From: Pawan Gupta Date: Thu, 14 Aug 2025 17:20:43 +0000 (-0700) Subject: x86/vmscape: Warn when STIBP is disabled with SMT X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=b7cc9887231526ca4fa89f3fa4119e47c2dc7b1e;p=users%2Fhch%2Fmisc.git x86/vmscape: Warn when STIBP is disabled with SMT Cross-thread attacks are generally harder as they require the victim to be co-located on a core. However, with VMSCAPE the adversary targets belong to the same guest execution, that are more likely to get co-located. In particular, a thread that is currently executing userspace hypervisor (after the IBPB) may still be targeted by a guest execution from a sibling thread. Issue a warning about the potential risk, except when: - SMT is disabled - STIBP is enabled system-wide - Intel eIBRS is enabled (which implies STIBP protection) Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen --- diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 1f8c1c51d057..fa32615db71d 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -3400,6 +3400,28 @@ void cpu_bugs_smt_update(void) break; } + switch (vmscape_mitigation) { + case VMSCAPE_MITIGATION_NONE: + case VMSCAPE_MITIGATION_AUTO: + break; + case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT: + case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER: + /* + * Hypervisors can be attacked across-threads, warn for SMT when + * STIBP is not already enabled system-wide. + * + * Intel eIBRS (!AUTOIBRS) implies STIBP on. + */ + if (!sched_smt_active() || + spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT || + spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT_PREFERRED || + (spectre_v2_in_eibrs_mode(spectre_v2_enabled) && + !boot_cpu_has(X86_FEATURE_AUTOIBRS))) + break; + pr_warn_once(VMSCAPE_MSG_SMT); + break; + } + mutex_unlock(&spec_ctrl_mutex); }