From c62fa117c32bd1abed9304c58e0da6940f8c7fc2 Mon Sep 17 00:00:00 2001 From: Johannes Wikner Date: Tue, 8 Oct 2024 12:58:03 +0200 Subject: x86/bugs: Do not use UNTRAIN_RET with IBPB on entry 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 Signed-off-by: Johannes Wikner Cc: --- arch/x86/kernel/cpu/bugs.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) (limited to 'arch') diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 4474c5a1cd9a..47a01d4028f6 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -1115,6 +1115,15 @@ 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); + setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT); mitigate_smt = true; @@ -2635,6 +2644,14 @@ 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 MITIGATION_IBPB_ENTRY.\n"); -- cgit v1.2.3