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KVM: x86: Mitigate the cross-thread return address predictions bug

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By default, KVM/SVM will intercept attempts by the guest to transition
out of C0. However, the KVM_CAP_X86_DISABLE_EXITS capability can be used
by a VMM to change this behavior. To mitigate the cross-thread return
address predictions bug (X86_BUG_SMT_RSB), a VMM must not be allowed to
override the default behavior to intercept C0 transitions.

Use a module parameter to control the mitigation on processors that are
vulnerable to X86_BUG_SMT_RSB. If the processor is vulnerable to the
X86_BUG_SMT_RSB bug and the module parameter is set to mitigate the bug,
KVM will not allow the disabling of the HLT, MWAIT and CSTATE exits.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <4019348b5e07148eb4d593380a5f6713b93c9a16.1675956146.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Tom Lendacky 2023-02-09 09:22:25 -06:00 committed by Paolo Bonzini
parent be8de49bea
commit 6f0f2d5ef8
1 changed files with 32 additions and 11 deletions
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43
arch/x86/kvm/x86.c
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@ -191,6 +191,10 @@ module_param(enable_pmu, bool, 0444);
bool __read_mostly eager_page_split = true; bool __read_mostly eager_page_split = true;
module_param(eager_page_split, bool, 0644); module_param(eager_page_split, bool, 0644);
/* Enable/disable SMT_RSB bug mitigation */
bool __read_mostly mitigate_smt_rsb;
module_param(mitigate_smt_rsb, bool, 0444);
/* /*
* Restoring the host value for MSRs that are only consumed when running in * Restoring the host value for MSRs that are only consumed when running in
* usermode, e.g. SYSCALL MSRs and TSC_AUX, can be deferred until the CPU * usermode, e.g. SYSCALL MSRs and TSC_AUX, can be deferred until the CPU
@ -4448,10 +4452,15 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
r = KVM_CLOCK_VALID_FLAGS; r = KVM_CLOCK_VALID_FLAGS;
break; break;
case KVM_CAP_X86_DISABLE_EXITS: case KVM_CAP_X86_DISABLE_EXITS:
r |= KVM_X86_DISABLE_EXITS_HLT | KVM_X86_DISABLE_EXITS_PAUSE | r = KVM_X86_DISABLE_EXITS_PAUSE;
KVM_X86_DISABLE_EXITS_CSTATE;
if(kvm_can_mwait_in_guest()) if (!mitigate_smt_rsb) {
r |= KVM_X86_DISABLE_EXITS_MWAIT; r |= KVM_X86_DISABLE_EXITS_HLT |
KVM_X86_DISABLE_EXITS_CSTATE;
if (kvm_can_mwait_in_guest())
r |= KVM_X86_DISABLE_EXITS_MWAIT;
}
break; break;
case KVM_CAP_X86_SMM: case KVM_CAP_X86_SMM:
if (!IS_ENABLED(CONFIG_KVM_SMM)) if (!IS_ENABLED(CONFIG_KVM_SMM))
@ -6227,15 +6236,26 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS) if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
break; break;
if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
kvm_can_mwait_in_guest())
kvm->arch.mwait_in_guest = true;
if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
kvm->arch.hlt_in_guest = true;
if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE) if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
kvm->arch.pause_in_guest = true; kvm->arch.pause_in_guest = true;
if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
kvm->arch.cstate_in_guest = true; #define SMT_RSB_MSG "This processor is affected by the Cross-Thread Return Predictions vulnerability. " \
"KVM_CAP_X86_DISABLE_EXITS should only be used with SMT disabled or trusted guests."
if (!mitigate_smt_rsb) {
if (boot_cpu_has_bug(X86_BUG_SMT_RSB) && cpu_smt_possible() &&
(cap->args[0] & ~KVM_X86_DISABLE_EXITS_PAUSE))
pr_warn_once(SMT_RSB_MSG);
if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
kvm_can_mwait_in_guest())
kvm->arch.mwait_in_guest = true;
if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
kvm->arch.hlt_in_guest = true;
if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
kvm->arch.cstate_in_guest = true;
}
r = 0; r = 0;
break; break;
case KVM_CAP_MSR_PLATFORM_INFO: case KVM_CAP_MSR_PLATFORM_INFO:
@ -13456,6 +13476,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_vmgexit_msr_protocol_exit);
static int __init kvm_x86_init(void) static int __init kvm_x86_init(void)
{ {
kvm_mmu_x86_module_init(); kvm_mmu_x86_module_init();
mitigate_smt_rsb &= boot_cpu_has_bug(X86_BUG_SMT_RSB) && cpu_smt_possible();
return 0; return 0;
} }
module_init(kvm_x86_init); module_init(kvm_x86_init);