Add character devices that expose PAPR-specific system parameters and VPD to user space.
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. It also is too low-level in many cases: a single logical operation may require multiple sys_rtas() calls in succession to complete. This carries the risk that a process may exit while leaving an operation unfinished. It also means that callers must coordinate their use of the syscall for functions that cannot tolerate multiple concurrent clients, such as ibm,get-vpd. The solution presented here is to add a pair of small pseries-specific "drivers," one for VPD and one for system parameters. The new drivers expose these facilities to user space in ways that are compatible with lockdown and require no coordination between their clients. Since the ibm,get-vpd call sequence performed by the papr-vpd driver must be serialized against all other uses of the function, the series begins by adding some new APIs to the core RTAS support code for this purpose. Both drivers could potentially support poll() methods to notify clients of changes to parameters or VPD that happen due to partition migration and other events. But that should be safe to leave for later, assuming there's any interest. I have made changes to librtas to prefer the new interfaces and verified that existing clients work correctly with the new code. A draft PR for that work is here: https://github.com/ibm-power-utilities/librtas/pull/36 The user-space ABI has not changed since v1 of this series. I expect to propose at least one more small driver in this style for platform dump retrieval in a separate submission in the future. --- Changes in v4: - Fix latent issue in rtas_token_to_function() which causes boot-time crashes. - More small preparatory changes: a function table iterator and additional symbolic constants for RTAS function return values. - Use symbolic constants for ibm,get-vpd statuses in papr-vpd.c. - Add commentary to papr_vpd_ioc_create_handle() explaining choice to retrieve all VPD at file handle creation time instead of deferring it to the read handler. - Rebase on current powerpc/next. - Link to v3: https://lore.kernel.org/r/20231025-papr-sys_rtas-vs-lockdown-v3-0-5eb04559e...@linux.ibm.com Changes in v3: - Add new rtas_function_lock()/unlock() APIs and convert existing code to use them. - Convert papr-vpd to use rtas_function_lock()/unlock() instead of having sys_rtas() obtain a driver-private mutex. - Rebase on current powerpc/next. - Link to v2: https://lore.kernel.org/r/20231013-papr-sys_rtas-vs-lockdown-v2-0-ead01ce01...@linux.ibm.com Changes in v2: - Fix unused-but-set variable warning in papr-sysparm code. - Rebase on powerpc/next branch. - Link to v1: https://lore.kernel.org/r/20231006-papr-sys_rtas-vs-lockdown-v1-0-3a36bfb66...@linux.ibm.com Changes in v1 vs initial RFC: - Add papr-sysparm driver and tests. - Add a papr-miscdev.h uapi header. - Prevent sys_rtas() from interfering with papr-vpd call sequences. - Handle -4 ("VPD changed") status in papr-vpd. - Include string_helpers.h in papr-vpd.c, per Michal Suchánek - Link to RFC: https://lore.kernel.org/r/20230822-papr-sys_rtas-vs-lockdown-v1-0-932623cf3...@linux.ibm.com --- Nathan Lynch (13): powerpc/rtas: Add for_each_rtas_function() iterator powerpc/rtas: Fall back to linear search on failed token->function lookup powerpc/rtas: Add function return status constants powerpc/rtas: Factor out function descriptor lookup powerpc/rtas: Facilitate high-level call sequences powerpc/rtas: Serialize firmware activation sequences powerpc/rtas: Warn if per-function lock isn't held powerpc/uapi: Export papr-miscdev.h header powerpc/pseries: Add papr-vpd character driver for VPD retrieval powerpc/pseries/papr-sysparm: Validate buffer object lengths powerpc/pseries/papr-sysparm: Expose character device to user space powerpc/selftests: Add test for papr-vpd powerpc/selftests: Add test for papr-sysparm Documentation/userspace-api/ioctl/ioctl-number.rst | 4 + arch/powerpc/include/asm/papr-sysparm.h | 17 +- arch/powerpc/include/asm/rtas.h | 27 +- arch/powerpc/include/uapi/asm/papr-miscdev.h | 9 + arch/powerpc/include/uapi/asm/papr-sysparm.h | 58 +++ arch/powerpc/include/uapi/asm/papr-vpd.h | 22 + arch/powerpc/kernel/rtas.c | 182 +++++-- arch/powerpc/platforms/pseries/Makefile | 1 + arch/powerpc/platforms/pseries/papr-sysparm.c | 201 +++++++- arch/powerpc/platforms/pseries/papr-vpd.c | 536 +++++++++++++++++++++ tools/testing/selftests/powerpc/Makefile | 2 + .../selftests/powerpc/papr_sysparm/.gitignore | 1 + .../selftests/powerpc/papr_sysparm/Makefile | 12 + .../selftests/powerpc/papr_sysparm/papr_sysparm.c | 164 +++++++ .../testing/selftests/powerpc/papr_vpd/.gitignore | 1 + tools/testing/selftests/powerpc/papr_vpd/Makefile | 12 + .../testing/selftests/powerpc/papr_vpd/papr_vpd.c | 352 ++++++++++++++ 17 files changed, 1556 insertions(+), 45 deletions(-) --- base-commit: 707df298cbde200b939c70be2577b20775fe3345 change-id: 20230817-papr-sys_rtas-vs-lockdown-5c54505db792 Best regards, -- Nathan Lynch <nath...@linux.ibm.com>
