x86/kvm/lapic: preserve gfn_to_hva_cache len on cache reinit

vcpu->arch.pv_eoi is accessible through both HV_X64_MSR_VP_ASSIST_PAGE and
MSR_KVM_PV_EOI_EN so on migration userspace may try to restore them in any
order. Values match, however, kvm_lapic_enable_pv_eoi() uses different
length: for Hyper-V case it's the whole struct hv_vp_assist_page, for KVM
native case it is 8. In case we restore KVM-native MSR last cache will
be reinitialized with len=8 so trying to access VP assist page beyond
8 bytes with kvm_read_guest_cached() will fail.

Check if we re-initializing cache for the same address and preserve length
in case it was greater.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -2647,14 +2647,22 @@ int kvm_hv_vapic_msr_read(struct kvm_vcpu *vcpu, u32 reg, u64 *data)
 int kvm_lapic_enable_pv_eoi(struct kvm_vcpu *vcpu, u64 data, unsigned long len)
 {
 	u64 addr = data & ~KVM_MSR_ENABLED;
+	struct gfn_to_hva_cache *ghc = &vcpu->arch.pv_eoi.data;
+	unsigned long new_len;
+
 	if (!IS_ALIGNED(addr, 4))
 		return 1;
 
 	vcpu->arch.pv_eoi.msr_val = data;
 	if (!pv_eoi_enabled(vcpu))
 		return 0;
-	return kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.pv_eoi.data,
-					 addr, len);
+
+	if (addr == ghc->gpa && len <= ghc->len)
+		new_len = ghc->len;
+	else
+		new_len = len;
+
+	return kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, addr, new_len);
 }
 
 void kvm_apic_accept_events(struct kvm_vcpu *vcpu)