This series is a follow-up to Joey's Permission Overlay Extension (POE) series [1] that recently landed on mainline. The goal is to improve the way we handle the register that governs which pkeys/POIndex are accessible (POR_EL0) during signal delivery. As things stand, we may unexpectedly fail to write the signal frame on the stack because POR_EL0 is not reset before the uaccess operations. See patch 3 for more details and the main changes this series brings.
A similar series landed recently for x86/MPK [2]; the present series aims at aligning arm64 with x86. Worth noting: once the signal frame is written, POR_EL0 is still set to POR_EL0_INIT, granting access to pkey 0 only. This means that a program that sets up an alternate signal stack with a non-zero pkey will need some assembly trampoline to set POR_EL0 before invoking the real signal handler, as discussed here [3]. This is not ideal, but it makes experimentation with pkeys in signal handlers possible while waiting for a potential interface to control the pkey state when delivering a signal. See Pierre's reply [4] for more information about use-cases and a potential interface.
The x86 series also added kselftests to ensure that no spurious SIGSEGV occurs during signal delivery regardless of which pkey is accessible at the point where the signal is delivered. This series adapts those kselftests to allow running them on arm64 (patch 4-5).
Finally patch 2 is a clean-up following feedback on Joey's series [5].
I have tested this series on arm64 and x86_64 (booting and running the protection_keys and pkey_sighandler_tests mm kselftests).
v1..v2: * In setup_rt_frame(), ensured that POR_EL0 is reset to its original value if we fail to deliver the signal (addresses Catalin's concern [6]). * Renamed *unpriv_access* to *user_access* in patch 3 (suggestion from Dave). * Made what patch 1-2 do explicit in the commit message body (suggestion from Dave).
- Kevin
[1] https://lore.kernel.org/linux-arm-kernel/20240822151113.1479789-1-joey.gouly... [2] https://lore.kernel.org/lkml/20240802061318.2140081-1-aruna.ramakrishna@orac... [3] https://lore.kernel.org/lkml/CABi2SkWxNkP2O7ipkP67WKz0-LV33e5brReevTTtba6oKU... [4] https://lore.kernel.org/linux-arm-kernel/87plns8owh.fsf@arm.com/ [5] https://lore.kernel.org/linux-arm-kernel/20241015114116.GA19334@willie-the-t... [6] https://lore.kernel.org/linux-arm-kernel/Zw6D2waVyIwYE7wd@arm.com/
Cc: akpm@linux-foundation.org Cc: anshuman.khandual@arm.com Cc: aruna.ramakrishna@oracle.com Cc: broonie@kernel.org Cc: catalin.marinas@arm.com Cc: dave.hansen@linux.intel.com Cc: dave.martin@arm.com Cc: jeffxu@chromium.org Cc: joey.gouly@arm.com Cc: pierre.langlois@arm.com Cc: shuah@kernel.org Cc: sroettger@google.com Cc: will@kernel.org Cc: linux-kselftest@vger.kernel.org Cc: x86@kernel.org
Kevin Brodsky (5): arm64: signal: Remove unused macro arm64: signal: Remove unnecessary check when saving POE state arm64: signal: Improve POR_EL0 handling to avoid uaccess failures selftests/mm: Use generic pkey register manipulation selftests/mm: Enable pkey_sighandler_tests on arm64
arch/arm64/kernel/signal.c | 95 +++++++++++++--- tools/testing/selftests/mm/Makefile | 8 +- tools/testing/selftests/mm/pkey-arm64.h | 1 + tools/testing/selftests/mm/pkey-x86.h | 2 + .../selftests/mm/pkey_sighandler_tests.c | 101 +++++++++++++----- 5 files changed, 162 insertions(+), 45 deletions(-)