[Mesa-dev] [PATCH 0/6] dma-buf: Add an API for exporting sync files (v12)

Christian König christian.koenig at amd.com
Thu Jun 17 07:37:36 UTC 2021

Am 16.06.21 um 20:30 schrieb Jason Ekstrand:
> On Tue, Jun 15, 2021 at 3:41 AM Christian König
> <ckoenig.leichtzumerken at gmail.com> wrote:
>> Hi Jason & Daniel,
>> maybe I should explain once more where the problem with this approach is
>> and why I think we need to get that fixed before we can do something
>> like this here.
>> To summarize what this patch here does is that it copies the exclusive
>> fence and/or the shared fences into a sync_file. This alone is totally
>> unproblematic.
>> The problem is what this implies. When you need to copy the exclusive
>> fence to a sync_file then this means that the driver is at some point
>> ignoring the exclusive fence on a buffer object.
> Not necessarily.  Part of the point of this is to allow for CPU waits
> on a past point in buffers timeline.  Today, we have poll() and
> GEM_WAIT both of which wait for the buffer to be idle from whatever
> GPU work is currently happening.  We want to wait on something in the
> past and ignore anything happening now.

Good point, yes that is indeed a valid use case.

> But, to the broader point, maybe?  I'm a little fuzzy on exactly where
> i915 inserts and/or depends on fences.
>> When you combine that with complex drivers which use TTM and buffer
>> moves underneath you can construct an information leak using this and
>> give userspace access to memory which is allocated to the driver, but
>> not yet initialized.
>> This way you can leak things like page tables, passwords, kernel data
>> etc... in large amounts to userspace and is an absolutely no-go for
>> security.
> Ugh...  Unfortunately, I'm really out of my depth on the implications
> going on here but I think I see your point.
>> That's why I'm said we need to get this fixed before we upstream this
>> patch set here and especially the driver change which is using that.
> Well, i915 has had uAPI for a while to ignore fences.

Yeah, exactly that's illegal.

At least the kernel internal fences like moving or clearing a buffer 
object needs to be taken into account before a driver is allowed to 
access a buffer.

Otherwise we have an information leak worth a CVE and that is certainly 
not something we want.

> Those changes are years in the past.  If we have a real problem here (not sure on
> that yet), then we'll have to figure out how to fix it without nuking
> uAPI.

Well, that was the basic idea of attaching flags to the fences in the 
dma_resv object.

In other words you clearly denote when you have to wait for a fence 
before accessing a buffer or you cause a security issue.


