linux-kernel - [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE

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Message-ID: <20240730-kcfi-v1-1-bbb948752a30@google.com>
Date: Tue, 30 Jul 2024 09:40:11 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Peter Zijlstra <peterz@...radead.org>, Miguel Ojeda <ojeda@...nel.org>, Kees Cook <kees@...nel.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Matthew Maurer <mmaurer@...gle.com>, 
	Alice Ryhl <aliceryhl@...gle.com>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

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.

Signed-off-by: Alice Ryhl <aliceryhl@...gle.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