[RFC PATCH 3/3] KVM: x86: Disallow writes to immutable feature MSRs after KVM_RUN

[Sean Christopherson <seanjc@google.com>]

Disallow writes to feature MSRs after KVM_RUN to prevent userspace from
changing the vCPU model after running the vCPU.  Similar to guest CPUID,
KVM uses feature MSRs to configure intercepts, determine what operations
are/aren't allowed, etc.  Changing the capabilities while the vCPU is
active will at best yield unpredictable guest behavior, and at worst
could be dangerous to KVM.

Allow writing the current value, e.g. so that userspace can blindly set
all MSRs when emulating RESET, and unconditionally allow writes to
MSR_IA32_UCODE_REV so that userspace can emulate patch loads.

Special case the VMX MSRs to keep the generic list small, i.e. so that
KVM can do a linear walk of the generic list without incurring meaningful
overhead.

Cc: Like Xu <like.xu.linux@gmail.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a1c65b77fb16..4da26a1f14c1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1541,6 +1541,26 @@ static u32 msr_based_features[ARRAY_SIZE(msr_based_features_all_except_vmx) +
 			      (KVM_LAST_EMULATED_VMX_MSR - KVM_FIRST_EMULATED_VMX_MSR + 1)];
 static unsigned int num_msr_based_features;
 
+/*
+ * All feature MSRs except uCode revID, which tracks the currently loaded uCode
+ * patch, are immutable once the vCPU model is defined.
+ */
+static bool kvm_is_immutable_feature_msr(u32 msr)
+{
+	int i;
+
+	if (msr >= KVM_FIRST_EMULATED_VMX_MSR && msr <= KVM_LAST_EMULATED_VMX_MSR)
+		return true;
+
+	for (i = 0; i < ARRAY_SIZE(msr_based_features_all_except_vmx); i++) {
+		if (msr == msr_based_features_all_except_vmx[i])
+			return msr != MSR_IA32_UCODE_REV;
+	}
+
+	return false;
+}
+
+
 static u64 kvm_get_arch_capabilities(void)
 {
 	u64 data = 0;
@@ -2136,6 +2156,23 @@ static int do_get_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 
 static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 {
+	u64 val;
+
+	/*
+	 * Disallow writes to immutable feature MSRs after KVM_RUN.  KVM does
+	 * not support modifying the guest vCPU model on the fly, e.g. changing
+	 * the nVMX capabilities while L2 is running is nonsensical.  Ignore
+	 * writes of the same value, e.g. to allow userspace to blindly stuff
+	 * all MSRs when emulating RESET.
+	 */
+	if (vcpu->arch.last_vmentry_cpu != -1 &&
+	    kvm_is_immutable_feature_msr(index)) {
+		if (do_get_msr(vcpu, index, &val) || *data != val)
+			return -EINVAL;
+
+		return 0;
+	}
+
 	return kvm_set_msr_ignored_check(vcpu, index, *data, true);
 }
 
-- 
2.37.1.559.g78731f0fdb-goog