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SubjectRe: [PATCH v2 00/14] x86: Add support for Clang CFI
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On 8/23/21 10:13 AM, 'Sami Tolvanen' via Clang Built Linux 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
>
> Version 2 depends on Clang >=14, where we fixed the issue with
> referencing static functions from inline assembly. Based on the
> feedback for v1, this version also changes the declaration of
> functions that are not callable from C to use an opaque type,
> which stops the compiler from replacing references to them. This
> avoids the need to sprinkle function_nocfi() macros in the kernel
> code.

How invasive are the changes in clang 14 necessary to make CFI work?
Would it be possible to backport them to LLVM 13?

-Tom

>
> 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-v2
>
> ---
> 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 (2):
> x86/extable: Do not mark exception callback as CFI
> x86, relocs: Ignore __typeid__ relocations
>
> Sami Tolvanen (12):
> objtool: Add CONFIG_CFI_CLANG support
> objtool: Add ASM_STACK_FRAME_NON_STANDARD
> linkage: Add DECLARE_ASM_FUNC_SYMBOL
> 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/mm/extable.c | 1 +
> arch/x86/power/Makefile | 2 ++
> arch/x86/purgatory/Makefile | 2 +-
> 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/linux/ftrace.h | 7 ++--
> include/linux/linkage.h | 13 +++++++
> include/linux/objtool.h | 6 ++++
> tools/include/linux/objtool.h | 6 ++++
> tools/objtool/arch/x86/decode.c | 16 +++++++++
> tools/objtool/elf.c | 51 +++++++++++++++++++++++++++
> tools/objtool/include/objtool/arch.h | 3 ++
> tools/objtool/include/objtool/elf.h | 2 +-
> 33 files changed, 166 insertions(+), 62 deletions(-)
>
>
> base-commit: d5ae8d7f85b7f6f6e60f1af8ff4be52b0926fde1
>

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