[PATCH 1/1] drm/amdkfd: Add IPC API

[Felix Kuehling]

Am 2020-07-13 um 11:28 p.m. schrieb Dave Airlie:
> On Tue, 14 Jul 2020 at 13:14, Felix Kuehling <Felix.Kuehling at amd.com> wrote:
>> This allows exporting and importing buffers. The API generates handles
>> that can be used with the HIP IPC API, i.e. big numbers rather than
>> file descriptors.
> First up why? I get the how.

The "why" is compatibility with HIP code ported from CUDA. The
equivalent CUDA IPC API works with handles that can be communicated
through e.g. a pipe or shared memory. You can't do that with file
descriptors.

https://docs.nvidia.com/cuda/cuda-runtime-api/group__CUDART__DEVICE.html#group__CUDART__DEVICE_1g8a37f7dfafaca652391d0758b3667539

https://docs.nvidia.com/cuda/cuda-runtime-api/group__CUDART__DEVICE.html#group__CUDART__DEVICE_1g01050a29fefde385b1042081ada4cde9


>
>> + * @share_handle is a 128 bit random number generated with
>> + * @get_random_bytes. This number should be very hard to guess.
>> + * Knowledge of the @share_handle implies authorization to access
>> + * the shared memory. User mode should treat it like a secret key.
>> + * It can be used to import this BO in a different process context
>> + * for IPC buffer sharing. The handle will be valid as long as the
>> + * underlying BO exists. If the same BO is exported multiple times,
> Do we have any examples of any APIs in the kernel that operate like
> this? That don't at least layer some sort of file permissions  and
> access control on top?

SystemV shared memory APIs (shmget, shmat) work similarly. There are
some permissions that can be specified by the exporter in shmget.
However, the handles are just numbers and much easier to guess (they are
32-bit integers). The most restrictive permissions would allow only the
exporting UID to attach to the shared memory segment.

I think DRM flink works similarly as well, with a global name IDR used
for looking up GEM objects using global object names.


>
> The reason fd's are good is that combined with unix sockets, you can't
> sniff it, you can't ptrace a process and find it, you can't write it
> out in a coredump and have someone access it later.

Arguably ptrace and core dumps give you access to all the memory
contents already. So you don't need the shared memory handle to access
memory in that case.


>
> To me this isn't secure design, it's obscure design, now I get to find
> you've likely shipped this in "downstream" ROCm already, and have
> customers running it.
>
> Maybe someone who knows security can ack merging this sort of uAPI
> design, I'm not confident in what it's doing is in any ways a good
> idea. I might have to ask some people to take a closer look.

Please do. We have tried to make this API as secure as possible within
the constraints of the user mode API we needed to implement.

Thanks,
  Felix


>
> Dave.