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

Jerome Glisse
2018-09-18T13:03:38Z

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.

Cheers,
Jérôme