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

[Alexandre Chartre]

On 11/16/20 7:34 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre
> <alexandre.chartre@xxxxxxxxxx> 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 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.
>
> Seems reasonable.

I finally remember why I have introduced a per-task PTI trampoline stack right now:
that's to be able to move the CR3 switch anywhere in the C handler. To do so, we need
a per-task stack to enter (and return) from the C handler as the handler can potentially
go to sleep.

Without a per-task trampoline stack, we would be limited to call the switch CR3 functions
from the assembly entry code before and after calling the C function handler (also called
from assembly).

alex.