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

Andy Lutomirski
2020-11-19T01:49:43Z

On Tue, Nov 17, 2020 at 8:59 AM Alexandre Chartre
<alexandre.chartre@oracle.com> wrote:
>
>
>
> On 11/17/20 4:52 PM, Andy Lutomirski wrote:
> > On Tue, Nov 17, 2020 at 7:07 AM Alexandre Chartre
> > <alexandre.chartre@oracle.com> wrote:
> >>
> >>
> >>
> >> On 11/16/20 7:34 PM, Andy Lutomirski wrote:
> >>> On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre
> >>> <alexandre.chartre@oracle.com> wrote:
> >>>>
> >>>>
> >>>> On 11/16/20 5:57 PM, Andy Lutomirski wrote:
> >>>>> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre
> >>>>> <alexandre.chartre@oracle.com> 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.
> >
>
> But the noinstr part of the handler can sleep, and if it does we will need to
> preserve the trampoline stack (even if we switch to the per-task kernel stack to
> execute the noinstr part).
>
> Example:
>
> #define DEFINE_IDTENTRY(func) \
> static __always_inline void __##func(struct pt_regs *regs); \
> \
> __visible noinstr void func(struct pt_regs *regs) \
> { \
> irqentry_state_t state; -+ \
> | \
> user_pagetable_escape(regs); | use trampoline stack (1)
> state = irqentry_enter(regs); | \
> instrumentation_begin(); -+ \
> run_idt(__##func, regs); |===| run __func() on kernel stack (this can sleep)
> instrumentation_end(); -+ \
> irqentry_exit(regs, state); | use trampoline stack (2)
> user_pagetable_return(regs); -+ \
> }
>
> Between (1) and (2) we need to preserve and use the same trampoline stack
> in case __func() went sleeping.
>

Why? Right now, we have the percpu entry stack, and we do just fine
if we enter on one percpu stack and exit from a different one. We
would need to call from asm to C on the entry stack, return back to
asm, and then switch stacks.