KVM, pkeys: save/restore PKRU when guest/host switches

Currently XSAVE state of host is not restored after VM-exit and PKRU
is managed by XSAVE so the PKRU from guest is still controlling the
memory access even if the CPU is running the code of host. This is
not safe as KVM needs to access the memory of userspace (e,g QEMU) to
do some emulation.

So we save/restore PKRU when guest/host switches.
Signed-off-by: Huaitong Han <huaitong.han@intel.com>
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -598,6 +598,10 @@ struct vcpu_vmx {
 	struct page *pml_pg;
 
 	u64 current_tsc_ratio;
+
+	bool guest_pkru_valid;
+	u32 guest_pkru;
+	u32 host_pkru;
 };
 
 enum segment_cache_field {
@@ -2107,6 +2111,7 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 	} while (cmpxchg(&pi_desc->control, old.control,
 			new.control) != old.control);
 }
+
 /*
  * Switches to specified vcpu, until a matching vcpu_put(), but assumes
  * vcpu mutex is already taken.
@@ -2167,6 +2172,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	}
 
 	vmx_vcpu_pi_load(vcpu, cpu);
+	vmx->host_pkru = read_pkru();
 }
 
 static void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
@@ -8639,6 +8645,9 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
 		vmx_set_interrupt_shadow(vcpu, 0);
 
+	if (vmx->guest_pkru_valid)
+		__write_pkru(vmx->guest_pkru);
+
 	atomic_switch_perf_msrs(vmx);
 	debugctlmsr = get_debugctlmsr();
 
@@ -8778,6 +8787,20 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 
 	vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);
 
+	/*
+	 * eager fpu is enabled if PKEY is supported and CR4 is switched
+	 * back on host, so it is safe to read guest PKRU from current
+	 * XSAVE.
+	 */
+	if (boot_cpu_has(X86_FEATURE_OSPKE)) {
+		vmx->guest_pkru = __read_pkru();
+		if (vmx->guest_pkru != vmx->host_pkru) {
+			vmx->guest_pkru_valid = true;
+			__write_pkru(vmx->host_pkru);
+		} else
+			vmx->guest_pkru_valid = false;
+	}
+
 	/*
 	 * the KVM_REQ_EVENT optimization bit is only on for one entry, and if
 	 * we did not inject a still-pending event to L1 now because of