Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive

Kenneth Lee
2018-09-21T10:07:37Z

On Thu, Sep 20, 2018 at 10:23:40AM -0400, Jerome Glisse wrote:
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> Date: Thu, 20 Sep 2018 10:23:40 -0400
> From: Jerome Glisse <jglisse@redhat.com>
> To: Kenneth Lee <liguozhu@hisilicon.com>
> CC: Kenneth Lee <nek.in.cn@gmail.com>, Alex Williamson
> <alex.williamson@redhat.com>, Herbert Xu <herbert@gondor.apana.org.au>,
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> linux-doc@vger.kernel.org, Sanjay Kumar <sanjay.k.kumar@intel.com>, Hao
> Fang <fanghao11@huawei.com>, linux-kernel@vger.kernel.org,
> linuxarm@huawei.com, iommu@lists.linux-foundation.org, "David S . Miller"
> <davem@davemloft.net>, linux-crypto@vger.kernel.org, Zhou Wang
> <wangzhou1@hisilicon.com>, Philippe Ombredanne <pombredanne@nexb.com>,
> Thomas Gleixner <tglx@linutronix.de>, Zaibo Xu <xuzaibo@huawei.com>,
> linux-accelerators@lists.ozlabs.org, Lu Baolu <baolu.lu@linux.intel.com>
> Subject: Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive
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> On Thu, Sep 20, 2018 at 01:55:43PM +0800, Kenneth Lee wrote:
> > On Tue, Sep 18, 2018 at 09:03:14AM -0400, Jerome Glisse wrote:
> > > On Tue, Sep 18, 2018 at 02:00:14PM +0800, Kenneth Lee wrote:
> > > > On Mon, Sep 17, 2018 at 08:37:45AM -0400, Jerome Glisse wrote:
> > > > > On Mon, Sep 17, 2018 at 04:39:40PM +0800, Kenneth Lee wrote:
> > > > > > On Sun, Sep 16, 2018 at 09:42:44PM -0400, Jerome Glisse wrote:
> > > > > > > So i want to summarize issues i have as this threads have dig deep into
> > > > > > > details. For this i would like to differentiate two cases first the easy
> > > > > > > one when relying on SVA/SVM. Then the second one when there is no SVA/SVM.
> > > > > >
> > > > > > Thank you very much for the summary.
> > > > > >
> > > > > > > In both cases your objectives as i understand them:
> > > > > > >
> > > > > > > [R1]- expose a common user space API that make it easy to share boiler
> > > > > > > plate code accross many devices (discovering devices, opening
> > > > > > > device, creating context, creating command queue ...).
> > > > > > > [R2]- try to share the device as much as possible up to device limits
> > > > > > > (number of independant queues the device has)
> > > > > > > [R3]- minimize syscall by allowing user space to directly schedule on the
> > > > > > > device queue without a round trip to the kernel
> > > > > > >
> > > > > > > I don't think i missed any.
> > > > > > >
> > > > > > >
> > > > > > > (1) Device with SVA/SVM
> > > > > > >
> > > > > > > For that case it is easy, you do not need to be in VFIO or part of any
> > > > > > > thing specific in the kernel. There is no security risk (modulo bug in
> > > > > > > the SVA/SVM silicon). Fork/exec is properly handle and binding a process
> > > > > > > to a device is just couple dozen lines of code.
> > > > > > >
> > > > > >
> > > > > > This is right...logically. But the kernel has no clear definition about "Device
> > > > > > with SVA/SVM" and no boiler plate for doing so. Then VFIO may become one of the
> > > > > > boiler plate.
> > > > > >
> > > > > > VFIO is one of the wrappers for IOMMU for user space. And maybe it is the only
> > > > > > one. If we add that support within VFIO, which solve most of the problem of
> > > > > > SVA/SVM, it will save a lot of work in the future.
> > > > >
> > > > > You do not need to "wrap" IOMMU for SVA/SVM. Existing upstream SVA/SVM user
> > > > > all do the SVA/SVM setup in couple dozen lines and i failed to see how it
> > > > > would require any more than that in your case.
> > > > >
> > > > >
> > > > > > I think this is the key confliction between us. So could Alex please say
> > > > > > something here? If the VFIO is going to take this into its scope, we can try
> > > > > > together to solve all the problem on the way. If it it is not, it is also
> > > > > > simple, we can just go to another way to fulfill this part of requirements even
> > > > > > we have to duplicate most of the code.
> > > > > >
> > > > > > Another point I need to emphasis here: because we have to replace the hardware
> > > > > > queue when fork, so it won't be very simple even in SVA/SVM case.
> > > > >
> > > > > I am assuming hardware queue can only be setup by the kernel and thus
> > > > > you are totaly safe forkwise as the queue is setup against a PASID and
> > > > > the child does not bind to any PASID and you use VM_DONTCOPY on the
> > > > > mmap of the hardware MMIO queue because you should really use that flag
> > > > > for that.
> > > > >
> > > > >
> > > > > > > (2) Device does not have SVA/SVM (or it is disabled)
> > > > > > >
> > > > > > > You want to still allow device to be part of your framework. However
> > > > > > > here i see fundamentals securities issues and you move the burden of
> > > > > > > being careful to user space which i think is a bad idea. We should
> > > > > > > never trus the userspace from kernel space.
> > > > > > >
> > > > > > > To keep the same API for the user space code you want a 1:1 mapping
> > > > > > > between device physical address and process virtual address (ie if
> > > > > > > device access device physical address A it is accessing the same
> > > > > > > memory as what is backing the virtual address A in the process.
> > > > > > >
> > > > > > > Security issues are on two things:
> > > > > > > [I1]- fork/exec, a process who opened any such device and created an
> > > > > > > active queue can transfer without its knowledge control of its
> > > > > > > commands queue through COW. The parent map some anonymous region
> > > > > > > to the device as a command queue buffer but because of COW the
> > > > > > > parent can be the first to copy on write and thus the child can
> > > > > > > inherit the original pages that are mapped to the hardware.
> > > > > > > Here parent lose control and child gain it.
> > > > > >
