[RFC PATCH v3 06/20] cfi: Add type helper macros

[Sami Tolvanen]

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 at google.com>
Reviewed-by: Kees Cook <keescook at chromium.org>
---
 include/linux/cfi_types.h | 57 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 include/linux/cfi_types.h

diff --git a/include/linux/cfi_types.h b/include/linux/cfi_types.h
new file mode 100644
index 000000000000..dd16e755a197
--- /dev/null
+++ b/include/linux/cfi_types.h
@@ -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 */
-- 
2.36.1.476.g0c4daa206d-goog