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

Nick Desaulniers
2021-09-30T18:39:01Z

On Thu, Sep 30, 2021 at 11:05 AM Sami Tolvanen <samitolvanen@google.com> 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@google.com>

>
> ---
> 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