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cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

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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@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Gatlin Newhouse <gatlin.newhouse@gmail.com>
Acked-by: Kees Cook <kees@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240801-kcfi-v2-1-c93caed3d121@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Alice Ryhl 2024-08-01 13:35:17 +00:00 committed by Miguel Ojeda
parent d077242d68
commit ce4a262098
2 changed files with 19 additions and 0 deletions
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3
Makefile
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@ -954,6 +954,9 @@ 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

16
arch/Kconfig
View File

@ -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