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

[Alexandre Chartre]

On 11/19/20 8:10 PM, Thomas Gleixner wrote:
> On Mon, Nov 16 2020 at 19:10, Alexandre Chartre wrote:
> > 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 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.
>
> Coming late, but this does not make any sense to me.
>
> Unless you map most of the kernel into the user page-table sleeping with
> the user page-table _cannot_ work. And if you do that you broke KPTI.
>
> You can neither pick arbitrary points in the C code of an exception
> handler to switch to the kernel mapping unless you mapped everything
> which might be touched before that into user space.
>
> How is that supposed to work?

Sorry I mixed up a few thing; I got confused with my own code which is not a
good sign...

It's not sleeping with the user page-table which, as you mentioned, doesn't
make sense, it's sleeping with the kernel page-table but with the PTI stack.

Basically, it is:
  - entering C code with (user page-table, PTI stack);
  - then it switches to the kernel page-table so we have (kernel page-table, PTI stack);
  - and then it switches to the kernel stack so we have (kernel page-table, kernel stack).

As this is all C code, some of which is executed with the PTI stack, we need the PTI stack
to be per-task so that the stack is preserved, in case that C code does a sleep/schedule
(no matter if this happens when using the PTI stack or the kernel stack).

alex.