linux-kernel - Re: [PATCH] cfi: encode cfi normalized integers + kasan/gcov bug in Kconfig

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Message-ID: <20240926000053.GA3188344@google.com>
Date: Thu, 26 Sep 2024 00:00:53 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Gatlin Newhouse <gatlin.newhouse@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, llvm@...ts.linux.dev,
	linux-kbuild@...r.kernel.org,
	kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] cfi: encode cfi normalized integers + kasan/gcov bug in
 Kconfig

Hi Alice,

On Wed, Sep 25, 2024 at 08:10:18AM +0000, Alice Ryhl wrote:
> An alternative solution is to inspect a binary created by clang or rustc
> to see whether the faulty CFI tags are in the binary. This would be a
> precise check, but it would involve hard-coding the *hashed* version of
> the CFI tag. This is because there's no way to get clang or rustc to
> output the unhased version of the CFI tag. Relying on the precise
> hashing algorithm using by CFI seems too fragile, so I have not pursued
> this option.

I suppose there would be no need to hardcode hashes in the test,
it's enough to verify that the hashes for the compiler-emitted
functions change when integer normalization is enabled. Still, I
agree that this doesn't sound worth it in this case.

> diff --git a/arch/Kconfig b/arch/Kconfig
> index ee58df8b1080..b8066bf43153 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -829,7 +829,7 @@ config CFI_CLANG
>  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)
> +	depends on HAVE_CFI_ICALL_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
> @@ -842,6 +842,22 @@ config CFI_ICALL_NORMALIZE_INTEGERS
>  
>  	  This option is necessary for using CFI with Rust. If unsure, say N.
>  
> +config HAVE_CFI_ICALL_NORMALIZE_INTEGERS
> +	def_bool !GCOV_KERNEL && !KASAN
> +	depends on CFI_CLANG
> +	depends on $(cc-option,-fsanitize=kcfi -fsanitize-cfi-icall-experimental-normalize-integers)

This looks reasonable to me. Thanks for the fix!

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami