Re: [PATCH v6 0/2] Add p2p via dmabuf to habanalabs

[Oded Gabbay]

On Thu, Sep 16, 2021 at 3:31 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Wed, Sep 15, 2021 at 10:45:36AM +0300, Oded Gabbay wrote:
> > On Tue, Sep 14, 2021 at 7:12 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > >
> > > On Tue, Sep 14, 2021 at 04:18:31PM +0200, Daniel Vetter wrote:
> > > > On Sun, Sep 12, 2021 at 07:53:07PM +0300, Oded Gabbay wrote:
> > > > > Hi,
> > > > > Re-sending this patch-set following the release of our user-space TPC
> > > > > compiler and runtime library.
> > > > >
> > > > > I would appreciate a review on this.
> > > >
> > > > I think the big open we have is the entire revoke discussions. Having the
> > > > option to let dma-buf hang around which map to random local memory ranges,
> > > > without clear ownership link and a way to kill it sounds bad to me.
> > > >
> > > > I think there's a few options:
> > > > - We require revoke support. But I've heard rdma really doesn't like that,
> > > >   I guess because taking out an MR while holding the dma_resv_lock would
> > > >   be an inversion, so can't be done. Jason, can you recap what exactly the
> > > >   hold-up was again that makes this a no-go?
> > >
> > > RDMA HW can't do revoke.
>
> Like why? I'm assuming when the final open handle or whatever for that MR
> is closed, you do clean up everything? Or does that MR still stick around
> forever too?
>
> > > So we have to exclude almost all the HW and several interesting use
> > > cases to enable a revoke operation.
> > >
> > > >   - For non-revokable things like these dma-buf we'd keep a drm_master
> > > >     reference around. This would prevent the next open to acquire
> > > >     ownership rights, which at least prevents all the nasty potential
> > > >     problems.
> > >
> > > This is what I generally would expect, the DMABUF FD and its DMA
> > > memory just floats about until the unrevokable user releases it, which
> > > happens when the FD that is driving the import eventually gets closed.
> > This is exactly what we are doing in the driver. We make sure
> > everything is valid until the unrevokable user releases it and that
> > happens only when the dmabuf fd gets closed.
> > And the user can't close it's fd of the device until he performs the
> > above, so there is no leakage between users.
>
> Maybe I got the device security model all wrong, but I thought Guadi is
> single user, and the only thing it protects is the system against the
> Gaudi device trhough iommu/device gart. So roughly the following can
> happen:
>
> 1. User A opens gaudi device, sets up dma-buf export
>
> 2. User A registers that with RDMA, or anything else that doesn't support
> revoke.
>
> 3. User A closes gaudi device
This can not happen without User A closing the FD of the dma-buf it exported.
We prevent User A from closing the device because when it exported the
dma-buf, the driver's code took a refcnt of the user's private
structure. You can see that in export_dmabuf_common() in the 2nd
patch. There is a call there to hl_ctx_get.
So even if User A calls close(device_fd), the driver won't let any
other user open the device until User A closes the fd of the dma-buf
object.

Moreover, once User A will close the dma-buf fd and the device is
released, the driver will scrub the device memory (this is optional
for systems who care about security).

And AFAIK, User A can't close the dma-buf fd once it registered it
with RDMA, without doing unregister.
This can be seen in ib_umem_dmabuf_get() which calls dma_buf_get()
which does fget(fd)


>
> 4. User B opens gaudi device, assumes that it has full control over the
> device and uploads some secrets, which happen to end up in the dma-buf
> region user A set up
>
> 5. User B extracts secrets.
>
> > > I still don't think any of the complexity is needed, pinnable memory
> > > is a thing in Linux, just account for it in mlocked and that is
> > > enough.
>
> It's not mlocked memory, it's mlocked memory and I can exfiltrate it.
> Mlock is fine, exfiltration not so much. It's mlock, but a global pool and
> if you didn't munlock then the next mlock from a completely different user
> will alias with your stuff.
>
> Or is there something that prevents that? Oded at least explain that gaudi
> works like a gpu from 20 years ago, single user, no security at all within
> the device.
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch