Re: [PATCH 2/3] x86/mm/pti: Don't clear permissions in pti_clone_pmd()

[Thomas Gleixner]

On Tue, 7 Aug 2018, Andy Lutomirski wrote:

> On Tue, Aug 7, 2018 at 11:34 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> > On 08/07/2018 03:24 AM, Joerg Roedel wrote:
> >> The function sets the global-bit on cloned PMD entries,
> >> which only makes sense when the permissions are identical
> >> between the user and the kernel page-table.
> >>
> >> Further, only write-permissions are cleared for entry-text
> >> and kernel-text sections, which are not writeable anyway.
> >
> > I think this patch is correct, but I'd be curious if Andy remembers why
> > we chose to clear _PAGE_RW on these things.  It might have been that we
> > were trying to say that the *entry* code shouldn't write to this stuff,
> > regardless of whether the normal kernel can.
> >
> > But, either way, I agree with the logic here that Global pages must
> > share permissions between both mappings, so feel free to add my Ack.  I
> > just want to make sure Andy doesn't remember some detail I'm forgetting.
> 
> I suspect it's because we used to (and maybe still do) initialize the
> user tables before mark_read_only().

We still do that because we need the entry stuff working for interrupts
early on. We now repeat the clone after mark_ro so the mask RW is not
longer required.

Thanks,

	tglx