KVM: x86: Fix potential race in KVM_GET_CLOCK

Sean noticed that KVM_GET_CLOCK was checking kvm_arch.use_master_clock
outside of the pvclock sync lock. This is problematic, as the clock
value written to the user may or may not actually correspond to a stable
TSC.

Fix the race by populating the entire kvm_clock_data structure behind
the pvclock_gtod_sync_lock.
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20210916181538.968978-4-oupton@google.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -2781,19 +2781,20 @@ static void kvm_update_masterclock(struct kvm *kvm)
 	kvm_end_pvclock_update(kvm);
 }
 
-u64 get_kvmclock_ns(struct kvm *kvm)
+static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 {
 	struct kvm_arch *ka = &kvm->arch;
 	struct pvclock_vcpu_time_info hv_clock;
 	unsigned long flags;
-	u64 ret;
 
 	spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
 	if (!ka->use_master_clock) {
 		spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
-		return get_kvmclock_base_ns() + ka->kvmclock_offset;
+		data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
+		return;
 	}
 
+	data->flags |= KVM_CLOCK_TSC_STABLE;
 	hv_clock.tsc_timestamp = ka->master_cycle_now;
 	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
 	spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
@@ -2805,13 +2806,26 @@ u64 get_kvmclock_ns(struct kvm *kvm)
 		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
 				   &hv_clock.tsc_shift,
 				   &hv_clock.tsc_to_system_mul);
-		ret = __pvclock_read_cycles(&hv_clock, rdtsc());
-	} else
-		ret = get_kvmclock_base_ns() + ka->kvmclock_offset;
+		data->clock = __pvclock_read_cycles(&hv_clock, rdtsc());
+	} else {
+		data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
+	}
 
 	put_cpu();
+}
 
-	return ret;
+u64 get_kvmclock_ns(struct kvm *kvm)
+{
+	struct kvm_clock_data data;
+
+	/*
+	 * Zero flags as it's accessed RMW, leave everything else uninitialized
+	 * as clock is always written and no other fields are consumed.
+	 */
+	data.flags = 0;
+
+	get_kvmclock(kvm, &data);
+	return data.clock;
 }
 
 static void kvm_setup_pvclock_page(struct kvm_vcpu *v,
@@ -5820,13 +5834,9 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
 static int kvm_vm_ioctl_get_clock(struct kvm *kvm, void __user *argp)
 {
 	struct kvm_clock_data data;
-	u64 now_ns;
-
-	now_ns = get_kvmclock_ns(kvm);
-	user_ns.clock = now_ns;
-	user_ns.flags = kvm->arch.use_master_clock ? KVM_CLOCK_TSC_STABLE : 0;
-	memset(&user_ns.pad, 0, sizeof(user_ns.pad));
 
+	memset(&data, 0, sizeof(data));
+	get_kvmclock(kvm, &data);
 	if (copy_to_user(argp, &data, sizeof(data)))
 		return -EFAULT;