[RFC PATCH v3 00/20] KCFI support

Sami Tolvanen
2022-06-10T23:35:16Z

KCFI is a proposed forward-edge control-flow integrity scheme for
Clang, which is more suitable for kernel use than the existing CFI
scheme used by CONFIG_CFI_CLANG. KCFI doesn't require LTO, doesn't
alter function references to point to a jump table, and won't break
function address equality. The latest LLVM patch is here:

https://reviews.llvm.org/D119296

This RFC series replaces the current arm64 CFI implementation with
KCFI and adds support for x86_64.

KCFI requires assembly functions that are indirectly called from C
code to be annotated with type identifiers. As type information is
only available in C, the compiler emits expected type identifiers into
the symbol table, so they can be referenced from assembly without
having to hardcode type hashes. Patch 6 adds helper macros for
annotating functions, and patches 8 and 18 add annotations.

In case of a type mismatch, KCFI always traps. To support error
handling, the compiler generates a .kcfi_traps section for x86_64,
which contains the locations of each trap, and for arm64, encodes
the necessary register information to the ESR. Patches 9 and 20 add
arch-specific error handlers.

To test this series, you'll need to compile your own Clang toolchain
with the patches linked above. You can also find the complete source
tree here:

https://github.com/samitolvanen/llvm-project/commits/kcfi-rfc-v3

This series is also available in GitHub:

https://github.com/samitolvanen/linux/commits/kcfi-rfc-v3

---
Changes in v3:
- Merged the patches that split CC_FLAGS_CFI from CC_FLAGS_LTO.

- Dropped the psci_initcall_t patch as Mark volunteered to send a
patch for this. Note that this patch is still needed to boot a
CFI kernel on certain arm64 systems:
https://lore.kernel.org/lkml/YoNhKaTT3EDukxXY@FVFF77S0Q05N/

- Added a patch to remove the now unnecessary workarounds with
CFI+ThinLTO in kallsyms.

- Added an lkdtm patch to ensure the test actually generates an
indirect call.

- Changed report_cfi_failure to clearly indicate if we failed to
decode target address.

- Switched to relative offsets for .kcfi_traps.

- On x86_64, moved CFI error handling from traps.c to cfi.c, and
as we only call memcpy indirectly w/ CONFIG_MODULES, ensured that
the compiler emits __kcfi_typeid_memcpy also without modules.

- On x86_64, added a check for the cmpl REX prefix to handle the
case where the compiler might not use r8-r15 registers for the
call target.

- On the compiler side, ensured that on x86_64 calls are emitted
immediately after the CFI check, fixed the __cfi_ preamble
linkage, and changed the compiler to emit relative offsets in
.kcfi_traps.

Changes in v2:
- Changed the compiler patch to encode arm64 target and type details
in the ESR, and updated the kernel error handling patch accordingly.

- Changed the compiler patch to embed the x86_64 type hash in a valid
instruction to avoid special casing objtool instruction decoding, and
added a __cfi_ symbol for the preamble. Changed the kernel error
handling and manual type annotations to match.

- Dropped the .kcfi_types section as that’s no longer needed by
objtool, and changed the objtool patch to simply ignore the __cfi_
preambles falling through.

- Dropped the .kcfi_traps section on arm64 as it’s no longer needed,
and moved the trap look-up code behind CONFIG_ARCH_USES_CFI_TRAPS,
which is selected only for x86_64.

- Dropped __nocfi attributes from arm64 code where CFI was disabled
due to address space confusion issues, and added type annotations to
relevant assembly functions.

- Dropped __nocfi from __init.

Sami Tolvanen (20):
treewide: Filter out CC_FLAGS_CFI
scripts/kallsyms: Ignore __kcfi_typeid_
cfi: Remove CONFIG_CFI_CLANG_SHADOW
cfi: Drop __CFI_ADDRESSABLE
cfi: Switch to -fsanitize=kcfi
cfi: Add type helper macros
lkdtm: Emit an indirect call for CFI tests
arm64: Add types to indirect called assembly functions
arm64: Add CFI error handling
arm64: Drop unneeded __nocfi attributes
init: Drop __nocfi from __init
treewide: Drop function_nocfi
treewide: Drop WARN_ON_FUNCTION_MISMATCH
treewide: Drop __cficanonical
objtool: Disable CFI warnings
kallsyms: Drop CONFIG_CFI_CLANG workarounds
x86/tools/relocs: Ignore __kcfi_typeid_ relocations
x86: Add types to indirectly called assembly functions
x86/purgatory: Disable CFI
x86: Add support for CONFIG_CFI_CLANG

Makefile | 13 +-
arch/Kconfig | 18 +-
arch/arm64/crypto/ghash-ce-core.S | 5 +-
arch/arm64/crypto/sm3-ce-core.S | 3 +-
arch/arm64/include/asm/brk-imm.h | 6 +
arch/arm64/include/asm/ftrace.h | 2 +-
arch/arm64/include/asm/mmu_context.h | 4 +-
arch/arm64/kernel/acpi_parking_protocol.c | 2 +-
arch/arm64/kernel/alternative.c | 2 +-
arch/arm64/kernel/cpu-reset.S | 5 +-
arch/arm64/kernel/cpufeature.c | 4 +-
arch/arm64/kernel/ftrace.c | 2 +-
arch/arm64/kernel/machine_kexec.c | 2 +-
arch/arm64/kernel/psci.c | 2 +-
arch/arm64/kernel/smp_spin_table.c | 2 +-
arch/arm64/kernel/traps.c | 47 ++-
arch/arm64/kernel/vdso/Makefile | 3 +-
arch/arm64/mm/proc.S | 5 +-
arch/x86/Kconfig | 2 +
arch/x86/crypto/blowfish-x86_64-asm_64.S | 5 +-
arch/x86/entry/vdso/Makefile | 3 +-
arch/x86/include/asm/cfi.h | 22 ++
arch/x86/include/asm/linkage.h | 12 +
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/cfi.c | 83 ++++++
arch/x86/kernel/traps.c | 4 +-
arch/x86/lib/memcpy_64.S | 3 +-
arch/x86/purgatory/Makefile | 4 +
arch/x86/tools/relocs.c | 1 +
drivers/firmware/efi/libstub/Makefile | 2 +
drivers/firmware/psci/psci.c | 4 +-
drivers/misc/lkdtm/cfi.c | 15 +-
drivers/misc/lkdtm/usercopy.c | 2 +-
include/asm-generic/bug.h | 16 -
include/asm-generic/vmlinux.lds.h | 37 +--
include/linux/cfi.h | 59 ++--
include/linux/cfi_types.h | 57 ++++
include/linux/compiler-clang.h | 14 +-
include/linux/compiler.h | 16 +-
include/linux/compiler_types.h | 4 -
include/linux/init.h | 6 +-
include/linux/module.h | 10 +-
include/linux/pci.h | 4 +-
kernel/cfi.c | 342 ++++------------------
kernel/kallsyms.c | 17 --
kernel/kthread.c | 3 +-
kernel/module/main.c | 49 +---
kernel/workqueue.c | 2 +-
scripts/kallsyms.c | 1 +
scripts/module.lds.S | 23 +-
tools/objtool/check.c | 7 +-
51 files changed, 420 insertions(+), 538 deletions(-)
create mode 100644 arch/x86/include/asm/cfi.h
create mode 100644 arch/x86/kernel/cfi.c
create mode 100644 include/linux/cfi_types.h

--
2.36.1.476.g0c4daa206d-goog