Message-ID: <20180920055543.GG207969@Turing-Arch-b>
Date:   Thu, 20 Sep 2018 13:55:43 +0800
From:   Kenneth Lee <liguozhu@...ilicon.com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     Kenneth Lee <nek.in.cn@...il.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        <kvm@...r.kernel.org>, Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>, <linux-doc@...r.kernel.org>,
        Sanjay Kumar <sanjay.k.kumar@...el.com>,
        "Hao Fang" <fanghao11@...wei.com>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>, <iommu@...ts.linux-foundation.org>,
        "David S . Miller" <davem@...emloft.net>,
        <linux-crypto@...r.kernel.org>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Zaibo Xu <xuzaibo@...wei.com>,
        <linux-accelerators@...ts.ozlabs.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive

On Tue, Sep 18, 2018 at 09:03:14AM -0400, Jerome Glisse wrote:
> Date: Tue, 18 Sep 2018 09:03:14 -0400
> From: Jerome Glisse <jglisse@...hat.com>
> To: Kenneth Lee <liguozhu@...ilicon.com>
> CC: Kenneth Lee <nek.in.cn@...il.com>, Alex Williamson
>  <alex.williamson@...hat.com>, Herbert Xu <herbert@...dor.apana.org.au>,
>  kvm@...r.kernel.org, Jonathan Corbet <corbet@....net>, Greg Kroah-Hartman
>  <gregkh@...uxfoundation.org>, Joerg Roedel <joro@...tes.org>,
>  linux-doc@...r.kernel.org, Sanjay Kumar <sanjay.k.kumar@...el.com>, Hao
>  Fang <fanghao11@...wei.com>, linux-kernel@...r.kernel.org,
>  linuxarm@...wei.com, iommu@...ts.linux-foundation.org, "David S . Miller"
>  <davem@...emloft.net>, linux-crypto@...r.kernel.org, Zhou Wang
>  <wangzhou1@...ilicon.com>, Philippe Ombredanne <pombredanne@...b.com>,
>  Thomas Gleixner <tglx@...utronix.de>, Zaibo Xu <xuzaibo@...wei.com>,
>  linux-accelerators@...ts.ozlabs.org, Lu Baolu <baolu.lu@...ux.intel.com>
> Subject: Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive
> User-Agent: Mutt/1.10.1 (2018-07-13)
> Message-ID: <20180918130314.GA3500@...hat.com>
> 
> 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".

> 
> Cheers,
> Jérôme

-- 
			-Kenneth(Hisilicon)

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