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

[Alexandre Chartre <alexandre.chartre@oracle.com>]

On 11/19/20 2:49 AM, Andy Lutomirski wrote:
> 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.
> 

That's the problem: I didn't want to return back to asm, so that the pagetable
switch can be done anywhere in the C handler.

So yes, returning to asm to switch the stack is the solution if we want to avoid
having per-task trampoline stack. The drawback is that this forces to do the
page-table switch at the beginning and end of the handler; the pagetable switch
cannot be moved deeper down into the C handler.

But that's probably a good first step (effectively just moving CR3 switch to C
without adding per-task trampoline stack). I will update the patches to do that,
and we can defer the per-task trampoline stack to later if there's an effective
need for it.

alex.