From: Jim Mattson jmattson@google.com

commit 2e7eab81425ad6c875f2ed47c0ce01e78afc38a5 upstream.

According to Intel's document on Indirect Branch Restricted Speculation, "Enabling IBRS does not prevent software from controlling the predicted targets of indirect branches of unrelated software executed later at the same predictor mode (for example, between two different user applications, or two different virtual machines). Such isolation can be ensured through use of the Indirect Branch Predictor Barrier (IBPB) command." This applies to both basic and enhanced IBRS.

Since L1 and L2 VMs share hardware predictor modes (guest-user and guest-kernel), hardware IBRS is not sufficient to virtualize IBRS. (The way that basic IBRS is implemented on pre-eIBRS parts, hardware IBRS is actually sufficient in practice, even though it isn't sufficient architecturally.)

For virtual CPUs that support IBRS, add an indirect branch prediction barrier on emulated VM-exit, to ensure that the predicted targets of indirect branches executed in L1 cannot be controlled by software that was executed in L2.

Since we typically don't intercept guest writes to IA32_SPEC_CTRL, perform the IBPB at emulated VM-exit regardless of the current IA32_SPEC_CTRL.IBRS value, even though the IBPB could technically be deferred until L1 sets IA32_SPEC_CTRL.IBRS, if IA32_SPEC_CTRL.IBRS is clear at emulated VM-exit.

This is CVE-2022-2196.

Fixes: 5c911beff20a ("KVM: nVMX: Skip IBPB when switching between vmcs01 and vmcs02") Cc: Sean Christopherson seanjc@google.com Signed-off-by: Jim Mattson jmattson@google.com Reviewed-by: Sean Christopherson seanjc@google.com Link: https://lore.kernel.org/r/20221019213620.1953281-3-jmattson@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Ovidiu Panait ovidiu.panait@eng.windriver.com --- arch/x86/kvm/vmx/nested.c | 11 +++++++++++ arch/x86/kvm/vmx/vmx.c | 6 ++++-- 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index cdebeceedbd0..f3c136548af6 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -4617,6 +4617,17 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,

vmx_switch_vmcs(vcpu, &vmx->vmcs01);

+ /* + * If IBRS is advertised to the vCPU, KVM must flush the indirect + * branch predictors when transitioning from L2 to L1, as L1 expects + * hardware (KVM in this case) to provide separate predictor modes. + * Bare metal isolates VMX root (host) from VMX non-root (guest), but + * doesn't isolate different VMCSs, i.e. in this case, doesn't provide + * separate modes for L2 vs L1. + */ + if (guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)) + indirect_branch_prediction_barrier(); + /* Update any VMCS fields that might have changed while L2 ran */ vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr); vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr); diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 0718658268fe..c849173b60c2 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -1332,8 +1332,10 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,

/* * No indirect branch prediction barrier needed when switching - * the active VMCS within a guest, e.g. on nested VM-Enter. - * The L1 VMM can protect itself with retpolines, IBPB or IBRS. + * the active VMCS within a vCPU, unless IBRS is advertised to + * the vCPU. To minimize the number of IBPBs executed, KVM + * performs IBPB on nested VM-Exit (a single nested transition + * may switch the active VMCS multiple times). */ if (!buddy || WARN_ON_ONCE(buddy->vmcs != prev)) indirect_branch_prediction_barrier();