Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 30, 2024 at 09:40:11AM +0000, Alice Ryhl wrote:
> Introduce a Kconfig option for enabling the experimental option to
> normalize integer types. This ensures that integer types of the same
> size and signedness are considered compatible by the Control Flow
> Integrity sanitizer.
> 
> This option exists for compatibility with Rust, as C and Rust do not
> have the same set of integer types. There are cases where C has two
> different integer types of the same size and alignment, but Rust only
> has one integer type of that size and alignment. When Rust calls into
> C functions using such types in their signature, this results in CFI
> failures.
> 
> This patch introduces a dedicated option for this because it is
> undesirable to have CONFIG_RUST affect CC_FLAGS in this way.

To be clear, any code compiled with this is incompatible with code
compiled without this, as the function signatures will differ, right?

Specifically, it will map things like 'unsigned long long' and 'unsigned
long' -- which are both u64 on LP64 targets to the same 'type', right?

I suppose it has been decided the security impact of this change is
minimal?

All in all, there is very little actual information provided here.

> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  Makefile     |  3 +++
>  arch/Kconfig | 11 +++++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index 2b5f9f098b6f..484c6900337e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -952,6 +952,9 @@ endif
>  
>  ifdef CONFIG_CFI_CLANG
>  CC_FLAGS_CFI	:= -fsanitize=kcfi
> +ifdef CONFIG_CFI_ICALL_NORMALIZE_INTEGERS
> +	CC_FLAGS_CFI	+= -fsanitize-cfi-icall-experimental-normalize-integers
> +endif
>  KBUILD_CFLAGS	+= $(CC_FLAGS_CFI)
>  export CC_FLAGS_CFI
>  endif
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 975dd22a2dbd..f6ecb15cb8ba 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -826,6 +826,17 @@ config CFI_CLANG
>  
>  	    https://clang.llvm.org/docs/ControlFlowIntegrity.html
>  
> +config CFI_ICALL_NORMALIZE_INTEGERS
> +	bool "Normalize CFI tags for integers"
> +	depends on CFI_CLANG
> +	depends on $(cc-option,-fsanitize=kcfi -fsanitize-cfi-icall-experimental-normalize-integers)
> +	help
> +	  This option normalizes the CFI tags for integer types so that all
> +	  integer types of the same size and signedness receive the same CFI
> +	  tag.
> +
> +	  This option is necessary for using CFI with Rust. If unsure, say N.
> +
>  config CFI_PERMISSIVE
>  	bool "Use CFI in permissive mode"
>  	depends on CFI_CLANG
> 
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
>