Cache SSH connections

The underlying SSH connections are kept open and are reused
across several SSH sessions. This is due to upstream issues in
which concurrent/parallel SSH connections may lead to instability.

golang/go#51926
golang/go#27140
Signed-off-by: Paulo Gomes <[email protected]>

go.mod

@@ -39,7 +39,7 @@ require (
 	github.com/onsi/gomega v1.18.1
 	github.com/otiai10/copy v1.7.0
 	github.com/spf13/pflag v1.0.5
-	golang.org/x/crypto v0.0.0-20220315160706-3147a52a75dd
+	golang.org/x/crypto v0.0.0-20220321153916-2c7772ba3064
 	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 	google.golang.org/api v0.73.0
 	gotest.tools v2.2.0+incompatible

go.sum

@@ -1153,6 +1153,8 @@ golang.org/x/crypto v0.0.0-20211117183948-ae814b36b871/go.mod h1:IxCIyHEi3zRg3s0
 golang.org/x/crypto v0.0.0-20220214200702-86341886e292/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
 golang.org/x/crypto v0.0.0-20220315160706-3147a52a75dd h1:XcWmESyNjXJMLahc3mqVQJcgSTDxFxhETVlfk9uGc38=
 golang.org/x/crypto v0.0.0-20220315160706-3147a52a75dd/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
+golang.org/x/crypto v0.0.0-20220321153916-2c7772ba3064 h1:S25/rfnfsMVgORT4/J61MJ7rdyseOZOyvLIrZEZ7s6s=
+golang.org/x/crypto v0.0.0-20220321153916-2c7772ba3064/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
 golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190306152737-a1d7652674e8/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190510132918-efd6b22b2522/go.mod h1:ZjyILWgesfNpC6sMxTJOJm9Kp84zZh5NQWvqDGG3Qr8=

main.go

@@ -228,7 +228,7 @@ func main() {
 	}()
 
 	if managed.Enabled() {
-		managed.InitManagedTransport()
+		managed.InitManagedTransport(ctrl.Log.WithName("managed-transport"))
 	}
 
 	setupLog.Info("starting manager")

pkg/git/libgit2/managed/http.go

@@ -209,10 +209,12 @@ func createClientRequest(targetUrl string, action git2go.SmartServiceAction, t *
 }
 
 func (t *httpSmartSubtransport) Close() error {
+	traceLog.Info("[http]: httpSmartSubtransport.Close()")
 	return nil
 }
 
 func (t *httpSmartSubtransport) Free() {
+	traceLog.Info("[http]: httpSmartSubtransport.Free()")
 }
 
 type httpSmartSubtransportStream struct {
@@ -277,6 +279,8 @@ func (self *httpSmartSubtransportStream) Write(buf []byte) (int, error) {
 
 func (self *httpSmartSubtransportStream) Free() {
 	if self.resp != nil {
+		traceLog.Info("[http]: httpSmartSubtransportStream.Free()")
+
 		// ensure body is fully processed and closed
 		// for increased likelihood of transport reuse in HTTP/1.x.
 		// it should not be a problem to do this more than once.
@@ -344,6 +348,7 @@ func (self *httpSmartSubtransportStream) sendRequest() error {
 		}
 
 		req.SetBasicAuth(userName, password)
+		traceLog.Info("[http]: new request", "method", req.Method, "URL", req.URL)
 		resp, err = self.client.Do(req)
 		if err != nil {
 			return err
@@ -363,6 +368,7 @@ func (self *httpSmartSubtransportStream) sendRequest() error {
 				return err
 			}
 
+			traceLog.Info("[http]: POST redirect", "URL", self.req.URL)
 			continue
 		}
 

pkg/git/libgit2/managed/init.go

@@ -19,6 +19,9 @@ package managed
 import (
 	"sync"
 	"time"
+
+	"github.com/fluxcd/pkg/runtime/logger"
+	"github.com/go-logr/logr"
 )
 
 var (
@@ -34,6 +37,9 @@ var (
 	// regardless of the current operation (i.e. connection,
 	// handshake, put/get).
 	fullHttpClientTimeOut time.Duration = 10 * time.Minute
+
+	debugLog logr.Logger
+	traceLog logr.Logger
 )
 
