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

[Andy Lutomirski]

On Tue, Nov 17, 2020 at 7:07 AM Alexandre Chartre
<alexandre.chartre@xxxxxxxxxx> wrote:
>
>
>
> 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).

The noinstr part of the C entry code won't sleep.

--Andy