virtionet: Fix 'path too long' issues with virtio-net unixgram Support

pkg/vf/virtionet.go

@@ -2,6 +2,7 @@ package vf
 
 import (
  "fmt"
+ "math/rand"
  "net"
  "os"
  "os/signal"
@@ -19,36 +20,45 @@ type VirtioNet struct {
  localAddr *net.UnixAddr
 }
 
-func localUnixSocketPath() (string, error) {
- homeDir, err := os.UserHomeDir()
+func localUnixSocketPath(dir string) (string, error) {
+ // unix socket endpoints are filesystem paths, but their max length is
+ // quite small (a bit over 100 bytes).
+ // In this function we try to build a filename which is relatively
+ // unique, not easily guessable (to prevent hostile collisions), and
+ // short (`os.CreateTemp` filenames are a bit too long)
+ //
+ // os.Getpid() is unique but guessable. We append a short 16 bit random
+ // number to it. We only use hex values to make the representation more
+ // compact
+ filename := filepath.Join(dir, fmt.Sprintf("vfkit-%x-%x.sock", os.Getpid(), rand.Int31n(0xffff))) //#nosec G404 -- no need for crypto/rand here
+
+ tmpFile, err := os.OpenFile(filename, os.O_CREATE|os.O_EXCL, 0600)
  if err != nil {
  return "", err
  }
- dir := filepath.Join(homeDir, "Library", "Application Support", "vfkit")
- if err := os.MkdirAll(dir, 0755); err != nil {
- return "", err
- }
- tmpFile, err := os.CreateTemp(dir, fmt.Sprintf("net-%d-*.sock", os.Getpid()))
- if err != nil {
- return "", err
- }
- // slightly racy, but this is in a directory only user-writable
+ // slightly racy, but hopefully this is in a directory only user-writable
  defer tmpFile.Close()
  defer os.Remove(tmpFile.Name())
 
  return tmpFile.Name(), nil
 }
 
 func (dev *VirtioNet) connectUnixPath() error {
+ // path for unixgram sockets must be less than 104 bytes on macOS
+ const maxUnixgramPathLen = 104
+
  remoteAddr := net.UnixAddr{
  Name: dev.UnixSocketPath,
  Net: "unixgram",
  }
- localSocketPath, err := localUnixSocketPath()
+ localSocketPath, err := localUnixSocketPath(filepath.Dir(dev.UnixSocketPath))
  if err != nil {
  return err
  }
- // FIXME: need to remove localSocketPath at process exit
+ if len(localSocketPath) >= maxUnixgramPathLen {
+ return fmt.Errorf("unixgram path '%s' is too long: %d >= %d bytes", localSocketPath, len(localSocketPath), maxUnixgramPathLen)
+ }
+ // FIXME: need to remove localSocketPath at process exit
  localAddr := net.UnixAddr{
  Name: localSocketPath,
  Net: "unixgram",