x86: Check corrupted return address when unwinding stack

If shadow stack is enabled, when unwinding stack, we count how many stack
frames we pop to reach the landing pad and adjust shadow stack by the same
amount.  When counting the stack frame, we compare the return address on
normal stack against the return address on shadow stack.  If they don't
match, return _URC_FATAL_PHASE2_ERROR for the corrupted return address on
normal stack.  Don't check the return address for

1. Non-catchable exception where exception_class == 0.  Process will be
terminated.
2. Zero return address which marks the outermost stack frame.
3. Signal stack frame since kernel puts a restore token on shadow stack.

	* unwind-generic.h (_Unwind_Frames_Increment): Add the EXC
	argument.
	* unwind.inc (_Unwind_RaiseException_Phase2): Pass EXC to
	_Unwind_Frames_Increment.
	(_Unwind_ForcedUnwind_Phase2): Likewise.
	* config/i386/shadow-stack-unwind.h (_Unwind_Frames_Increment):
	Take the EXC argument.  Return _URC_FATAL_PHASE2_ERROR if the
	return address on normal stack doesn't match the return address
	on shadow stack.

libgcc/config/i386/shadow-stack-unwind.h

@@ -54,10 +54,39 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    aligned.  If the original shadow stack is 8 byte aligned, we just
    need to pop 2 slots, one restore token, from shadow stack.  Otherwise,
    we need to pop 3 slots, one restore token + 4 byte padding, from
-   shadow stack.  */
-#ifndef __x86_64__
+   shadow stack.
+
+   When popping a stack frame, we compare the return address on normal
+   stack against the return address on shadow stack.  If they don't match,
+   return _URC_FATAL_PHASE2_ERROR for the corrupted return address on
+   normal stack.  Don't check the return address for
+   1. Non-catchable exception where exception_class == 0.  Process will
+      be terminated.
+   2. Zero return address which marks the outermost stack frame.
+   3. Signal stack frame since kernel puts a restore token on shadow
+      stack.
+ */
 #undef _Unwind_Frames_Increment
-#define _Unwind_Frames_Increment(context, frames)	\
+#ifdef __x86_64__
+#define _Unwind_Frames_Increment(exc, context, frames)	\
+    {							\
+      frames++;						\
+      if (exc->exception_class != 0			\
+	  && _Unwind_GetIP (context) != 0		\
+	  && !_Unwind_IsSignalFrame (context))		\
+	{						\
+	  _Unwind_Word ssp = _get_ssp ();		\
+	  if (ssp != 0)					\
+	    {						\
+	      ssp += 8 * frames;			\
+	      _Unwind_Word ra = *(_Unwind_Word *) ssp;	\
+	      if (ra != _Unwind_GetIP (context))	\
+		return _URC_FATAL_PHASE2_ERROR;		\
+	    }						\
+	}						\
+    }
+#else
+#define _Unwind_Frames_Increment(exc, context, frames)	\
   if (_Unwind_IsSignalFrame (context))			\
     do							\
       {							\
@@ -83,5 +112,19 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
       }							\
     while (0);						\
   else							\
-    frames++;
+    {							\
+      frames++;						\
+      if (exc->exception_class != 0			\
+	  && _Unwind_GetIP (context) != 0)		\
+	{						\
+	  _Unwind_Word ssp = _get_ssp ();		\
+	  if (ssp != 0)					\
+	    {						\
+	      ssp += 4 * frames;			\
+	      _Unwind_Word ra = *(_Unwind_Word *) ssp;	\
+	      if (ra != _Unwind_GetIP (context))	\
+		return _URC_FATAL_PHASE2_ERROR;		\
+	    }						\
+	}						\
+    }
 #endif

libgcc/unwind-generic.h

@@ -292,6 +292,6 @@ EXCEPTION_DISPOSITION _GCC_specific_handler (PEXCEPTION_RECORD, void *,
 #define _Unwind_Frames_Extra(frames)
 
 /* Increment frame count.  */
-#define _Unwind_Frames_Increment(context, frames) frames++
+#define _Unwind_Frames_Increment(exc, context, frames) frames++
 
 #endif /* unwind.h */

libgcc/unwind.inc

@@ -73,7 +73,7 @@ _Unwind_RaiseException_Phase2(struct _Unwind_Exception *exc,
       gcc_assert (!match_handler);
 
       uw_update_context (context, &fs);
-      _Unwind_Frames_Increment (context, frames);
+      _Unwind_Frames_Increment (exc, context, frames);
     }
 
   *frames_p = frames;
@@ -191,7 +191,7 @@ _Unwind_ForcedUnwind_Phase2 (struct _Unwind_Exception *exc,
       /* Update cur_context to describe the same frame as fs, and discard
 	 the previous context if necessary.  */
       uw_advance_context (context, &fs);
-      _Unwind_Frames_Increment (context, frames);
+      _Unwind_Frames_Increment (exc, context, frames);
     }
 
   *frames_p = frames;