> > > > > > This is indeed an issue. But it remains an issue only if you continue to use the
> > > > > > queue and the memory after fork. We can use at_fork kinds of gadget to fix it in
> > > > > > user space.
> > > > >
> > > > > Trusting user space is a no go from my point of view.
> > > >
> > > > Can we dive deeper on this? Maybe we have different understanding on "Trusting
> > > > user space". As my understanding, "trusting user space" means "no matter what
> > > > the user process does, it should only hurt itself and anything give to it, no
> > > > the kernel and the other process".
> > > >
> > > > In our case, we create a channel between a process and the hardware. The process
> > > > can do whateven it like to its own memory the channel itself. It won't hurt the
> > > > other process and the kernel. And if the process fork a child and give the
> > > > channel to the child, it should the freedom on those resource remain within the
> > > > parent and the child. We are not trust another else.
> > > >
> > > > So do you refer to something else here?
> > > >
> > >
> > > I am refering to COW giving control to the child on to what happens
> > > in the parent from device point of view. A process hurting itself is
> > > fine, but if process now has to do special steps to protect from
> > > its child ie make sure that its childs can not hurt it, then i see
> > > that as a kernel bug. We can not ask user space process to know about
> > > all the thousands things that needs to be done to avoid issues with
> > > each device driver that the process may use (process can be totaly
> > > ignorant it is using a device if that device is use by a library it
> > > links to).
> > >
> > >
> > > Maybe what needs to happen will explain it better. So if userspace
> > > wants to be secure and protect itself from its child taking over the
> > > device through COW:
> > >
> > > - parent opened a device and is using it
> > >
> > > ... when parent wants to fork/exec it must:
> > >
> > > - parent _must_ flush device command queue and wait for the
> > > device to finish all pending jobs
> > >
> > > - parent _must_ unmap all range mapped to the device
> > >
> > > - parent should first close device file (unless you force set
> > > the CLOEXEC flag in the kernel)/it could also just flush
> > > but if you are not mapping the device command queue with
> > > VM_DONTCOPY then you should really be closing the device
> > >
> > > - now parent can fork/exec
> > >
> > > - parent must force COW ie write at least one byte to _all_
> > > pages in the range it wants to use with the device
> > >
> > > - parent re-open the device and re-initialize everything
> > >
> > >
> > > So this is putting quite a burden on a number of steps the parent
> > > _must_ do in order to keep control of memory exposed to the device.
> > > Not doing so can potentialy lead (it depends on who does the COW
> > > first) to the child taking control of memory use by the device,
> > > memory which was mapped by the parent before the child was created.
> > >
> > > Forcing CLOEXEC and VM_DONTCOPY somewhat help to simplify this,
> > > but you still need to stop, flush, unmap, before fork/exec and then
> > > re-init everything after.
> > >
> > >
> > > This is only when not using SVA/SVM, SVA/SVM is totaly fine from
> > > that point of view, no issues whatsoever.
> > >
> > > The solution i outlined in previous email do not have that above
> > > issue either, no need to rely on user space doing that dance.
> >
> > Thank you. I get the point. I'm now trying to see if I can solve the problem by
> > seting the vma to VM_SHARED when the portiong is "shared to the hardware".
> >
>
> FYI you can not convert a private anonymous vma to a share one it is
> illegal AFAIK at least i never heard of it and i am pretty sure the
> mm code would break if that happens. The user space is the one that
> decide what flags a vma has, not the kernel. Modulo few flags like
> DONTCOPY that can be force set by device driver for their vma ie vma
> of an mmap against the device file.
>
> If you don't like my solution here is another one but it is ugly and
> i think it is a bad idea. Again this is for the non SVA/SVM case and
> it assumes that the command queue is a mmap() of the device file:
> (A) register mmu_notifier
> (B) on _every_ invalidate range callback (_no matter_ what is the
> range) you zap the command queue mapped to user space (this is
> because you can't tell if the callback happens for a fork or
> something else) wait for the hardware queue to finish and clear
> all the iommu/dma mapping and you unpin all the pages ie
> put_page()
> (C) in device file vma page fault handler (vm_operations_struct.
> fault) you redo all the GUP and redo all the iommu/dma mapping
> and you remap the command queue to the userspace
>
> In (C) you can remap different command queue if you are in the child
> than in the parent (just look at current->mm and compare it to the
> one the command queue was created against).
>
> Note that this solution will be much __slower__ than what i described
> in my previous email. You will see that mmu notifier callbacks happens
> often and for tons of reasons and you will be _constantly_ undoing and
> redoing tons of work.
>
> This can be mitigated if you can differentiate reasons behind a mmu
> notifier callback. I posted patchset to do that a while ago and i
> intend to post it again in the next month or so. But this would still
> be a bad idea and solution i described previously is much more sane.
>
> Trying to pretend you can have the same thing as SVA/SVM without SVA
> is not a good idea. The non SVA case can still expose same API (like
> i described previously) but should go through kernel for _every_
> hardware submission (you can batch multiple commands in one submission).
> Not doing so is way too risky from my POV.
>
> Cheers,
> Jérôme

You are quite right. I tried all the way to find a leak in the mm system and
fail to. I will tried other way or maybe discard the non-SVA scenario.

Cheers,
--
-Kenneth(Hisilicon)