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

Alexandre Chartre
2020-11-17T17:00:46Z

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.