From: Peter Zijlstra peterz@infradead.org
commit 0b53c374b9eff2255a386f1f1cfb9a928e52a5ae upstream.
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 Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/Makefile | 2 ++ arch/x86/include/asm/nospec-branch.h | 2 ++ arch/x86/lib/retpoline.S | 13 +++++++++++++ 3 files changed, 17 insertions(+)
--- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -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 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -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) \ --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -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)