new file mode 100644
@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Clang Control Flow Integrity (CFI) type definitions.
+ */
+#ifndef _LINUX_CFI_TYPES_H
+#define _LINUX_CFI_TYPES_H
+
+#ifdef CONFIG_CFI_CLANG
+#include <linux/linkage.h>
+
+#ifdef __ASSEMBLY__
+/*
+ * Use the __kcfi_typeid_<function> type identifier symbol to
+ * annotate indirectly called assembly functions. The compiler emits
+ * these symbols for all address-taken function declarations in C
+ * code.
+ */
+#ifndef __CFI_TYPE
+#define __CFI_TYPE(name) \
+ .4byte __kcfi_typeid_##name
+#endif
+
+#define SYM_TYPED_ENTRY(name, fname, linkage, align...) \
+ linkage(name) ASM_NL \
+ align ASM_NL \
+ __CFI_TYPE(fname) ASM_NL \
+ name:
+
+#define __SYM_TYPED_FUNC_START_ALIAS(name, fname) \
+ SYM_TYPED_ENTRY(name, fname, SYM_L_GLOBAL, SYM_A_ALIGN)
+
+#define __SYM_TYPED_FUNC_START(name, fname) \
+ SYM_TYPED_ENTRY(name, fname, SYM_L_GLOBAL, SYM_A_ALIGN)
+
+#endif /* __ASSEMBLY__ */
+
+#else /* CONFIG_CFI_CLANG */
+
+#ifdef __ASSEMBLY__
+#define __SYM_TYPED_FUNC_START_ALIAS(name, fname) \
+ SYM_FUNC_START_ALIAS(name)
+
+#define __SYM_TYPED_FUNC_START(name, fname) \
+ SYM_FUNC_START(name)
+#endif /* __ASSEMBLY__ */
+
+#endif /* CONFIG_CFI_CLANG */
+
+#ifdef __ASSEMBLY__
+#define SYM_TYPED_FUNC_START_ALIAS(name) \
+ __SYM_TYPED_FUNC_START_ALIAS(name, name)
+
+#define SYM_TYPED_FUNC_START(name) \
+ __SYM_TYPED_FUNC_START(name, name)
+#endif /* __ASSEMBLY__ */
+
+#endif /* _LINUX_CFI_TYPES_H */
With CONFIG_CFI_CLANG, assembly functions called indirectly from C code must be annotated with type identifiers to pass CFI checking. In order to make this easier, the compiler emits a __kcfi_typeid_<function> symbol for each address-taken function declaration in C, which contains the expected type identifier that we can refer to in assembly code. Add typed versions of SYM_FUNC_START and SYM_FUNC_START_ALIAS, which emit the type identifier before the function. Architectures that support KCFI can define their own __CFI_TYPE macro to override the default preamble format. As an example, for the x86_64 blowfish_dec_blk function, the compiler emits the following type symbol: $ readelf -sW vmlinux | grep __kcfi_typeid_blowfish_dec_blk 121794: ffffffffef478db5 0 NOTYPE WEAK DEFAULT ABS __kcfi_typeid_blowfish_dec_blk And SYM_FUNC_START will generate the following preamble based on the __CFI_TYPE definition for the architecture: $ objdump -dr arch/x86/crypto/blowfish-x86_64-asm_64.o ... 00000000000003f7 <__cfi_blowfish_dec_blk>: 3f7: cc int3 3f8: cc int3 3f9: 8b 04 25 00 00 00 00 mov 0x0,%eax 3fc: R_X86_64_32S __kcfi_typeid_blowfish_dec_blk 400: cc int3 401: cc int3 0000000000000402 <blowfish_dec_blk>: ... Note that the address of all assembly functions annotated with SYM_FUNC_START* must be taken in C code that's linked into the binary or the missing __kcfi_typeid_ symbol will result in a linker error with CONFIG_CFI_CLANG. If the code that contains the indirect call is not always compiled in, __ADDRESSABLE(functionname) can be used to ensure that the __kcfi_typeid_ symbol is emitted. Signed-off-by: Sami Tolvanen <samitolvanen@google.com> Reviewed-by: Kees Cook <keescook@chromium.org> --- include/linux/cfi_types.h | 57 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) create mode 100644 include/linux/cfi_types.h