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

[Thomas Gleixner]

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.

Thanks,

	tglx