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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 a typed version of SYM_FUNC_START, which emits 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
120204: 00000000ef478db5 0 NOTYPE WEAK DEFAULT ABS
__kcfi_typeid_blowfish_dec_blk
And SYM_TYPED_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
...
0000000000000400 <__cfi_blowfish_dec_blk>:
...
40b: b8 00 00 00 00 mov $0x0,%eax
40c: R_X86_64_32 __kcfi_typeid_blowfish_dec_blk
0000000000000410 <blowfish_dec_blk>:
...
Note that the address of all assembly functions annotated with
SYM_TYPED_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>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-7-samitolvanen@google.com
45 lines
1.1 KiB
C
45 lines
1.1 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* Clang Control Flow Integrity (CFI) type definitions.
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*/
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#ifndef _LINUX_CFI_TYPES_H
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#define _LINUX_CFI_TYPES_H
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#ifdef __ASSEMBLY__
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#include <linux/linkage.h>
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#ifdef CONFIG_CFI_CLANG
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/*
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* Use the __kcfi_typeid_<function> type identifier symbol to
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* annotate indirectly called assembly functions. The compiler emits
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* these symbols for all address-taken function declarations in C
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* code.
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*/
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#ifndef __CFI_TYPE
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#define __CFI_TYPE(name) \
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.4byte __kcfi_typeid_##name
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#endif
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#define SYM_TYPED_ENTRY(name, linkage, align...) \
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linkage(name) ASM_NL \
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align ASM_NL \
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__CFI_TYPE(name) ASM_NL \
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name:
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#define SYM_TYPED_START(name, linkage, align...) \
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SYM_TYPED_ENTRY(name, linkage, align)
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#else /* CONFIG_CFI_CLANG */
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#define SYM_TYPED_START(name, linkage, align...) \
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SYM_START(name, linkage, align)
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#endif /* CONFIG_CFI_CLANG */
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#ifndef SYM_TYPED_FUNC_START
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#define SYM_TYPED_FUNC_START(name) \
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SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)
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#endif
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#endif /* __ASSEMBLY__ */
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#endif /* _LINUX_CFI_TYPES_H */
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