[PATCH 5/6] KVM: SVM: Add missing save/restore handling of LBR MSRs

Yosry Ahmed
2025-11-08T00:46:01Z

MSR_IA32_DEBUGCTLMSR and LBR MSRs are currently not enumerated by
KVM_GET_MSR_INDEX_LIST, and LBR MSRs cannot be set with KVM_SET_MSRS. So
save/restore is completely broken.

Fix it by adding the MSRs to msrs_to_save_base, and allowing writes to
LBR MSRs from userspace only (as they are read-only MSRs). Additionally,
to correctly restore L1's LBRs while L2 is running, make sure the LBRs
are copied from the captured VMCB01 save area in svm_copy_vmrun_state().

Fixes: 24e09cbf480a ("KVM: SVM: enable LBR virtualization")
Cc: stable@vger.kernel.org

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
arch/x86/kvm/svm/nested.c | 3 +++
arch/x86/kvm/svm/svm.c | 20 ++++++++++++++++++++
arch/x86/kvm/x86.c | 3 +++
3 files changed, 26 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index e7861392f2fcd..955de5d90f48e 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1057,6 +1057,9 @@ void svm_copy_vmrun_state(struct vmcb_save_area *to_save,
to_save->isst_addr = from_save->isst_addr;
to_save->ssp = from_save->ssp;
}
+
+ if (lbrv)
+ svm_copy_lbrs(to_save, from_save);
}

void svm_copy_vmloadsave_state(struct vmcb *to_vmcb, struct vmcb *from_vmcb)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 9eb112f0e61f0..45faf892a0431 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2988,6 +2988,26 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
vmcb_mark_dirty(svm->vmcb, VMCB_LBR);
svm_update_lbrv(vcpu);
break;
+ case MSR_IA32_LASTBRANCHFROMIP:
+ if (!msr->host_initiated)
+ return 1;
+ svm->vmcb->save.br_from = data;
+ break;
+ case MSR_IA32_LASTBRANCHTOIP:
+ if (!msr->host_initiated)
+ return 1;
+ svm->vmcb->save.br_to = data;
+ break;
+ case MSR_IA32_LASTINTFROMIP:
+ if (!msr->host_initiated)
+ return 1;
+ svm->vmcb->save.last_excp_from = data;
+ break;
+ case MSR_IA32_LASTINTTOIP:
+ if (!msr->host_initiated)
+ return 1;
+ svm->vmcb->save.last_excp_to = data;
+ break;
case MSR_VM_HSAVE_PA:
/*
* Old kernels did not validate the value written to
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9c2e28028c2b5..1d36a0f782c49 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -348,6 +348,9 @@ static const u32 msrs_to_save_base[] = {
MSR_IA32_U_CET, MSR_IA32_S_CET,
MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP, MSR_IA32_PL2_SSP,
MSR_IA32_PL3_SSP, MSR_IA32_INT_SSP_TAB,
+ MSR_IA32_DEBUGCTLMSR,
+ MSR_IA32_LASTBRANCHFROMIP, MSR_IA32_LASTBRANCHTOIP,
+ MSR_IA32_LASTINTFROMIP, MSR_IA32_LASTINTTOIP,
};

static const u32 msrs_to_save_pmu[] = {
--
2.51.2.1041.gc1ab5b90ca-goog