Am 16.06.21 um 20:30 schrieb Jason Ekstrand:
On Tue, Jun 15, 2021 at 3:41 AM Christian König ckoenig.leichtzumerken@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.
Christian.
--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.fre... IGT tests: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork....
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@amd.com Cc: Michel Dänzer michel@daenzer.net Cc: Dave Airlie airlied@redhat.com Cc: Bas Nieuwenhuizen bas@basnieuwenhuizen.nl Cc: Daniel Stone daniels@collabora.com Cc: mesa-dev@lists.freedesktop.org Cc: wayland-devel@lists.freedesktop.org Test-with: 20210524205225.872316-1-jason@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(-)