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

    --Andy

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