> --Jason
>> Regards,
>> Christian.
>> Am 10.06.21 um 23:09 schrieb Jason Ekstrand:
>>> Modern userspace APIs like Vulkan are built on an explicit
>>> synchronization model.  This doesn't always play nicely with the
>>> implicit synchronization used in the kernel and assumed by X11 and
>>> Wayland.  The client -> compositor half of the synchronization isn't too
>>> bad, at least on intel, because we can control whether or not i915
>>> synchronizes on the buffer and whether or not it's considered written.
>>> The harder part is the compositor -> client synchronization when we get
>>> the buffer back from the compositor.  We're required to be able to
>>> provide the client with a VkSemaphore and VkFence representing the point
>>> in time where the window system (compositor and/or display) finished
>>> using the buffer.  With current APIs, it's very hard to do this in such
>>> a way that we don't get confused by the Vulkan driver's access of the
>>> buffer.  In particular, once we tell the kernel that we're rendering to
>>> the buffer again, any CPU waits on the buffer or GPU dependencies will
>>> wait on some of the client rendering and not just the compositor.
>>> This new IOCTL solves this problem by allowing us to get a snapshot of
>>> the implicit synchronization state of a given dma-buf in the form of a
>>> sync file.  It's effectively the same as a poll() or I915_GEM_WAIT only,
>>> instead of CPU waiting directly, it encapsulates the wait operation, at
>>> the current moment in time, in a sync_file so we can check/wait on it
>>> later.  As long as the Vulkan driver does the sync_file export from the
>>> dma-buf before we re-introduce it for rendering, it will only contain
>>> fences from the compositor or display.  This allows to accurately turn
>>> it into a VkFence or VkSemaphore without any over- synchronization.
>>> This patch series actually contains two new ioctls.  There is the export
>>> one mentioned above as well as an RFC for an import ioctl which provides
>>> the other half.  The intention is to land the export ioctl since it seems
>>> like there's no real disagreement on that one.  The import ioctl, however,
>>> has a lot of debate around it so it's intended to be RFC-only for now.
>>> Mesa MR: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fmesa%2Fmesa%2F-%2Fmerge_requests%2F4037&data=04%7C01%7Cchristian.koenig%40amd.com%7Cb094e69c94814727939508d930f4ca94%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637594650220923783%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=xUwaiuw8Qt3d37%2F6NYOHU3K%2FMFwsvg79rno9zTNodRs%3D&reserved=0
>>> IGT tests: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork.freedesktop.org%2Fseries%2F90490%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7Cb094e69c94814727939508d930f4ca94%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637594650220923783%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=wygYaeVg%2BXmfeEUC45lWH5GgNBukl0%2B%2FMpT5u9LKYDI%3D&reserved=0
>>> v10 (Jason Ekstrand, Daniel Vetter):
>>>    - Add reviews/acks
>>>    - Add a patch to rename _rcu to _unlocked
>>>    - Split things better so import is clearly RFC status
>>> v11 (Daniel Vetter):
>>>    - Add more CCs to try and get maintainers
>>>    - Add a patch to document DMA_BUF_IOCTL_SYNC
>>>    - Generally better docs
>>>    - Use separate structs for import/export (easier to document)
>>>    - Fix an issue in the import patch
>>> v12 (Daniel Vetter):
>>>    - Better docs for DMA_BUF_IOCTL_SYNC
>>> v12 (Christian König):
>>>    - Drop the rename patch in favor of Christian's series
>>>    - Add a comment to the commit message for the dma-buf sync_file export
>>>      ioctl saying why we made it an ioctl on dma-buf
>>> Cc: Christian König <christian.koenig at amd.com>
>>> Cc: Michel Dänzer <michel at daenzer.net>
>>> Cc: Dave Airlie <airlied at redhat.com>
>>> Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
>>> Cc: Daniel Stone <daniels at collabora.com>
>>> Cc: mesa-dev at lists.freedesktop.org
>>> Cc: wayland-devel at lists.freedesktop.org
>>> Test-with: 20210524205225.872316-1-jason at jlekstrand.net
>>> Christian König (1):
>>>     dma-buf: Add dma_fence_array_for_each (v2)
>>> Jason Ekstrand (5):
>>>     dma-buf: Add dma_resv_get_singleton (v6)
>>>     dma-buf: Document DMA_BUF_IOCTL_SYNC (v2)
>>>     dma-buf: Add an API for exporting sync files (v12)
>>>     RFC: dma-buf: Add an extra fence to dma_resv_get_singleton_unlocked
>>>     RFC: dma-buf: Add an API for importing sync files (v7)
>>>    Documentation/driver-api/dma-buf.rst |   8 ++
>>>    drivers/dma-buf/dma-buf.c            | 103 +++++++++++++++++++++++++
>>>    drivers/dma-buf/dma-fence-array.c    |  27 +++++++
>>>    drivers/dma-buf/dma-resv.c           | 110 +++++++++++++++++++++++++++
>>>    include/linux/dma-fence-array.h      |  17 +++++
>>>    include/linux/dma-resv.h             |   2 +
>>>    include/uapi/linux/dma-buf.h         | 103 ++++++++++++++++++++++++-
>>>    7 files changed, 369 insertions(+), 1 deletion(-)

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