[PATCH V3 8/9] x86/speculation: Add return address unwind hints to retpoline and RSB stuffing

[Alexandre Chartre]

The retpoline and RSB stuffing code modify the stack to change return
addresses of intra-function calls. Add return address unwind hints so
that objtool can be aware of these tweaks. This requires a little
refactoring of the __FILL_RETURN_BUFFER macro for adding hints to
assembly and C code.

Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
---
 arch/x86/include/asm/nospec-branch.h | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index b0db793fd083..20594ea99c21 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -9,6 +9,7 @@
 #include <asm/alternative-asm.h>
 #include <asm/cpufeatures.h>
 #include <asm/msr-index.h>
+#include <asm/unwind_hints.h>
 
 /*
  * This should be used immediately before a retpoline alternative. It tells
@@ -43,7 +44,7 @@
  * the optimal version â two calls, each with their own speculation
  * trap should their return address end up getting used, in a loop.
  */
-#define __FILL_RETURN_BUFFER(reg, nr, sp)	\
+#define __FILL_RETURN_BUFFER_BEGIN(reg, nr, sp)	\
 	mov	$(nr/2), reg;			\
 771:						\
 	call	772f;				\
@@ -58,12 +59,19 @@
 	lfence;					\
 	jmp	775b;				\
 774:						\
-	dec	reg;				\
+	dec	reg;
+
+#define __FILL_RETURN_BUFFER_END(sp)		\
 	add	$(BITS_PER_LONG/8) * 2, sp;	\
 	jnz	771b;
 
 #ifdef __ASSEMBLY__
 
+#define __FILL_RETURN_BUFFER(reg, nr, sp)	\
+	__FILL_RETURN_BUFFER_BEGIN(reg, nr, sp)	\
+	UNWIND_HINT_RADDR_DELETE 2;		\
+	__FILL_RETURN_BUFFER_END(sp)
+
 /*
  * This should be used immediately before an indirect jump/call. It tells
  * objtool the subsequent indirect jump/call is vouched safe for retpoline
@@ -88,6 +96,7 @@
 	lfence
 	jmp	.Lspec_trap_\@
 .Ldo_rop_\@:
+	UNWIND_HINT_RADDR_ALTER
 	mov	\reg, (%_ASM_SP)
 	ret
 .endm
@@ -237,6 +246,11 @@ enum ssb_mitigation {
 extern char __indirect_thunk_start[];
 extern char __indirect_thunk_end[];
 
+#define __FILL_RETURN_BUFFER(reg, nr, sp)			\
+	__stringify(__FILL_RETURN_BUFFER_BEGIN(reg, nr, sp))	\
+	UNWIND_HINT_RADDR_DELETE(2)				\
+	__stringify(__FILL_RETURN_BUFFER_END(sp))
+
 /*
  * On VMEXIT we must ensure that no RSB predictions learned in the guest
  * can be followed in the host, by overwriting the RSB completely. Both
@@ -250,7 +264,7 @@ static inline void vmexit_fill_RSB(void)
 
 	asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
 		      ALTERNATIVE("jmp 910f",
-				  __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
+				  __FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1),
 				  X86_FEATURE_RETPOLINE)
 		      "910:"
 		      : "=r" (loops), ASM_CALL_CONSTRAINT
-- 
2.18.2