This is a proposal for adding chardev-based access to a select subset of RTAS functions on the pseries platform.
The problem: important platform features are enabled on Linux VMs through the powerpc-specific rtas() syscall in combination with writeable mappings of /dev/mem. In typical usage, this is encapsulated behind APIs provided by the librtas library. This paradigm is incompatible with lockdown, which prohibits /dev/mem access. The solution I'm working on is to add a small pseries-specific "driver" for each functional area, exposing the relevant features to user space in ways that are compatible with lockdown. In most of these areas, I believe it's possible to change librtas to prefer the new chardev interfaces without disrupting existing users. I've broken down the affected functions into the following areas and priorities: High priority: * VPD retrieval. * System parameters: retrieval and update. Medium priority: * Platform dump retrieval. * Light path diagnostics (get/set-dynamic-indicator, get-dynamic-sensor-state, get-indices). Low priority (may never happen): * Error injection: would have to be carefully restricted. * Physical attestation: no known users. * LPAR perftools: no known users. Out of scope: * DLPAR (configure-connector et al): involves device tree updates which must be handled entirely in-kernel for lockdown. This is the object of a separate effort. See https://github.com/ibm-power-utilities/librtas/issues/29 for more details. In this RFC, I've included a single driver for VPD retrieval. Clients use ioctl() to obtain a file descriptor-based handle for the VPD they want. I think this could be a good model for the other areas too, but I'd like to get opinions on it. In the next iteration I expect to add a separate driver for system parameters. For reference, I floated a different approach for system parameters here: https://lore.kernel.org/linuxppc-dev/20220730000458.130938-1-nath...@linux.ibm.com/ --- Nathan Lynch (2): powerpc/pseries: papr-vpd char driver for VPD retrieval powerpc/selftests: add test for papr-vpd Documentation/userspace-api/ioctl/ioctl-number.rst | 2 + arch/powerpc/include/uapi/asm/papr-vpd.h | 29 ++ arch/powerpc/platforms/pseries/Makefile | 1 + arch/powerpc/platforms/pseries/papr-vpd.c | 353 +++++++++++++++++++++ tools/testing/selftests/powerpc/Makefile | 1 + .../testing/selftests/powerpc/papr_vpd/.gitignore | 1 + tools/testing/selftests/powerpc/papr_vpd/Makefile | 12 + .../testing/selftests/powerpc/papr_vpd/papr_vpd.c | 351 ++++++++++++++++++++ 8 files changed, 750 insertions(+) --- base-commit: d77497508a229529830850ba07e1e52596463d21 change-id: 20230817-papr-sys_rtas-vs-lockdown-5c54505db792 Best regards, -- Nathan Lynch <nath...@linux.ibm.com>
