Messages in this thread | | | Subject | Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack | From | Alexandre Chartre <> | Date | Thu, 19 Nov 2020 09:05:53 +0100 |
| |
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.
| |