Re: [PATCH RFC 3/4] x86/pti: don't mark the user PGD with _PAGE_NX.

[Dave Hansen]

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?