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-Tag: v6.6.106~2 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1e1dc1f9edb92effe32751921778164ed8fb3056;p=thirdparty%2Fkernel%2Fstable.git x86/vmscape: Warn when STIBP is disabled with SMT Commit b7cc9887231526ca4fa89f3fa4119e47c2dc7b1e upstream. 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 Signed-off-by: Borislav Petkov (AMD) Signed-off-by: Greg Kroah-Hartman --- diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 41a543e88c375..315926ccea0fa 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -2942,6 +2942,8 @@ static void __init vmscape_select_mitigation(void) #undef pr_fmt #define pr_fmt(fmt) fmt +#define VMSCAPE_MSG_SMT "VMSCAPE: SMT on, STIBP is required for full protection. See https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/vmscape.html for more details.\n" + void cpu_bugs_smt_update(void) { mutex_lock(&spec_ctrl_mutex); @@ -3013,6 +3015,28 @@ 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); }