x86/bugs: Skip RSB fill at VMEXIT

entry_ibpb() is designed to follow Intel's IBPB specification regardless
of CPU. This includes invalidating RSB entries.

Hence, if IBPB on VMEXIT has been selected, entry_ibpb() as part of the
RET untraining in the VMEXIT path will take care of all BTB and RSB
clearing so there's no need to explicitly fill the RSB anymore.

  [ bp: Massage commit message. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Johannes Wikner <kwikner@ethz.ch>
Cc: <stable@kernel.org>

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index d1915427b4ffcb1141452b0fe5bb8872ab2ae1da..4474c5a1cd9a70da2e4554e5861c245c4e034994 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1117,6 +1117,14 @@ do_cmd_auto:
                setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
                setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
                mitigate_smt = true;
+
+               /*
+                * There is no need for RSB filling: entry_ibpb() ensures
+                * all predictions, including the RSB, are invalidated,
+                * regardless of IBPB implementation.
+                */
+               setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
+
                break;
 
        case RETBLEED_MITIGATION_STUFF:
@@ -2638,6 +2646,13 @@ static void __init srso_select_mitigation(void)
                        if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
                                setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
                                srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
+
+                               /*
+                                * There is no need for RSB filling: entry_ibpb() ensures
+                                * all predictions, including the RSB, are invalidated,
+                                * regardless of IBPB implementation.
+                                */
+                               setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
                        }
                } else {
                        pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");