Re: [RFC PATCH 0/4] Linaro restricted heap

[Jens Wiklander]

Hi,

On Tue, Sep 24, 2024 at 01:13:18PM -0500, Andrew Davis wrote:
> On 9/23/24 1:33 AM, Dmitry Baryshkov wrote:
> > Hi,
> > 
> > On Fri, Aug 30, 2024 at 09:03:47AM GMT, Jens Wiklander wrote:
> > > Hi,
> > > 
> > > This patch set is based on top of Yong Wu's restricted heap patch set [1].
> > > It's also a continuation on Olivier's Add dma-buf secure-heap patch set [2].
> > > 
> > > The Linaro restricted heap uses genalloc in the kernel to manage the heap
> > > carvout. This is a difference from the Mediatek restricted heap which
> > > relies on the secure world to manage the carveout.
> > > 
> > > I've tried to adress the comments on [2], but [1] introduces changes so I'm
> > > afraid I've had to skip some comments.
> > 
> > I know I have raised the same question during LPC (in connection to
> > Qualcomm's dma-heap implementation). Is there any reason why we are
> > using generic heaps instead of allocating the dma-bufs on the device
> > side?
> > 
> > In your case you already have TEE device, you can use it to allocate and
> > export dma-bufs, which then get imported by the V4L and DRM drivers.
> > 
> 
> This goes to the heart of why we have dma-heaps in the first place.
> We don't want to burden userspace with having to figure out the right
> place to get a dma-buf for a given use-case on a given hardware.
> That would be very non-portable, and fail at the core purpose of
> a kernel: to abstract hardware specifics away.
> 
> Worse, the actual interface for dma-buf exporting changes from
> framework to framework (getting a dma-buf from DRM is different
> than V4L, and there would be yet another API for TEE, etc..)
> 
> Most subsystem don't need an allocator, they work just fine
> simply being only dma-bufs importers. Recent example being the
> IIO subsystem[0], for which some early posting included an
> allocator, but in the end, all that was needed was to consume
> buffers.
> 
> For devices that don't actually contain memory there is no
> reason to be an exporter. What most want is just to consume
> normal system memory. Or system memory with some constraints
> (e.g. contiguous, coherent, restricted, etc..).
> 
> > I have a feeling (I might be completely wrong here) that by using
> > generic dma-buf heaps we can easily end up in a situation when the
> > userspace depends heavily on the actual platform being used (to map the
> > platform to heap names). I think we should instead depend on the
> > existing devices (e.g. if there is a TEE device, use an IOCTL to
> > allocate secured DMA BUF from it, otherwise check for QTEE device,
> > otherwise check for some other vendor device).
> > 
> > The mental experiment to check if the API is correct is really simple:
> > Can you use exactly the same rootfs on several devices without
> > any additional tuning (e.g. your QEMU, HiKey, a Mediatek board, Qualcomm
> > laptop, etc)?
> > 
> 
> This is a great north star to follow. And exactly the reason we should
> *not* be exposing device specific constraints to userspace. The constrains
> change based on the platform. So a userspace would have to also pick
> a different set of constraints based on each platform.
> 
> Userspace knows which subsystems it will attach a buffer, and the
> kernel knows what constraints those devices have on a given platform.
> Ideal case is then allocate from the one exporter, attach to various
> devices, and have the constraints solved at map time by the exporter
> based on the set of attached devices.
> 
> For example, on one platform the display needs contiguous buffers,
> but on a different platform the display can scatter-gather. So
> what heap should our generic application allocate from when it
> wants a buffer consumable by the display, CMA or System?
> Answer *should* be always use the generic exporter, and that
> exporter then picks the right backing type based on the platform.
> 
> Userspace shouldn't be dealing with any of these constraints
> (looking back, adding the CMA heap was probably incorrect,
> and the System heap should have been the only one. Idea back
> then was a userspace helper would show up to do the constraint
> solving and pick the right heap. That has yet to materialize and
> folks are still just hardcoding which heap to use..).
> 
> Same for this restricted heap, I'd like to explore if we can
> enhance the System heap such that when attached to the TEE framework,
> the backing memory is either made restricted by fire-walling,
> or allocating from a TEE carveout (based on platform).

