[PATCH v8 00/12] add support for Clang's Shadow Call Stack

From: Sami Tolvanen
Date: Tue Feb 18 2020 - 19:08:26 EST

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:


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

Changes in v8:
- Added __noscs to __hyp_text instead of filtering SCS flags from
the entire arch/arm64/kvm/hyp directory.
- Added a patch to filter out -ffixed-x18 and SCS flags from the
EFI stub.

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
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
- 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

Sami Tolvanen (12):
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
efi/libstub: disable SCS

Makefile | 6 +
arch/Kconfig | 35 ++++
arch/arm64/Kconfig | 5 +
arch/arm64/Makefile | 4 +
arch/arm64/include/asm/kvm_hyp.h | 2 +-
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/mm/proc.S | 14 ++
drivers/base/node.c | 6 +
drivers/firmware/efi/libstub/Makefile | 3 +
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 +
34 files changed, 662 insertions(+), 7 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: 2b72104b8c12504176fb5fc1442d6e54e31e338b