x86/entry: Have entry_ibpb() invalidate return predictions

entry_ibpb() should invalidate all indirect predictions, including return
target predictions. Not all IBPB implementations do this, in which case the
fallback is RSB filling.

Prevent SRSO-style hijacks of return predictions following IBPB, as the return
target predictor can be corrupted before the IBPB completes.

  [ bp: Massage. ]
Signed-off-by: Johannes Wikner <kwikner@ethz.ch>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>

arch/x86/entry/entry.S

@@ -9,6 +9,8 @@
 #include <asm/unwind_hints.h>
 #include <asm/segment.h>
 #include <asm/cache.h>
+#include <asm/cpufeatures.h>
+#include <asm/nospec-branch.h>
 
 #include "calling.h"
 
@@ -19,6 +21,9 @@ SYM_FUNC_START(entry_ibpb)
 	movl	$PRED_CMD_IBPB, %eax
 	xorl	%edx, %edx
 	wrmsr
+
+	/* Make sure IBPB clears return stack preductions too. */
+	FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_BUG_IBPB_NO_RET
 	RET
 SYM_FUNC_END(entry_ibpb)
 /* For KVM */