KVM: x86: Reject loading KVM if host.PAT[0] != WB

Reject KVM if entry '0' in the host's IA32_PAT MSR is not programmed to
writeback (WB) memtype.  KVM subtly relies on IA32_PAT entry '0' to be
programmed to WB by leaving the PAT bits in shadow paging and NPT SPTEs
as '0'.  If something other than WB is in PAT[0], at _best_ guests will
suffer very poor performance, and at worst KVM will crash the system by
breaking cache-coherency expecations (e.g. using WC for guest memory).
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220715230016.3762909-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -9141,6 +9141,7 @@ static struct notifier_block pvclock_gtod_notifier = {
 int kvm_arch_init(void *opaque)
 {
 	struct kvm_x86_init_ops *ops = opaque;
+	u64 host_pat;
 	int r;
 
 	if (kvm_x86_ops.hardware_enable) {
@@ -9179,6 +9180,20 @@ int kvm_arch_init(void *opaque)
 		goto out;
 	}
 
+	/*
+	 * KVM assumes that PAT entry '0' encodes WB memtype and simply zeroes
+	 * the PAT bits in SPTEs.  Bail if PAT[0] is programmed to something
+	 * other than WB.  Note, EPT doesn't utilize the PAT, but don't bother
+	 * with an exception.  PAT[0] is set to WB on RESET and also by the
+	 * kernel, i.e. failure indicates a kernel bug or broken firmware.
+	 */
+	if (rdmsrl_safe(MSR_IA32_CR_PAT, &host_pat) ||
+	    (host_pat & GENMASK(2, 0)) != 6) {
+		pr_err("kvm: host PAT[0] is not WB\n");
+		r = -EIO;
+		goto out;
+	}
+
 	r = -ENOMEM;
 
 	x86_emulator_cache = kvm_alloc_emulator_cache();