Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

[Daniel Vetter]

On Wed, Apr 25, 2018 at 09:07:07AM +0300, Oleksandr Andrushchenko wrote:
> On 04/24/2018 11:35 PM, Dongwon Kim wrote:
> > Had a meeting with Daniel and talked about bringing out generic
> > part of hyper-dmabuf to the userspace, which means we most likely
> > reuse IOCTLs defined in xen-zcopy for our use-case if we follow
> > his suggestion.
> I will still have kernel side API, so backends/frontends implemented
> in the kernel can access that functionality as well.
> > 
> > So assuming we use these IOCTLs as they are,
> > Several things I would like you to double-check..
> > 
> > 1. returning gref as is to the user space is still unsafe because
> > it is a constant, easy to guess and any process that hijacks it can easily
> > exploit the buffer. So I am wondering if it's possible to keep dmabuf-to
> > -gref or gref-to-dmabuf in kernel space and add other layers on top
> > of those in actual IOCTLs to add some safety.. We introduced flink like
> > hyper_dmabuf_id including random number but many says even that is still
> > not safe.
> Yes, it is generally unsafe. But even if we have implemented
> the approach you have in hyper-dmabuf or similar, what stops
> malicious software from doing the same with the existing gntdev UAPI?
> No need to brute force new UAPI if there is a simpler one.
> That being said, I'll put security aside at the first stage,
> but of course we can start investigating ways to improve
> (I assume you already have use-cases where security issues must
> be considered, so, probably you can tell more on what was investigated
> so far).

Maybe a bit more context here:

So in graphics we have this old flink approach for buffer sharing with
processes, and it's unsafe because way too easy to guess the buffer
handles. And anyone with access to the graphics driver can then import
that buffer object. We switched to file descriptor passing to make sure
only the intended recipient can import a buffer.

So at the vm->vm level it sounds like grefs are safe, because they're only
for a specific other guest (or sets of guests, not sure about). That means
security is only within the OS. For that you need to make sure that
unpriviledge userspace simply can't ever access a gref. If that doesn't
work out, then I guess we should improve the xen gref stuff to have a more
secure cookie.

> > 2. maybe we could take hypervisor-independent process (e.g. SGT<->page)
> > out of xen-zcopy and put those in a new helper library.
> I believe this can be done, but at the first stage I would go without
> that helper library, so it is clearly seen what can be moved to it later
> (I know that you want to run ACRN as well, but can I run it on ARM? ;)

There's already helpers for walking sgtables and adding pages/enumerating
pages. I don't think we need more.

> > 3. please consider the case where original DMA-BUF's first offset
> > and last length are not 0 and PAGE_SIZE respectively. I assume current
> > xen-zcopy only supports page-aligned buffer with PAGE_SIZE x n big.
> Hm, what is the use-case for that?

dma-buf is always page-aligned. That's a hard constraint of the linux
dma-buf interface spec.
-Daniel

> > thanks,
> > DW
> Thank you,
> Oleksandr
> > On Tue, Apr 24, 2018 at 02:59:39PM +0300, Oleksandr Andrushchenko wrote:
> > > On 04/24/2018 02:54 PM, Daniel Vetter wrote:
> > > > On Mon, Apr 23, 2018 at 03:10:35PM +0300, Oleksandr Andrushchenko wrote:
> > > > > On 04/23/2018 02:52 PM, Wei Liu wrote:
> > > > > > On Fri, Apr 20, 2018 at 02:25:20PM +0300, Oleksandr Andrushchenko wrote:
> > > > > > > > >       the gntdev.
> > > > > > > > > 
> > > > > > > > > I think this is generic enough that it could be implemented by a
> > > > > > > > > device not tied to Xen. AFAICT the hyper_dma guys also wanted
> > > > > > > > > something similar to this.
> > > > > > > > You can't just wrap random userspace memory into a dma-buf. We've just had
> > > > > > > > this discussion with kvm/qemu folks, who proposed just that, and after a
> > > > > > > > bit of discussion they'll now try to have a driver which just wraps a
> > > > > > > > memfd into a dma-buf.
> > > > > > > So, we have to decide either we introduce a new driver
> > > > > > > (say, under drivers/xen/xen-dma-buf) or extend the existing
> > > > > > > gntdev/balloon to support dma-buf use-cases.
> > > > > > > 
> > > > > > > Can anybody from Xen community express their preference here?
> > > > > > > 
> > > > > > Oleksandr talked to me on IRC about this, he said a few IOCTLs need to
> > > > > > be added to either existing drivers or a new driver.
> > > > > > 
> > > > > > I went through this thread twice and skimmed through the relevant
> > > > > > documents, but I couldn't see any obvious pros and cons for either
> > > > > > approach. So I don't really have an opinion on this.
> > > > > > 
> > > > > > But, assuming if implemented in existing drivers, those IOCTLs need to
> > > > > > be added to different drivers, which means userspace program needs to
> > > > > > write more code and get more handles, it would be slightly better to
> > > > > > implement a new driver from that perspective.
> > > > > If gntdev/balloon extension is still considered:
> > > > > 
> > > > > All the IOCTLs will be in gntdev driver (in current xen-zcopy terminology):
> > > I was lazy to change dumb to dma-buf, so put this notice ;)
> > > > >   - DRM_ICOTL_XEN_ZCOPY_DUMB_FROM_REFS
> > > > >   - DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS
> > > > >   - DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE
> > > > s/DUMB/DMA_BUF/ please. This is generic dma-buf, it has nothing to do with
> > > > the dumb scanout buffer support in the drm/gfx subsystem. This here can be
> > > > used for any zcopy sharing among guests (as long as your endpoints
> > > > understands dma-buf, which most relevant drivers do).
> > > Of course, please see above
> > > > -Daniel
> > > > 
> > > > > Balloon driver extension, which is needed for contiguous/DMA
> > > > > buffers, will be to provide new *kernel API*, no UAPI is needed.
> > > > > 
> > > > > > Wei.
> > > > > Thank you,
> > > > > Oleksandr
> > > > > _______________________________________________
> > > > > dri-devel mailing list
> > > > > dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch