Adds support for a process to change its DEXCR value. The implementation is somewhat conservative; SBHE (speculative branch hint enable) is not exposed as an editable aspect because its effects can bleed over to other threads.
As explained in the third patch, this series changes the reset/inherit behaviour on exec. Previously there was a bitmask that tracked which aspects to copy from the current state vs resetting to a fixed default. This allows unprivileged processes to disable ROP protection for setuid binaries though, and is generally a weird interface to work with. The actual intent (and new implementation) tracks the exec value as an independent value that doesn't use the parent's DEXCR at all. The parent can control this reset value separately to its own DEXCR value. The other interesting part is the prctl interface. I've made the _SET, _CLEAR, _SET_ONEXEC, and _CLEAR_ONEXEC controls each a separate flag. This makes it easier to re-use with the getter prctl, as opposed to making set/clear a boolean value with a separate flag for if it's 'on-exec'. With separate flags you can return both the current and on-exec state in the getter in the same way you'd prepare it for the setter. There are still more features that can be added. A global switch to disable ROP protection could be useful as an option to prevent performance degradation. Also a prctl to randomise the hash key could be useful for when userspace knows a fork is not going to run any parent hashes. These features could be added in a future series (or the next version of this one :) ) though. Benjamin Gray (9): selftests/powerpc/dexcr: Add -no-pie to hashchk tests powerpc/dexcr: Track the DEXCR per-process powerpc/dexcr: Reset DEXCR value across exec powerpc/dexcr: Add DEXCR prctl interface selftests/powerpc/dexcr: Add DEXCR prctl interface test selftests/powerpc/dexcr: Attempt to enable NPHIE in hashchk selftest selftests/powerpc/dexcr: Add DEXCR config details to lsdexcr selftests/powerpc/dexcr: Add chdexcr utility Documentation: Document PowerPC kernel dynamic DEXCR interface Documentation/arch/powerpc/dexcr.rst | 141 +++++++++++- arch/powerpc/include/asm/processor.h | 13 +- arch/powerpc/kernel/Makefile | 1 + arch/powerpc/kernel/dexcr.c | 120 ++++++++++ arch/powerpc/kernel/process.c | 17 ++ arch/powerpc/kernel/ptrace/ptrace-view.c | 7 +- include/uapi/linux/prctl.h | 16 ++ kernel/sys.c | 16 ++ .../selftests/powerpc/dexcr/.gitignore | 2 + .../testing/selftests/powerpc/dexcr/Makefile | 8 +- .../testing/selftests/powerpc/dexcr/chdexcr.c | 110 +++++++++ tools/testing/selftests/powerpc/dexcr/dexcr.c | 40 ++++ tools/testing/selftests/powerpc/dexcr/dexcr.h | 57 +++++ .../selftests/powerpc/dexcr/dexcr_test.c | 213 ++++++++++++++++++ .../selftests/powerpc/dexcr/hashchk_test.c | 8 +- .../testing/selftests/powerpc/dexcr/lsdexcr.c | 103 ++++++--- 16 files changed, 823 insertions(+), 49 deletions(-) create mode 100644 arch/powerpc/kernel/dexcr.c create mode 100644 tools/testing/selftests/powerpc/dexcr/chdexcr.c create mode 100644 tools/testing/selftests/powerpc/dexcr/dexcr_test.c -- 2.44.0