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

    alex.

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