Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack

[Alexandre Chartre]

On 11/16/20 5:57 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
> <alexandre.chartre@xxxxxxxxxx> wrote:
> > When entering the kernel from userland, use the per-task PTI stack
> > instead of the per-cpu trampoline stack. Like the trampoline stack,
> > the PTI stack is mapped both in the kernel and in the user page-table.
> > Using a per-task stack which is mapped into the kernel and the user
> > page-table instead of a per-cpu stack will allow executing more code
> > before switching to the kernel stack and to the kernel page-table.
>
> Why?
 
When executing more code in the kernel, we are likely to reach a point
where we need to sleep while we are using the user page-table, so we need
to be using a per-thread stack.

> I can't immediately evaluate how nasty the page table setup is because
> it's not in this patch.

The page-table is the regular page-table as introduced by PTI. It is just
augmented with a few additional mapping which are in patch 11 (x86/pti:
Extend PTI user mappings).

>  But AFAICS the only thing that this enables is sleeping with user pagetables.

That's precisely the point, it allows to sleep with the user page-table.

> Do we really need to do that?

Actually, probably not with this particular patchset, because I do the page-table
switch at the very beginning and end of the C handler. I had some code where I
moved the page-table switch deeper in the kernel handler where you definitively
can sleep (for example, if you switch back to the user page-table before
exit_to_user_mode_prepare()).

So a first step should probably be to not introduce the per-task PTI trampoline stack,
and stick with the existing trampoline stack. The per-task PTI trampoline stack can
be introduced later when the page-table switch is moved deeper in the C handler and
we can effectively sleep while using the user page-table.

alex.