platform/kvm: Disable async preemption while mapping initial virtual regions.

All new mappings are handled from the SIGSYS signal handler and we have to
guarantee that the signal handler doesn't race with the code that handles
initial regions.

PiperOrigin-RevId: 453521202

pkg/sentry/platform/kvm/machine.go

@@ -318,6 +318,10 @@ func newMachine(vm int) (*machine, error) {
 		}
 	}
 
+	// handleBluepillFault takes the slot spinlock and it is called from
+	// seccompMmapHandler, so here we have to guarantee that mmap is not
+	// called while we hold the slot spinlock.
+	disableAsyncPreemption()
 	applyVirtualRegions(func(vr virtualRegion) {
 		if excludeVirtualRegion(vr) {
 			return // skip region.
@@ -331,6 +335,7 @@ func newMachine(vm int) (*machine, error) {
 		mapRegion(vr, 0)
 
 	})
+	enableAsyncPreemption()
 
 	// Initialize architecture state.
 	if err := m.initArchState(); err != nil {

pkg/sentry/platform/kvm/machine_unsafe.go

@@ -240,3 +240,23 @@ func seccompMmapHandler(context unsafe.Pointer) {
 	}
 	seccompMmapHandlerCnt.Add(-1)
 }
+
+// disableAsyncPreemption disables asynchronous preemption of go-routines.
+func disableAsyncPreemption() {
+	set := linux.MakeSignalSet(linux.SIGURG)
+	_, _, errno := unix.RawSyscall6(unix.SYS_RT_SIGPROCMASK, linux.SIG_BLOCK,
+		uintptr(unsafe.Pointer(&set)), 0, linux.SignalSetSize, 0, 0)
+	if errno != 0 {
+		panic(fmt.Sprintf("sigprocmask failed: %d", errno))
+	}
+}
+
+// enableAsyncPreemption enables asynchronous preemption of go-routines.
+func enableAsyncPreemption() {
+	set := linux.MakeSignalSet(linux.SIGURG)
+	_, _, errno := unix.RawSyscall6(unix.SYS_RT_SIGPROCMASK, linux.SIG_UNBLOCK,
+		uintptr(unsafe.Pointer(&set)), 0, linux.SignalSetSize, 0, 0)
+	if errno != 0 {
+		panic(fmt.Sprintf("sigprocmask failed: %d", errno))
+	}
+}