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Date:   Mon, 23 Aug 2021 10:16:09 -0700
From:   Tom Stellard <tstellar@...hat.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI

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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