silly mistake and > > thereby undoing the protection is surely not what we want. > > > > Need to find a free time slot to think about that. > > This does get immensely easier if we choose a mode at exec() (or fork() > even) and never change it. The prctl() _could_ just be a flag to tell > what your children should do.
Switching PTI on/off for a whole process would be nightmarish.
The simplest model is indeed child inheritance tree propagation - plus perhaps the ability for a thread to change its *own* PTI status, which obviously doesn't create any deep "process lookup" or cross-CPU complications.
( Note that here I only mean "simple to implement" - we might decide to not offer the ABI. )
Thanks,
Ingo
| |