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

[Alexandre Chartre]

On 5/14/19 5:23 PM, Andy Lutomirski wrote:
> On Tue, May 14, 2019 at 2:42 AM Alexandre Chartre
> <alexandre.chartre@xxxxxxxxxx> wrote:
> > On 5/14/19 10:34 AM, Andy Lutomirski wrote:
> > > > On May 14, 2019, at 1:25 AM, Alexandre Chartre <alexandre.chartre@xxxxxxxxxx> 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
> > > > > > <alexandre.chartre@xxxxxxxxxx> 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 donât 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": https://lkml.org/lkml/2018/11/22/1240
> >
> > 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 youâll have is vmalloc: the kernel stack is in the
> > > vmap range, and, if you allow access to vmap memory at all, youâll
> > > 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.


alex.