 // InitManagedTransport initialises HTTP(S) and SSH managed transport
@@ -47,9 +53,14 @@ var (
 //
 // This function will only register managed transports once, subsequent calls
 // leads to no-op.
-func InitManagedTransport() error {
+func InitManagedTransport(log logr.Logger) error {
 	var err error
+
 	once.Do(func() {
+		log.Info("Enabling experimental managed transport")
+		debugLog = log.V(logger.DebugLevel)
+		traceLog = log.V(logger.TraceLevel)
+
 		if err = registerManagedHTTP(); err != nil {
 			return
 		}

pkg/git/libgit2/managed/managed_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/fluxcd/pkg/gittestserver"
 	"github.com/fluxcd/pkg/ssh"
 	"github.com/fluxcd/source-controller/pkg/git"
+	"github.com/go-logr/logr"
 
 	git2go "github.com/libgit2/git2go/v33"
 	. "github.com/onsi/gomega"
@@ -247,7 +248,7 @@ func TestManagedTransport_E2E(t *testing.T) {
 	defer server.StopSSH()
 
 	// Force managed transport to be enabled
-	InitManagedTransport()
+	InitManagedTransport(logr.Discard())
 
 	repoPath := "test.git"
 	err = server.InitRepo("../testdata/git/repo", git.DefaultBranch, repoPath)
@@ -312,7 +313,7 @@ func TestManagedTransport_HandleRedirect(t *testing.T) {
 	defer os.RemoveAll(tmpDir)
 
 	// Force managed transport to be enabled
-	InitManagedTransport()
+	InitManagedTransport(logr.Discard())
 
 	// GitHub will cause a 301 and redirect to https
 	repo, err := git2go.Clone("http://github.com/stefanprodan/podinfo", tmpDir, &git2go.CloneOptions{

pkg/git/libgit2/managed/ssh.go

@@ -53,6 +53,7 @@ import (
 	"net/url"
 	"runtime"
 	"strings"
+	"sync"
 	"time"
 
 	"golang.org/x/crypto/ssh"
@@ -63,6 +64,17 @@ import (
 // registerManagedSSH registers a Go-native implementation of
 // SSH transport that doesn't rely on any lower-level libraries
 // such as libssh2.
+//
+// The underlying SSH connections are kept open and are reused
+// across several SSH sessions. This is due to upstream issues in
+// which concurrent/parallel SSH connections may lead to instability.
+//
+// Connections are created on first attempt to use a given remote. The
+// connection is removed from the cache on the first failed session related
+// operation.
+//
+// https://github.com/golang/go/issues/51926
+// https://github.com/golang/go/issues/27140
 func registerManagedSSH() error {
 	for _, protocol := range []string{"ssh", "ssh+git", "git+ssh"} {
 		_, err := git2go.NewRegisteredSmartTransport(protocol, false, sshSmartSubtransportFactory)
@@ -90,6 +102,9 @@ type sshSmartSubtransport struct {
 	currentStream *sshSmartSubtransportStream
 }
 
+var aMux sync.RWMutex
+var sshClients map[string]*ssh.Client = make(map[string]*ssh.Client)
+
 func (t *sshSmartSubtransport) Action(urlString string, action git2go.SmartServiceAction) (git2go.SmartSubtransportStream, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
@@ -136,7 +151,14 @@ func (t *sshSmartSubtransport) Action(urlString string, action git2go.SmartServi
 	}
 	defer cred.Free()
 
-	sshConfig, err := getSSHConfigFromCredential(cred)
+	var addr string
+	if u.Port() != "" {
+		addr = fmt.Sprintf("%s:%s", u.Hostname(), u.Port())
+	} else {
+		addr = fmt.Sprintf("%s:22", u.Hostname())
+	}
+
+	ckey, sshConfig, err := cacheKeyAndConfig(addr, cred)
 	if err != nil {
 		return nil, err
 	}
@@ -157,52 +179,66 @@ func (t *sshSmartSubtransport) Action(urlString string, action git2go.SmartServi
 		return t.transport.SmartCertificateCheck(cert, true, hostname)
 	}
 
-	var addr string
-	if u.Port() != "" {
-		addr = fmt.Sprintf("%s:%s", u.Hostname(), u.Port())
-	} else {
-		addr = fmt.Sprintf("%s:22", u.Hostname())
+	aMux.RLock()
+	if c, ok := sshClients[ckey]; ok {
+		traceLog.Info("[ssh]: cache hit", "remoteAddress", addr)
+		t.client = c
 	}
+	aMux.RUnlock()
+
+	if t.client == nil {
+		traceLog.Info("[ssh]: cache miss", "remoteAddress", addr)
+
+		aMux.Lock()
+		defer aMux.Unlock()
+
+		// In some scenarios the ssh handshake can hang indefinitely at
+		// golang.org/x/crypto/ssh.(*handshakeTransport).kexLoop.
+		//
+		// xref: https://github.com/golang/go/issues/51926
+		done := make(chan error, 1)
+		go func() {
+			t.client, err = ssh.Dial("tcp", addr, sshConfig)
+			done <- err
+		}()
+
+		dialTimeout := sshConfig.Timeout + (30 * time.Second)
 
-	// In some scenarios the ssh handshake can hang indefinitely at
-	// golang.org/x/crypto/ssh.(*handshakeTransport).kexLoop.
-	//
-	// xref: https://github.com/golang/go/issues/51926
-	done := make(chan error, 1)
-	go func() {
-		t.client, err = ssh.Dial("tcp", addr, sshConfig)
-		done <- err
-	}()
-
-	select {
-	case doneErr := <-done:
-		if doneErr != nil {
-			err = fmt.Errorf("ssh.Dial: %w", doneErr)
+		select {
+		case doneErr := <-done:
+			if doneErr != nil {
+				err = fmt.Errorf("ssh.Dial: %w", doneErr)
+			}
+		case <-time.After(dialTimeout):
+			err = fmt.Errorf("timed out waiting for ssh.Dial after %s", dialTimeout)
 		}
-	case <-time.After(sshConfig.Timeout + (5 * time.Second)):
-		err = fmt.Errorf("timed out waiting for ssh.Dial")
-	}
 
-	if err != nil {
-		return nil, err
+		if err != nil {
+			return nil, err
+		}
+
+		sshClients[ckey] = t.client
 	}
 
-	t.session, err = t.client.NewSession()
-	if err != nil {
+	traceLog.Info("[ssh]: creating new ssh session")
+	if t.session, err = t.client.NewSession(); err != nil {
+		discardCachedSshClient(ckey)
 		return nil, err
 	}
 
-	t.stdin, err = t.session.StdinPipe()
-	if err != nil {
+	if t.stdin, err = t.session.StdinPipe(); err != nil {
+		discardCachedSshClient(ckey)
 		return nil, err
 	}
 
-	t.stdout, err = t.session.StdoutPipe()
-	if err != nil {
+	if t.stdout, err = t.session.StdoutPipe(); err != nil {
+		discardCachedSshClient(ckey)
 		return nil, err
 	}
 
+	traceLog.Info("[ssh]: run on remote", "cmd", cmd)
 	if err := t.session.Start(cmd); err != nil {
+		discardCachedSshClient(ckey)
 		return nil, err
 	}
 
@@ -216,6 +252,8 @@ func (t *sshSmartSubtransport) Action(urlString string, action git2go.SmartServi
 
 func (t *sshSmartSubtransport) Close() error {
 	var returnErr error
+
+	traceLog.Info("[ssh]: sshSmartSubtransport.Close()")
 	t.currentStream = nil
 	if t.client != nil {
 		if err := t.stdin.Close(); err != nil {
@@ -224,12 +262,12 @@ func (t *sshSmartSubtransport) Close() error {
 		t.client = nil
 	}
 	if t.session != nil {
-
 		// In some scenarios session.Wait() can hang indefinitely.
 		// Here we force a timeout to skip the wait and continue
 		// with closing the session.
 		done := make(chan error, 1)
 		go func() {
+			traceLog.Info("[ssh]: session.Wait()")
 			err := t.session.Wait()
 			done <- err
 		}()
@@ -243,6 +281,7 @@ func (t *sshSmartSubtransport) Close() error {
 			returnErr = fmt.Errorf("timed out waiting for session.Wait()")
 		}
 
+		traceLog.Info("[ssh]: session.Close()")
 		if err := t.session.Close(); err != nil {
 			returnErr = fmt.Errorf("cannot close session: %w", err)
 		}
@@ -252,6 +291,7 @@ func (t *sshSmartSubtransport) Close() error {
 }
 
 func (t *sshSmartSubtransport) Free() {
+	traceLog.Info("[ssh]: sshSmartSubtransport.Free()")
 }
 
 type sshSmartSubtransportStream struct {
@@ -267,19 +307,20 @@ func (stream *sshSmartSubtransportStream) Write(buf []byte) (int, error) {
 }
 
 func (stream *sshSmartSubtransportStream) Free() {
+	traceLog.Info("[ssh]: sshSmartSubtransportStream.Free()")
 }
 
-func getSSHConfigFromCredential(cred *git2go.Credential) (*ssh.ClientConfig, error) {
+func cacheKeyAndConfig(remoteAddress string, cred *git2go.Credential) (string, *ssh.ClientConfig, error) {
 	username, _, privatekey, passphrase, err := cred.GetSSHKey()
 	if err != nil {
-		return nil, err
+		return "", nil, err
 	}
 
 	var pemBytes []byte
 	if cred.Type() == git2go.CredentialTypeSSHMemory {
 		pemBytes = []byte(privatekey)
 	} else {
-		return nil, fmt.Errorf("file based SSH credential is not supported")
+		return "", nil, fmt.Errorf("file based SSH credential is not supported")
 	}
 
 	var key ssh.Signer
@@ -290,12 +331,46 @@ func getSSHConfigFromCredential(cred *git2go.Credential) (*ssh.ClientConfig, err
 	}
 
 	if err != nil {
-		return nil, err
+		return "", nil, err
 	}
 
-	return &ssh.ClientConfig{
+	ck := cacheKey(remoteAddress, username, key.PublicKey().Marshal())
+	cfg := &ssh.ClientConfig{
 		User:    username,
 		Auth:    []ssh.AuthMethod{ssh.PublicKeys(key)},
 		Timeout: sshConnectionTimeOut,
-	}, nil
+	}
+
+	return ck, cfg, nil
+}
+
+// cacheKey generates a cache key that is multi-tenancy safe.
+//
+// Stablishing multiple and concurrent ssh connections leads to stability
+// issues documented above. However, the caching/sharing of already stablished
+// connections could represent a vector for users to bypass the ssh authentication
+// mechanism.
+//
+// cacheKey tries to ensure that connections are only shared by users that
+// have the exact same remoteAddress and credentials. An important assumption
+// here is that public key is always derived from the private key in cacheKeyAndConfig.
+func cacheKey(remoteAddress, userName string, pubKey []byte) string {
+	h := sha256.New()
+
+	v := fmt.Sprintf("%s-%s-%v", remoteAddress, userName, pubKey)
+
+	h.Write([]byte(v))
+	return fmt.Sprintf("%x", h.Sum(nil))
+}
+
+// discardCachedSshClient discards the cached ssh client, forcing the next git operation
+// to create a new one via ssh.Dial.
+func discardCachedSshClient(key string) {
+	aMux.Lock()
+	defer aMux.Unlock()
+
+	if _, found := sshClients[key]; found {
+		traceLog.Info("[ssh]: discard cached ssh client")
+		delete(sshClients, key)
+	}
 }