Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 17 Nov 2020 07:52:33 -0800 | Subject | Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack |
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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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