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 20:37:03 +0100 |
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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. > > Where is the code that allocates and frees these stacks hiding? I > think I should at least read it.
Stacks are allocated/freed with the task stack, this code is unchanged (see alloc_thread_stack_node()). The trick is that I have doubled the THREAD_SIZE (patch 8 "x86/pti: Introduce per-task PTI trampoline stack"). Half the stack is a used as the kernel stack (mapped only in the kernel page-table), the other half is used as the PTI stack (mapped in the kernel and user page-table). The mapping to the user page-table is done in mm_map_task() in fork.c (patch 11 "x86/pti: Extend PTI user mappings").
alex.
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