x86/bugs: Do not use UNTRAIN_RET with IBPB on entry

BugLink: https://bugs.launchpad.net/bugs/2088231

commit c62fa11 upstream.

Since X86_FEATURE_ENTRY_IBPB will invalidate all harmful predictions
with IBPB, no software-based untraining of returns is needed anymore.
Currently, this change affects retbleed and SRSO mitigations so if
either of the mitigations is doing IBPB and the other one does the
software sequence, the latter is not needed anymore.

  [ bp: Massage commit message. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Johannes Wikner <kwikner@ethz.ch>
Cc: <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

Author: sktt

arch/x86/kernel/cpu/bugs.c

@@ -1092,6 +1092,15 @@ static void __init retbleed_select_mitigation(void)
 
 	case RETBLEED_MITIGATION_IBPB:
 		setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+
+		/*
+		 * IBPB on entry already obviates the need for
+		 * software-based untraining so clear those in case some
+		 * other mitigation like SRSO has selected them.
+		 */
+		setup_clear_cpu_cap(X86_FEATURE_UNRET);
+		setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
+
 		mitigate_smt = true;
 
 		/*
@@ -2599,6 +2608,14 @@ static void __init srso_select_mitigation(void)
 			if (has_microcode) {
 				setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
 				srso_mitigation = SRSO_MITIGATION_IBPB;
+
+				/*
+				 * IBPB on entry already obviates the need for
+				 * software-based untraining so clear those in case some
+				 * other mitigation like Retbleed has selected them.
+				 */
+				setup_clear_cpu_cap(X86_FEATURE_UNRET);
+				setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
 			}
 		} else {
 			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");