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SubjectRe: [RFC KVM 18/27] kvm/isolation: function to copy page table entries for percpu buffer

On 5/14/19 5:23 PM, Andy Lutomirski wrote:
> On Tue, May 14, 2019 at 2:42 AM Alexandre Chartre
> <> wrote:
>> On 5/14/19 10:34 AM, Andy Lutomirski wrote:
>>>> On May 14, 2019, at 1:25 AM, Alexandre Chartre <> wrote:
>>>>> On 5/14/19 9:09 AM, Peter Zijlstra wrote:
>>>>>> On Mon, May 13, 2019 at 11:18:41AM -0700, Andy Lutomirski wrote:
>>>>>> On Mon, May 13, 2019 at 7:39 AM Alexandre Chartre
>>>>>> <> wrote:
>>>>>>> pcpu_base_addr is already mapped to the KVM address space, but this
>>>>>>> represents the first percpu chunk. To access a per-cpu buffer not
>>>>>>> allocated in the first chunk, add a function which maps all cpu
>>>>>>> buffers corresponding to that per-cpu buffer.
>>>>>>> Also add function to clear page table entries for a percpu buffer.
>>>>>> This needs some kind of clarification so that readers can tell whether
>>>>>> you're trying to map all percpu memory or just map a specific
>>>>>> variable. In either case, you're making a dubious assumption that
>>>>>> percpu memory contains no secrets.
>>>>> I'm thinking the per-cpu random pool is a secrit. IOW, it demonstrably
>>>>> does contain secrits, invalidating that premise.
>>>> The current code unconditionally maps the entire first percpu chunk
>>>> (pcpu_base_addr). So it assumes it doesn't contain any secret. That is
>>>> mainly a simplification for the POC because a lot of core information
>>>> that we need, for example just to switch mm, are stored there (like
>>>> cpu_tlbstate, current_task...).
>>> I don’t think you should need any of this.
>> At the moment, the current code does need it. Otherwise it can't switch from
>> kvm mm to kernel mm: switch_mm_irqs_off() will fault accessing "cpu_tlbstate",
>> and then the page fault handler will fail accessing "current" before calling
>> the kvm page fault handler. So it will double fault or loop on page faults.
>> There are many different places where percpu variables are used, and I have
>> experienced many double fault/page fault loop because of that.
> Now you're experiencing what working on the early PTI code was like :)
> This is why I think you shouldn't touch current in any of this.
>>>> If the entire first percpu chunk effectively has secret then we will
>>>> need to individually map only buffers we need. The kvm_copy_percpu_mapping()
>>>> function is added to copy mapping for a specified percpu buffer, so
>>>> this used to map percpu buffers which are not in the first percpu chunk.
>>>> Also note that mapping is constrained by PTE (4K), so mapped buffers
>>>> (percpu or not) which do not fill a whole set of pages can leak adjacent
>>>> data store on the same pages.
>>> I would take a different approach: figure out what you need and put it in its
>>> own dedicated area, kind of like cpu_entry_area.
>> That's certainly something we can do, like Julian proposed with "Process-local
>> memory allocations":
>> That's fine for buffers allocated from KVM, however, we will still need some
>> core kernel mappings so the thread can run and interrupts can be handled.
>>> One nasty issue you’ll have is vmalloc: the kernel stack is in the
>>> vmap range, and, if you allow access to vmap memory at all, you’ll
>>> need some way to ensure that *unmap* gets propagated. I suspect the
>>> right choice is to see if you can avoid using the kernel stack at all
>>> in isolated mode. Maybe you could run on the IRQ stack instead.
>> I am currently just copying the task stack mapping into the KVM page table
>> (patch 23) when a vcpu is created:
>> err = kvm_copy_ptes(tsk->stack, THREAD_SIZE);
>> And this seems to work. I am clearing the mapping when the VM vcpu is freed,
>> so I am making the assumption that the same task is used to create and free
>> a vcpu.
> vCPUs are bound to an mm but not a specific task, right? So I think
> this is wrong in both directions.

I know, that was yet another shortcut for the POC, I assume there's a 1:1
mapping between a vCPU and task, but I think that's fair with qemu.

> Suppose a vCPU is created, then the task exits, the stack mapping gets
> freed (the core code tries to avoid this, but it does happen), and a
> new stack gets allocated at the same VA with different physical pages.
> Now you're toast :) On the flip side, wouldn't you crash if a vCPU is
> created and then run on a different thread?

Yes, that's why I have a safety net: before entering KVM isolation I always
check that the current task is mapped in the KVM address space, if not it
gets mapped.

> How important is the ability to enable IRQs while running with the KVM
> page tables?

I can't say, I would need to check but we probably need IRQs at least for
some timers. Sounds like you would really prefer IRQs to be disabled.


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