Re: [PATCH 30/39] x86/mm/pti: Clone entry-text again in pti_finalize()

[Joerg Roedel]

On Fri, Jul 13, 2018 at 04:21:45PM -0700, Andy Lutomirski wrote:
> On Wed, Jul 11, 2018 at 4:29 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> > From: Joerg Roedel <jroedel@xxxxxxx>
> >
> > The mapping for entry-text might have changed in the kernel
> > after it was cloned to the user page-table. Clone again
> > to update the user page-table to bring the mapping in sync
> > with the kernel again.
> 
> Can't we just defer pti_init() until after mark_readonly()?  What am I missing?

I tried that:

	https://lore.kernel.org/lkml/1530618746-23116-1-git-send-email-joro@xxxxxxxxxx/

But while testing it turned out that the kernel potentially executes
user-space code already before mark_readonly() has ran. This happens
when some initcall requests a module and the initrd is already
populated. Then usermode-helper kicks in and runs a userspace binary
already. When pti_init() has not run yet the user-space page-table is
completly empty, causing a triple fault when we switch to the user cr3
on the way to user-space.


Regards,

	Joerg