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 20:55:33 +0100 |
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On 11/19/20 8:10 PM, Thomas Gleixner wrote: > On Mon, Nov 16 2020 at 19:10, Alexandre Chartre 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 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. > > Coming late, but this does not make any sense to me. > > Unless you map most of the kernel into the user page-table sleeping with > the user page-table _cannot_ work. And if you do that you broke KPTI. > > You can neither pick arbitrary points in the C code of an exception > handler to switch to the kernel mapping unless you mapped everything > which might be touched before that into user space. > > How is that supposed to work? >
Sorry I mixed up a few thing; I got confused with my own code which is not a good sign...
It's not sleeping with the user page-table which, as you mentioned, doesn't make sense, it's sleeping with the kernel page-table but with the PTI stack.
Basically, it is: - entering C code with (user page-table, PTI stack); - then it switches to the kernel page-table so we have (kernel page-table, PTI stack); - and then it switches to the kernel stack so we have (kernel page-table, kernel stack).
As this is all C code, some of which is executed with the PTI stack, we need the PTI stack to be per-task so that the stack is preserved, in case that C code does a sleep/schedule (no matter if this happens when using the PTI stack or the kernel stack).
alex.
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