linux-kernel - Re: [PATCH] cfi: fix conditions in HAVE_CFI_ICALL_NORMALIZE

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Message-ID: <CAGSQo03ZOppScZT_1DuWYsnveKvYHdc-K_djHi=AB4CKjAbgyQ@mail.gmail.com>
Date: Tue, 8 Oct 2024 10:46:29 -0700
From: Matthew Maurer <mmaurer@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Sami Tolvanen <samitolvanen@...gle.com>, Kees Cook <kees@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] cfi: fix conditions in HAVE_CFI_ICALL_NORMALIZE_INTEGERS

This makes sense, as some folks have a Rust compiler they know has the
fix, but build system detection for it isn't there yet. This lets them
override availability if needed.

That said, we should definitely be sure to get this back to a
non-configurable toggle once the LLVM version detection is in.

Reviewed-By: Matthew Maurer <mmaurer@...gle.com>

On Tue, Oct 8, 2024 at 10:42 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> The CFI_ICALL_NORMALIZE_INTEGERS option is incompatible with KASAN
> because LLVM will emit some constructors when using KASAN that are
> assigned incorrect CFI tags. These constructors are emitted due to use
> of -fsanitize=kernel-address or -fsanitize=kernel-hwaddress that are
> respectively passed when KASAN_GENERIC or KASAN_SW_TAGS are enabled.
> However, the KASAN_HW_TAGS option relies on hardware support for MTE
> instead and does not pass either flag. (Note also that KASAN_HW_TAGS
> does not `select CONSTRUCTORS`.)
>
> Additionally, the option is configured to have a prompt and gated behind
> EXPERT. The previous method for a user override of the option did not
> actually work. This is expected to be temporary, as I intend to add a
> precise detection check for 6.13 - I did not included that here to avoid
> adding a RUSTC_LLVM_VERSION config in a fix.
>
> Fixes: 4c66f8307ac0 ("cfi: encode cfi normalized integers + kasan/gcov bug in Kconfig")
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  arch/Kconfig | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 8af374ea1adc..2632de28c05a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -852,8 +852,9 @@ 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
> +       bool "Are normalized CFI tags for integers available?"
> +       default !GCOV_KERNEL && !KASAN_GENERIC && !KASAN_SW_TAGS
> +       depends on EXPERT || (!GCOV_KERNEL && !KASAN_GENERIC && !KASAN_SW_TAGS)
>         depends on $(cc-option,-fsanitize=kcfi -fsanitize-cfi-icall-experimental-normalize-integers)
>         help
>           Is CFI_ICALL_NORMALIZE_INTEGERS supported with the set of compilers
>
> ---
> base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
> change-id: 20241008-cfi-fix-llvm-gate-115e48d6acc9
>
> Best regards,
> --
> Alice Ryhl <aliceryhl@...gle.com>
>