Messages in this thread | | | Subject | Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack | From | Alexandre Chartre <> | Date | Mon, 16 Nov 2020 19:10:22 +0100 |
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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.
alex.
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