So the exporter (you mentioned System heap) will somehow know how to
interact with the TEE subsystem to allocate suitable memory?

I suppose the memory could be from a static carveout, dynamic restricted
memory allocation, or how to turn normal memory into restricted memory
(fire-walling), depending on the platform.

> 
> This will mean more inter-subsystem coordination, but we can
> iterate on these in kernel interfaces. We cannot iterate on
> userspace interfaces, those have to be correct the first time.

Good point, this approach should make it easier for userspace.

Thanks,
Jens

> 
> Andrew
> 
> [0] https://www.kernel.org/doc/html/next/iio/iio_dmabuf_api.html
> 
> > > 
> > > This can be tested on QEMU with the following steps:
> > > repo init -u https://github.com/jenswi-linaro/manifest.git -m qemu_v8.xml \
> > >          -b prototype/sdp-v1
> > > repo sync -j8
> > > cd build
> > > make toolchains -j4
> > > make all -j$(nproc)
> > > make run-only
> > > # login and at the prompt:
> > > xtest --sdp-basic
> > > 
> > > https://optee.readthedocs.io/en/latest/building/prerequisites.html
> > > list dependencies needed to build the above.
> > > 
> > > The tests are pretty basic, mostly checking that a Trusted Application in
> > > the secure world can access and manipulate the memory.
> > 
> > - Can we test that the system doesn't crash badly if user provides
> >    non-secured memory to the users which expect a secure buffer?
> > 
> > - At the same time corresponding entities shouldn't decode data to the
> >    buffers accessible to the rest of the sytem.
> > 
> > > 
> > > Cheers,
> > > Jens
> > > 
> > > [1] https://lore.kernel.org/dri-devel/20240515112308.10171-1-yong.wu@xxxxxxxxxxxx/
> > > [2] https://lore.kernel.org/lkml/20220805135330.970-1-olivier.masse@xxxxxxx/
> > > 
> > > Changes since Olivier's post [2]:
> > > * Based on Yong Wu's post [1] where much of dma-buf handling is done in
> > >    the generic restricted heap
> > > * Simplifications and cleanup
> > > * New commit message for "dma-buf: heaps: add Linaro restricted dmabuf heap
> > >    support"
> > > * Replaced the word "secure" with "restricted" where applicable
> > > 
> > > Etienne Carriere (1):
> > >    tee: new ioctl to a register tee_shm from a dmabuf file descriptor
> > > 
> > > Jens Wiklander (2):
> > >    dma-buf: heaps: restricted_heap: add no_map attribute
> > >    dma-buf: heaps: add Linaro restricted dmabuf heap support
> > > 
> > > Olivier Masse (1):
> > >    dt-bindings: reserved-memory: add linaro,restricted-heap
> > > 
> > >   .../linaro,restricted-heap.yaml               |  56 ++++++
> > >   drivers/dma-buf/heaps/Kconfig                 |  10 ++
> > >   drivers/dma-buf/heaps/Makefile                |   1 +
> > >   drivers/dma-buf/heaps/restricted_heap.c       |  17 +-
> > >   drivers/dma-buf/heaps/restricted_heap.h       |   2 +
> > >   .../dma-buf/heaps/restricted_heap_linaro.c    | 165 ++++++++++++++++++
> > >   drivers/tee/tee_core.c                        |  38 ++++
> > >   drivers/tee/tee_shm.c                         | 104 ++++++++++-
> > >   include/linux/tee_drv.h                       |  11 ++
> > >   include/uapi/linux/tee.h                      |  29 +++
> > >   10 files changed, 426 insertions(+), 7 deletions(-)
> > >   create mode 100644 Documentation/devicetree/bindings/reserved-memory/linaro,restricted-heap.yaml
> > >   create mode 100644 drivers/dma-buf/heaps/restricted_heap_linaro.c
> > > 
> > > -- 
> > > 2.34.1
> > > 
> >