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. The security impact of this flag is minimal. When Sami Tolvanen looked into it, he found that integer normalization reduced the number of unique type hashes in the kernel by ~1%, which is acceptable. 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 signedness, but Rust only has one integer type of that size and signedness. When Rust calls into C functions using such types in their signature, this results in CFI failures. One example is 'unsigned long long' and 'unsigned long' which are both 64-bit on LP64 targets, so on those targets this flag will give both types the same CFI tag. This flag changes the ABI heavily. It is not applied automatically when CONFIG_RUST is turned on to make sure that the CONFIG_RUST option does not change the ABI of C code. For example, some build may need to make other changes atomically with toggling this flag. Having it be a separate option makes it possible to first turn on normalized integer tags, and then later turn on CONFIG_RUST. Similarly, when turning on CONFIG_RUST in a build, you may need a few attempts where the RUST=y commit gets reverted a few times. It is inconvenient if reverting RUST=y also requires reverting the changes you made to support normalized integer tags. To avoid having this flag impact builds that don't care about this, the next patch in this series will make CONFIG_RUST turn on this option using `select` rather than `depends on`. Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> --- Makefile | 3 +++ arch/Kconfig | 16 ++++++++++++++++ 2 files changed, 19 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..ee58df8b1080 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -826,6 +826,22 @@ 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. + + The option is separate from CONFIG_RUST because it affects the ABI. + When working with build systems that care about the ABI, it is + convenient to be able to turn on this flag first, before Rust is + turned on. + + 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
