[PATCH v4 1/8] KVM: nVMX: Keep KVM updates to BNDCFGS ctrl bits across MSR write

[Oliver Upton <oupton@xxxxxxxxxx>]

Since commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls
when guest MPX disabled"), KVM has taken ownership of the "load
IA32_BNDCFGS" and "clear IA32_BNDCFGS" VMX entry/exit controls. The ABI
is that these bits must be set in the IA32_VMX_TRUE_{ENTRY,EXIT}_CTLS
MSRs if the guest's CPUID supports MPX, and clear otherwise.

However, commit aedbaf4f6afd ("KVM: x86: Extract
kvm_update_cpuid_runtime() from kvm_update_cpuid()") partially broke KVM
ownership of the aforementioned bits. Before, kvm_update_cpuid() was
exercised frequently when running a guest and constantly applied its own
changes to the BNDCFGS bits. Now, the BNDCFGS bits are only ever
updated after a KVM_SET_CPUID/KVM_SET_CPUID2 ioctl, meaning that a
subsequent MSR write from userspace will clobber these values.

Uphold the old ABI by reapplying KVM's tweaks to the BNDCFGS bits after
an MSR write from userspace.

Fixes: aedbaf4f6afd ("KVM: x86: Extract kvm_update_cpuid_runtime() from kvm_update_cpuid()")
Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/nested.c | 9 +++++++++
 arch/x86/kvm/vmx/vmx.c    | 2 +-
 arch/x86/kvm/vmx/vmx.h    | 2 ++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 1dfe23963a9e..a13f8f4e3d82 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1291,6 +1291,15 @@ vmx_restore_control_msr(struct vcpu_vmx *vmx, u32 msr_index, u64 data)
 
 	*lowp = data;
 	*highp = data >> 32;
+
+	/*
+	 * Ensure KVM fiddling with these MSRs is preserved after userspace
+	 * write.
+	 */
+	if (msr_index == MSR_IA32_VMX_TRUE_ENTRY_CTLS ||
+	    msr_index == MSR_IA32_VMX_TRUE_EXIT_CTLS)
+		nested_vmx_entry_exit_ctls_update(&vmx->vcpu);
+
 	return 0;
 }
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b325f99b2177..3a97220c5f78 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7246,7 +7246,7 @@ static void nested_vmx_cr_fixed1_bits_update(struct kvm_vcpu *vcpu)
 #undef cr4_fixed1_update
 }
 
-static void nested_vmx_entry_exit_ctls_update(struct kvm_vcpu *vcpu)
+void nested_vmx_entry_exit_ctls_update(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 7f2c82e7f38f..e134e2763502 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -423,6 +423,8 @@ static inline void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr,
 
 void vmx_update_cpu_dirty_logging(struct kvm_vcpu *vcpu);
 
+void nested_vmx_entry_exit_ctls_update(struct kvm_vcpu *vcpu);
+
 /*
  * Note, early Intel manuals have the write-low and read-high bitmap offsets
  * the wrong way round.  The bitmaps control MSRs 0x00000000-0x00001fff and
-- 
2.35.1.574.g5d30c73bfb-goog