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Date:   Fri, 17 Sep 2021 14:25:19 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Oded Gabbay <ogabbay@kernel.org>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Leon Romanovsky <leonro@nvidia.com>,
        Christoph Hellwig <hch@lst.de>,
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        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
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Subject: Re: [PATCH v6 0/2] Add p2p via dmabuf to habanalabs
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On Thu, Sep 16, 2021 at 03:44:25PM +0300, Oded Gabbay wrote:
> On Thu, Sep 16, 2021 at 3:31 PM Daniel Vetter <daniel@ffwll.ch> 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@ziepe.ca> 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)

Yeah that's essentially what I was looking for. This is defacto
hand-rolling the drm_master owner tracking stuff. As long as we have
something like this in place it should be fine I think.
-Daniel

> > 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch