x86/kvm: Fix SETcc emulation for return thunks

Prepare the SETcc fastop stuff for when RET can be larger still.

The tricky bit here is that the expressions should not only be
constant C expressions, but also absolute GAS expressions. This means
no ?: and 'true' is ~0.

Also ensure em_setcc() has the same alignment as the actual FOP_SETCC()
ops, this ensures there cannot be an alignment hole between em_setcc()
and the first op.

Additionally, add a .skip directive to the FOP_SETCC() macro to fill
any remaining space with INT3 traps; however the primary purpose of
this directive is to generate AS warnings when the remaining space
goes negative. Which is a very good indication the alignment magic
went side-ways.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kvm/emulate.c

@@ -325,13 +325,15 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
 #define FOP_RET(name) \
 	__FOP_RET(#name)
 
-#define FOP_START(op) \
+#define __FOP_START(op, align) \
 	extern void em_##op(struct fastop *fake); \
 	asm(".pushsection .text, \"ax\" \n\t" \
 	    ".global em_" #op " \n\t" \
-	    ".align " __stringify(FASTOP_SIZE) " \n\t" \
+	    ".align " __stringify(align) " \n\t" \
 	    "em_" #op ":\n\t"
 
+#define FOP_START(op) __FOP_START(op, FASTOP_SIZE)
+
 #define FOP_END \
 	    ".popsection")
 
@@ -437,14 +439,13 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
  *
  * ENDBR			[4 bytes; CONFIG_X86_KERNEL_IBT]
  * SETcc %al			[3 bytes]
- * RET         [1 byte]
+ * RET | JMP __x86_return_thunk	[1,5 bytes; CONFIG_RETPOLINE]
  * INT3				[1 byte; CONFIG_SLS]
- *
- * Which gives possible sizes 4, 5, 8 or 9.  When rounded up to the
- * next power-of-two alignment they become 4, 8 or 16 resp.
  */
-#define SETCC_LENGTH	(ENDBR_INSN_SIZE + 4 + IS_ENABLED(CONFIG_SLS))
-#define SETCC_ALIGN	(4 << IS_ENABLED(CONFIG_SLS) << HAS_KERNEL_IBT)
+#define RET_LENGTH	(1 + (4 * IS_ENABLED(CONFIG_RETPOLINE)) + \
+			 IS_ENABLED(CONFIG_SLS))
+#define SETCC_LENGTH	(ENDBR_INSN_SIZE + 3 + RET_LENGTH)
+#define SETCC_ALIGN	(4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))
 static_assert(SETCC_LENGTH <= SETCC_ALIGN);
 
 #define FOP_SETCC(op) \
@@ -453,9 +454,10 @@ static_assert(SETCC_LENGTH <= SETCC_ALIGN);
 	#op ": \n\t" \
 	ASM_ENDBR \
 	#op " %al \n\t" \
-	__FOP_RET(#op)
+	__FOP_RET(#op) \
+	".skip " __stringify(SETCC_ALIGN) " - (.-" #op "), 0xcc \n\t"
 
-FOP_START(setcc)
+__FOP_START(setcc, SETCC_ALIGN)
 FOP_SETCC(seto)
 FOP_SETCC(setno)
 FOP_SETCC(setc)