Don't panic on KVM init if no memory for new vCPU.

This should be an indication to the user to set greater memory limits, a clean
exit with an error should do that.

PiperOrigin-RevId: 761270698

Author: konstantin-s-bogom

pkg/sentry/platform/kvm/machine.go

@@ -227,11 +227,11 @@ type dieState struct {
 // createVCPU creates and returns a new vCPU.
 //
 // Precondition: mu must be held.
-func (m *machine) createVCPU(id int) *vCPU {
+func (m *machine) createVCPU(id int) (*vCPU, error) {
 	// Create the vCPU.
 	fd, errno := hostsyscall.RawSyscall(unix.SYS_IOCTL, uintptr(m.fd), KVM_CREATE_VCPU, uintptr(id))
 	if errno != 0 {
-		panic(fmt.Sprintf("error creating new vCPU(id=%d): %v", id, errno))
+		return nil, fmt.Errorf("error creating new vCPU(id=%d): %w", id, errno)
 	}
 
 	c := &vCPU{
@@ -259,7 +259,7 @@ func (m *machine) createVCPU(id int) *vCPU {
 		panic(fmt.Sprintf("error initializing vCPU(id=%d) state: %v", id, err))
 	}
 
-	return c // Done.
+	return c, nil // Done.
 }
 
 // forceMappingEntireAddressSpace forces mapping the entire process address

pkg/sentry/platform/kvm/machine_amd64.go

@@ -50,10 +50,12 @@ func (m *machine) initArchState() error {
 
 	// Initialize all vCPUs to minimize kvm ioctl-s allowed by seccomp filters.
 	m.mu.Lock()
+	defer m.mu.Unlock()
 	for i := 0; i < m.maxVCPUs; i++ {
-		m.createVCPU(i)
+		if _, err := m.createVCPU(i); err != nil {
+			return err
+		}
 	}
-	m.mu.Unlock()
 
 	return nil
 }

pkg/sentry/platform/kvm/machine_arm64_unsafe.go

@@ -49,15 +49,19 @@ func (m *machine) initArchState() error {
 		panic(fmt.Sprintf("error setting KVM_ARM_PREFERRED_TARGET failed: %v", errno))
 	}
 
-	// Initialize all vCPUs on ARM64, while this does not happen on x86_64.
-	// The reason for the difference is that ARM64 and x86_64 have different KVM timer mechanisms.
-	// If we create vCPU dynamically on ARM64, the timer for vCPU would mess up for a short time.
+	// Initialize all vCPUs on ARM64 (we also do this on x86_64, but for
+	// ARM64 it is especially important as it has a different KVM timer
+	// mechanism).
+	// If we create vCPU dynamically on ARM64, the timer for vCPU would mess
+	// up for a short time.
 	// For more detail, please refer to https://github.com/google/gvisor/issues/5739
 	m.mu.Lock()
+	defer m.mu.Unlock()
 	for i := 0; i < m.maxVCPUs; i++ {
-		m.createVCPU(i)
+		if _, err := m.createVCPU(i); err != nil {
+			return err
+		}
 	}
-	m.mu.Unlock()
 	return nil
 }