Re: [PATCH v4 00/15] x86: Add support for Clang CFI

[Nick Desaulniers]

On Thu, Sep 30, 2021 at 11:05 AM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
>
> This series adds support for Clang's Control-Flow Integrity (CFI)
> checking to x86_64. With CFI, the compiler injects a runtime
> check before each indirect function call to ensure the target is
> a valid function with the correct static type. This restricts
> possible call targets and makes it more difficult for an attacker
> to exploit bugs that allow the modification of stored function
> pointers. For more details, see:
>
>   https://clang.llvm.org/docs/ControlFlowIntegrity.html
>
> Note that v4 is based on tip/master. The first two patches contain
> objtool support for CFI, the remaining patches change function
> declarations to use opaque types, fix type mismatch issues that
> confuse the compiler, and disable CFI where it can't be used.
>
> You can also pull this series from
>
>   https://github.com/samitolvanen/linux.git x86-cfi-v4

I tested this series with near ToT clang with CONFIG_LTO_CLANG_THIN=y
and our small buildroot-based userspace image in QEMU, then again with
CONFIG_LTO_CLANG_FULL=y. No new warnings from objtool, everything
seemed fine.  FWIW,

Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

>
> ---
> Changes in v4:
> - Dropped the extable patch after the code was refactored in -tip.
>
> - Switched to __section() instead of open-coding the attribute.
>
> - Added an explicit ifdef for filtering out CC_FLAGS_CFI in
>   purgatory for readability.
>
> - Added a comment to arch_cfi_jump_reloc_offset() in objtool.
>
> Changes in v3:
> - Dropped Clang requirement to >= 13 after the missing compiler
>   fix was backported there.
>
> - Added DEFINE_CFI_IMMEDIATE_RETURN_STUB to address the issue
>   with tp_stub_func in kernel/tracepoint.c.
>
> - Renamed asm_func_t to asm_func_ptr.
>
> - Changed extable handlers to use __cficanonical instead of
>   disabling CFI for fixup_exception.
>
> Changes in v2:
> - Dropped the first objtool patch as the warnings were fixed in
>   separate patches.
>
> - Changed fix_cfi_relocs() in objtool to not rely on jump table
>   symbols, and to return an error if it can't find a relocation.
>
> - Fixed a build issue with ASM_STACK_FRAME_NON_STANDARD().
>
> - Dropped workarounds for inline assembly references to
>   address-taken static functions with CFI as this was fixed in
>   the compiler.
>
> - Changed the C declarations of non-callable functions to use
>   opaque types and dropped the function_nocfi() patches.
>
> - Changed ARCH_SUPPORTS_CFI_CLANG to depend on Clang >=14 for
>   the compiler fixes.
>
>
> Kees Cook (1):
>   x86, relocs: Ignore __typeid__ relocations
>
> Sami Tolvanen (14):
>   objtool: Add CONFIG_CFI_CLANG support
>   objtool: Add ASM_STACK_FRAME_NON_STANDARD
>   linkage: Add DECLARE_ASM_FUNC_SYMBOL
>   cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB
>   tracepoint: Exclude tp_stub_func from CFI checking
>   ftrace: Use an opaque type for functions not callable from C
>   lkdtm: Disable UNSET_SMEP with CFI
>   lkdtm: Use an opaque type for lkdtm_rodata_do_nothing
>   x86: Use an opaque type for functions not callable from C
>   x86/purgatory: Disable CFI
>   x86, module: Ignore __typeid__ relocations
>   x86, cpu: Use LTO for cpu.c with CFI
>   x86, kprobes: Fix optprobe_template_func type mismatch
>   x86, build: Allow CONFIG_CFI_CLANG to be selected
>
>  arch/x86/Kconfig                      |  1 +
>  arch/x86/include/asm/ftrace.h         |  2 +-
>  arch/x86/include/asm/idtentry.h       | 10 +++---
>  arch/x86/include/asm/page_64.h        |  7 ++--
>  arch/x86/include/asm/paravirt_types.h |  3 +-
>  arch/x86/include/asm/processor.h      |  2 +-
>  arch/x86/include/asm/proto.h          | 25 ++++++-------
>  arch/x86/include/asm/uaccess_64.h     |  9 ++---
>  arch/x86/kernel/alternative.c         |  2 +-
>  arch/x86/kernel/ftrace.c              |  2 +-
>  arch/x86/kernel/kprobes/opt.c         |  4 +--
>  arch/x86/kernel/module.c              |  4 +++
>  arch/x86/kernel/paravirt.c            |  4 +--
>  arch/x86/kvm/emulate.c                |  4 +--
>  arch/x86/kvm/kvm_emulate.h            |  9 ++---
>  arch/x86/power/Makefile               |  2 ++
>  arch/x86/purgatory/Makefile           |  4 +++
>  arch/x86/tools/relocs.c               |  7 ++++
>  arch/x86/xen/enlighten_pv.c           |  6 ++--
>  arch/x86/xen/xen-ops.h                | 10 +++---
>  drivers/misc/lkdtm/bugs.c             |  2 +-
>  drivers/misc/lkdtm/lkdtm.h            |  2 +-
>  drivers/misc/lkdtm/perms.c            |  2 +-
>  drivers/misc/lkdtm/rodata.c           |  2 +-
>  include/asm-generic/vmlinux.lds.h     | 11 ++++++
>  include/linux/cfi.h                   | 13 +++++++
>  include/linux/ftrace.h                |  7 ++--
>  include/linux/linkage.h               | 13 +++++++
>  include/linux/objtool.h               |  6 ++++
>  kernel/cfi.c                          | 24 ++++++++++++-
>  kernel/tracepoint.c                   |  5 +--
>  tools/include/linux/objtool.h         |  6 ++++
>  tools/objtool/arch/x86/decode.c       | 17 +++++++++
>  tools/objtool/elf.c                   | 51 +++++++++++++++++++++++++++
>  tools/objtool/include/objtool/arch.h  |  3 ++
>  tools/objtool/include/objtool/elf.h   |  2 +-
>  36 files changed, 217 insertions(+), 66 deletions(-)
>
>
> base-commit: d4bfebd9ef497ee0afb498f6028a5074a6ccf307
> --
> 2.33.0.800.g4c38ced690-goog
>


-- 
Thanks,
~Nick Desaulniers