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x86/Kconfig: Introduce function padding

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Now that all functions are 16 byte aligned, add 16 bytes of NOP
padding in front of each function. This prepares things for software
call stack tracking and kCFI/FineIBT.

This significantly increases kernel .text size, around 5.1% on a
x86_64-defconfig-ish build.

However, per the random access argument used for alignment, these 16
extra bytes are code that wouldn't be used. Performance measurements
back this up by showing no significant performance regressions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111146.950884492@infradead.org
This commit is contained in:
Thomas Gleixner 2022-09-15 13:11:18 +02:00 committed by Peter Zijlstra
parent 8f7c0d8b23
commit bea75b3389
5 changed files with 79 additions and 5 deletions
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20
arch/x86/Kconfig
View File

@ -2446,9 +2446,27 @@ config CC_HAS_SLS
config CC_HAS_RETURN_THUNK
def_bool $(cc-option,-mfunction-return=thunk-extern)
config CC_HAS_ENTRY_PADDING
def_bool $(cc-option,-fpatchable-function-entry=16,16)
config FUNCTION_PADDING_CFI
int
default 59 if FUNCTION_ALIGNMENT_64B
default 27 if FUNCTION_ALIGNMENT_32B
default 11 if FUNCTION_ALIGNMENT_16B
default 3 if FUNCTION_ALIGNMENT_8B
default 0
# Basically: FUNCTION_ALIGNMENT - 5*CFI_CLANG
# except Kconfig can't do arithmetic :/
config FUNCTION_PADDING_BYTES
int
default FUNCTION_PADDING_CFI if CFI_CLANG
default FUNCTION_ALIGNMENT
config HAVE_CALL_THUNKS
def_bool y
depends on RETHUNK && OBJTOOL
depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL
config CALL_THUNKS
def_bool n

6
arch/x86/Makefile
View File

@ -208,6 +208,12 @@ ifdef CONFIG_SLS
KBUILD_CFLAGS += -mharden-sls=all
endif
ifdef CONFIG_CALL_THUNKS
PADDING_CFLAGS := -fpatchable-function-entry=$(CONFIG_FUNCTION_PADDING_BYTES),$(CONFIG_FUNCTION_PADDING_BYTES)
KBUILD_CFLAGS += $(PADDING_CFLAGS)
export PADDING_CFLAGS
endif
KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE)
ifdef CONFIG_LTO_CLANG

3
arch/x86/entry/vdso/Makefile
View File

@ -95,7 +95,7 @@ ifneq ($(RETPOLINE_VDSO_CFLAGS),)
endif
endif
$(vobjs): KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
$(vobjs): KBUILD_CFLAGS := $(filter-out $(PADDING_CFLAGS) $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) $(RANDSTRUCT_CFLAGS) $(GCC_PLUGINS_CFLAGS) $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS)) $(CFL)
$(vobjs): KBUILD_AFLAGS += -DBUILD_VDSO
#
@ -158,6 +158,7 @@ KBUILD_CFLAGS_32 := $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(RETPOLINE_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(CC_FLAGS_CFI),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 := $(filter-out $(PADDING_CFLAGS),$(KBUILD_CFLAGS_32))
KBUILD_CFLAGS_32 += -m32 -msoft-float -mregparm=0 -fpic
KBUILD_CFLAGS_32 += -fno-stack-protector
KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls)

51
arch/x86/include/asm/linkage.h
View File

@ -15,8 +15,19 @@
#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT, 0x90;
#define __ALIGN_STR __stringify(__ALIGN)
#define ASM_FUNC_ALIGN __ALIGN_STR
#define __FUNC_ALIGN __ALIGN
#if defined(CONFIG_CALL_THUNKS) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
#define FUNCTION_PADDING .skip CONFIG_FUNCTION_ALIGNMENT, 0x90;
#else
#define FUNCTION_PADDING
#endif
#if (CONFIG_FUNCTION_ALIGNMENT > 8) && !defined(__DISABLE_EXPORTS) && !defined(BULID_VDSO)
# define __FUNC_ALIGN __ALIGN; FUNCTION_PADDING
#else
# define __FUNC_ALIGN __ALIGN
#endif
#define ASM_FUNC_ALIGN __stringify(__FUNC_ALIGN)
#define SYM_F_ALIGN __FUNC_ALIGN
#ifdef __ASSEMBLY__
@ -45,11 +56,45 @@
#endif /* __ASSEMBLY__ */
/*
* Depending on -fpatchable-function-entry=N,N usage (CONFIG_CALL_THUNKS) the
* CFI symbol layout changes.
*
* Without CALL_THUNKS:
*
* .align FUNCTION_ALIGNMENT
* __cfi_##name:
* .skip FUNCTION_PADDING, 0x90
* .byte 0xb8
* .long __kcfi_typeid_##name
* name:
*
* With CALL_THUNKS:
*
* .align FUNCTION_ALIGNMENT
* __cfi_##name:
* .byte 0xb8
* .long __kcfi_typeid_##name
* .skip FUNCTION_PADDING, 0x90
* name:
*
* In both cases the whole thing is FUNCTION_ALIGNMENT aligned and sized.
*/
#ifdef CONFIG_CALL_THUNKS
#define CFI_PRE_PADDING
#define CFI_POST_PADDING .skip CONFIG_FUNCTION_PADDING_BYTES, 0x90;
#else
#define CFI_PRE_PADDING .skip CONFIG_FUNCTION_PADDING_BYTES, 0x90;
#define CFI_POST_PADDING
#endif
#define __CFI_TYPE(name) \
SYM_START(__cfi_##name, SYM_L_LOCAL, SYM_A_NONE) \
.fill 11, 1, 0x90 ASM_NL \
CFI_PRE_PADDING \
.byte 0xb8 ASM_NL \
.long __kcfi_typeid_##name ASM_NL \
CFI_POST_PADDING \
SYM_FUNC_END(__cfi_##name)
/* SYM_TYPED_FUNC_START -- use for indirectly called globals, w/ CFI type */

4
include/linux/bpf.h
View File

@ -984,7 +984,11 @@ int arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_func
}
#ifdef CONFIG_X86_64
#ifdef CONFIG_CALL_THUNKS
#define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5+CONFIG_FUNCTION_PADDING_BYTES,CONFIG_FUNCTION_PADDING_BYTES)))
#else
#define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5)))
#endif
#else
#define BPF_DISPATCHER_ATTRIBUTES
#endif