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

commit c62fa117c32bd1abed9304c58e0da6940f8c7fc2 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>

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 23f884a5d12c803f8bf8b8b725fc9138d4a8c438..0b7f6bcbb8ea941813ee0b5795517656e2fc126c 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1061,6 +1061,15 @@ do_cmd_auto:
 
        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;
 
                /*
@@ -2461,6 +2470,14 @@ 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");