| Message ID | 20200128184934.77625-1-samitolvanen@google.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | add support for Clang's Shadow Call Stack | expand |
Hi Sami, On 28/01/2020 18:49, Sami Tolvanen wrote: > 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 inactive 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. I found I had to add: | KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS)) to drivers/firmware/efi/libstub/Makefile, to get this going. I don't think there is much point supporting SCS for the EFIstub, its already isolated from the rest of the kernel's C code by the __efistub symbol prefix machinery, and trying to use it would expose us to buggy firmware at a point we can't handle it! I can send a patch if its easier for you, Thanks, James
On Tue, Feb 11, 2020 at 5:57 AM James Morse <james.morse@arm.com> wrote: > I found I had to add: > | KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS)) > > to drivers/firmware/efi/libstub/Makefile, to get this going. Ah, good catch! > I don't think there is much point supporting SCS for the EFIstub, its already isolated > from the rest of the kernel's C code by the __efistub symbol prefix machinery, and trying > to use it would expose us to buggy firmware at a point we can't handle it! Yes, fully agreed. > I can send a patch if its easier for you, It's not a problem, I will include a patch for this in v8. Sami
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 inactive 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. 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 in the kernel stack to alter control flow, such as function graph tracing, although it may be possible to later change these features 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 v7: - Changed irq_stack_entry/exit to store the shadow stack pointer in x24 instead of x20 as kernel_entry uses x20-x23 to store data that can be used later. Updated the comment as well. - Changed the Makefile in arch/arm64/kvm/hyp to also filter out -ffixed-x18. - Changed SHADOW_CALL_STACK to depend on !FUNCTION_GRAPH_TRACER instead of not selecting HAVE_FUNCTION_GRAPH_TRACER with SCS. - Removed ifdefs from the EFI wrapper and updated the comment to explain why we are restoring x18. - Rebased as Ard's x18 patches that were part of this series have already been merged. Changes in v6: - Updated comment in the EFI RT wrapper to include the explanation from the commit message. - Fixed the SHADOW_CALL_STACK_VMAP config option and the compilation errors in scs_init_irq() - Updated the comment in entry.S to Mark's suggestion - Fixed the WARN_ON in scs_init() to trip only when the return value for cpuhp_setup_state() is < 0. - Removed ifdefs from the code in arch/arm64/kernel/scs.c and added separate shadow stacks for the SDEI handler Changes in v5: - Updated the comment in __scs_base() to Mark's suggestion - Changed all instances of uintptr_t to unsigned long - Added allocation poisoning for KASAN to catch unintentional shadow stack accesses; moved set_set_magic before poisoning and switched scs_used() and scs_corrupted() to access the buffer using READ_ONCE_NOCHECK() instead - Changed scs_free() to check for NULL instead of zero - Renamed SCS_CACHE_SIZE to NR_CACHED_SCS - Added a warning if cpuhp_setup_state fails in scs_init() - Dropped patches disabling kretprobes after confirming there's no functional conflict with SCS instrumentation - Added an explanation to the commit message why function graph tracing and SCS are incompatible - Removed the ifdefs from arch/arm64/mm/proc.S and added comments explaining why we are saving and restoring x18 - Updated scs_check_usage format to include process information Changes in v4: - Fixed authorship for Ard's patches - Added missing commit messages - Commented code that clears SCS from thread_info - Added a comment about SCS_END_MAGIC being non-canonical Changes in v3: - Switched to filter-out for removing SCS flags in Makefiles - Changed the __noscs attribute to use __no_sanitize__("...") instead of no_sanitize("...") - Cleaned up inline function definitions and moved task_scs() into a macro - Cleaned up scs_free() and scs_magic() - Moved SCS initialization into dup_task_struct() and removed the now unused scs_task_init() - Added comments to __scs_base() and scs_task_reset() to better document design choices - Changed copy_page to make the offset and bias explicit 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 Sami Tolvanen (11): add support for Clang's Shadow Call Stack (SCS) scs: add accounting scs: add support for stack usage debugging scs: disable when function graph tracing is enabled 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: disable SCS for hypervisor code arm64: implement Shadow Call Stack arm64: scs: add shadow stacks for SDEI Makefile | 6 + arch/Kconfig | 35 ++++ arch/arm64/Kconfig | 5 + arch/arm64/Makefile | 4 + arch/arm64/include/asm/scs.h | 39 +++++ 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/efi-rt-wrapper.S | 11 +- arch/arm64/kernel/entry.S | 46 ++++- arch/arm64/kernel/head.S | 9 + arch/arm64/kernel/irq.c | 2 + arch/arm64/kernel/process.c | 2 + arch/arm64/kernel/scs.c | 114 +++++++++++++ arch/arm64/kernel/sdei.c | 7 + arch/arm64/kernel/smp.c | 4 + arch/arm64/kernel/vdso/Makefile | 2 +- arch/arm64/kvm/hyp/Makefile | 3 + arch/arm64/mm/proc.S | 14 ++ 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 | 57 +++++++ init/init_task.c | 8 + kernel/Makefile | 1 + kernel/fork.c | 9 + kernel/sched/core.c | 2 + kernel/scs.c | 246 +++++++++++++++++++++++++++ mm/page_alloc.c | 6 + mm/vmstat.c | 3 + 33 files changed, 661 insertions(+), 6 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 base-commit: b0be0eff1a5ab77d588b76bd8b1c92d5d17b3f73