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x86/retpoline: Use -mfunction-return

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Utilize -mfunction-return=thunk-extern when available to have the
compiler replace RET instructions with direct JMPs to the symbol
__x86_return_thunk. This does not affect assembler (.S) sources, only C
sources.

-mfunction-return=thunk-extern has been available since gcc 7.3 and
clang 15.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
This commit is contained in:
Peter Zijlstra 2022-06-14 23:15:36 +02:00 committed by Borislav Petkov
parent 00e1533325
commit 0b53c374b9
3 changed files with 17 additions and 0 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

2
arch/x86/Makefile
View file

@ -15,11 +15,13 @@ endif
ifdef CONFIG_CC_IS_GCC
RETPOLINE_CFLAGS := $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch-cs-prefix)
RETPOLINE_CFLAGS += $(call cc-option,-mfunction-return=thunk-extern)
RETPOLINE_VDSO_CFLAGS := $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
endif
ifdef CONFIG_CC_IS_CLANG
RETPOLINE_CFLAGS := -mretpoline-external-thunk
RETPOLINE_VDSO_CFLAGS := -mretpoline
RETPOLINE_CFLAGS += $(call cc-option,-mfunction-return=thunk-extern)
endif
export RETPOLINE_CFLAGS
export RETPOLINE_VDSO_CFLAGS

2
arch/x86/include/asm/nospec-branch.h
View file

@ -123,6 +123,8 @@
typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
extern retpoline_thunk_t __x86_indirect_thunk_array[];
extern void __x86_return_thunk(void);
#ifdef CONFIG_RETPOLINE
#define GEN(reg) \

13
arch/x86/lib/retpoline.S
View file

@ -67,3 +67,16 @@ SYM_CODE_END(__x86_indirect_thunk_array)
#define GEN(reg) EXPORT_THUNK(reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN
/*
* This function name is magical and is used by -mfunction-return=thunk-extern
* for the compiler to generate JMPs to it.
*/
SYM_CODE_START(__x86_return_thunk)
UNWIND_HINT_EMPTY
ANNOTATE_NOENDBR
ret
int3
SYM_CODE_END(__x86_return_thunk)
__EXPORT_THUNK(__x86_return_thunk)