This patch series adds support for Clang's Shadow Call Stack (SCS) mitigation, which uses a separately allocated shadow stack to protect against return address overwrites. More information can be found here: https://clang.llvm.org/docs/ShadowCallStack.html SCS provides better protection against traditional buffer overflows than CONFIG_STACKPROTECTOR_*, but it should be noted that SCS security guarantees in the kernel differ from the ones documented for user space. The kernel must store addresses of shadow stacks used by other tasks and interrupt handlers in memory, which means an attacker capable reading and writing arbitrary memory may be able to locate them and hijack control flow by modifying shadow stacks that are not currently in use. SCS is currently supported only on arm64, where the compiler requires the x18 register to be reserved for holding the current task's shadow stack pointer. Because of this, the series includes patches from Ard to remove x18 usage from assembly code. With -fsanitize=shadow-call-stack, the compiler injects instructions to all non-leaf C functions to store the return address to the shadow stack, and unconditionally load it again before returning. As a result, SCS is currently incompatible with features that rely on modifying function return addresses to alter control flow, such as function graph tracing and kretprobes, although it may be possible to later change these feature to modify the shadow stack instead. A copy of the return address is still kept in the kernel stack for compatibility with stack unwinding, for example. SCS has a minimal performance overhead, but allocating shadow stacks increases kernel memory usage. The feature is therefore mostly useful on hardware that lacks support for PAC instructions. Changes in v2: - Changed Ard's KVM patch to use x29 instead of x18 for the guest context, which makes restore_callee_saved_regs cleaner - Updated help text (and commit messages) to point out differences in security properties compared to user space SCS - Cleaned up config options: removed the ROP protection choice, replaced the CC_IS_CLANG dependency with an arch-specific cc-option test, and moved disabling of incompatible config options to an arch-specific Kconfig - Added CC_FLAGS_SCS, which are filtered out where needed instead of using DISABLE_SCS - Added a __has_feature guard around __noscs for older clang versions - Changed the shadow stack overflow check for vmapped SCS to use SCS_SIZE to avoid surprises when changing configs - Renamed SCS_GFP to GFP_SCS - Dropped the patch to reserve x18 unconditionally, it's now only reserved with SCS - Added a clarification why restoring x18 in the EFI RT wrapper is safe - Added a missing change to arch/arm64/include/asm/suspend.h, and a comment to arch/arm64/mm/proc.S to remind that struct cpu_suspend_ctx must be kept in sync with the code - Moved x18 loading/storing during a context switch to cpu_switch_to(), renamed scs_thread_switch() to scs_overflow_check(), and removed the now unused scs_load() - Added compile-time initialization for init_shadow_call_stack and removed scs_set_init_magic() Ard Biesheuvel (2): arm64/lib: copy_page: avoid x18 register in assembler code arm64: kernel: avoid x18 as an arbitrary temp register Sami Tolvanen (15): arm64: mm: don't use x18 in idmap_kpti_install_ng_mappings arm64: kvm: stop treating register x18 as caller save add support for Clang's Shadow Call Stack (SCS) scs: add accounting scs: add support for stack usage debugging kprobes: fix compilation without CONFIG_KRETPROBES arm64: disable function graph tracing with SCS arm64: disable kretprobes with SCS arm64: reserve x18 from general allocation with SCS arm64: preserve x18 when CPU is suspended arm64: efi: restore x18 if it was corrupted arm64: vdso: disable Shadow Call Stack arm64: kprobes: fix kprobes without CONFIG_KRETPROBES arm64: disable SCS for hypervisor code arm64: implement Shadow Call Stack Makefile | 6 + arch/Kconfig | 33 +++++ arch/arm64/Kconfig | 9 +- arch/arm64/Makefile | 4 + arch/arm64/include/asm/scs.h | 45 ++++++ arch/arm64/include/asm/stacktrace.h | 4 + arch/arm64/include/asm/suspend.h | 2 +- arch/arm64/include/asm/thread_info.h | 3 + arch/arm64/kernel/Makefile | 1 + arch/arm64/kernel/asm-offsets.c | 3 + arch/arm64/kernel/cpu-reset.S | 4 +- arch/arm64/kernel/efi-rt-wrapper.S | 7 +- arch/arm64/kernel/entry.S | 28 ++++ arch/arm64/kernel/head.S | 9 ++ arch/arm64/kernel/irq.c | 2 + arch/arm64/kernel/probes/kprobes.c | 2 + arch/arm64/kernel/process.c | 2 + arch/arm64/kernel/scs.c | 39 +++++ arch/arm64/kernel/smp.c | 4 + arch/arm64/kernel/vdso/Makefile | 2 +- arch/arm64/kvm/hyp/Makefile | 3 + arch/arm64/kvm/hyp/entry.S | 41 +++-- arch/arm64/lib/copy_page.S | 38 ++--- arch/arm64/mm/proc.S | 72 +++++---- drivers/base/node.c | 6 + fs/proc/meminfo.c | 4 + include/linux/compiler-clang.h | 6 + include/linux/compiler_types.h | 4 + include/linux/mmzone.h | 3 + include/linux/scs.h | 78 ++++++++++ init/init_task.c | 8 + kernel/Makefile | 1 + kernel/fork.c | 9 ++ kernel/kprobes.c | 38 ++--- kernel/sched/core.c | 2 + kernel/sched/sched.h | 1 + kernel/scs.c | 214 +++++++++++++++++++++++++++ mm/page_alloc.c | 6 + mm/vmstat.c | 3 + 39 files changed, 649 insertions(+), 97 deletions(-) create mode 100644 arch/arm64/include/asm/scs.h create mode 100644 arch/arm64/kernel/scs.c create mode 100644 include/linux/scs.h create mode 100644 kernel/scs.c -- 2.24.0.rc0.303.g954a862665-goog
