ssh: add ServerConfig.NoClientAuthCallback

It was possible to accept auth type "none" before, but not dynamically
at runtime as a function of the ConnMetadata like the other auth types'
callback hooks.

See golang/go#51994
and https://go-review.googlesource.com/c/crypto/+/395314

Change-Id: I83ea80901d4977d8f78523e3d1e16e0a7df5b172
(cherry picked from commit 4a431fab27b09acb1458fbb8709e12b2760e58a2)

Author: bradfitz

ssh/server.go

@@ -68,8 +68,16 @@ type ServerConfig struct {
 
 	// NoClientAuth is true if clients are allowed to connect without
 	// authenticating.
+	// To determine NoClientAuth at runtime, set NoClientAuth to true
+	// and the optional NoClientAuthCallback to a non-nil value.
 	NoClientAuth bool
 
+	// NoClientAuthCallback, if non-nil, is called when a user
+	// attempts to authenticate with auth method "none".
+	// NoClientAuth must also be set to true for this be used, or
+	// this func is unused.
+	NoClientAuthCallback func(ConnMetadata) (*Permissions, error)
+
 	// MaxAuthTries specifies the maximum number of authentication attempts
 	// permitted per connection. If set to a negative number, the number of
 	// attempts are unlimited. If set to zero, the number of attempts are limited
@@ -455,7 +463,11 @@ userAuthLoop:
 		switch userAuthReq.Method {
 		case "none":
 			if config.NoClientAuth {
-				authErr = nil
+				if config.NoClientAuthCallback != nil {
+					perms, authErr = config.NoClientAuthCallback(s)
+				} else {
+					authErr = nil
+				}
 			}
 
 			// allow initial attempt of 'none' without penalty

ssh/session_test.go

@@ -780,3 +780,54 @@ func TestHostKeyAlgorithms(t *testing.T) {
 		t.Fatal("succeeded connecting with unknown hostkey algorithm")
 	}
 }
+
+func TestServerClientAuthCallback(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	userCh := make(chan string, 1)
+
+	serverConf := &ServerConfig{
+		NoClientAuth: true,
+		NoClientAuthCallback: func(conn ConnMetadata) (*Permissions, error) {
+			userCh <- conn.User()
+			return nil, nil
+		},
+	}
+	const someUsername = "some-username"
+
+	serverConf.AddHostKey(testSigners["ecdsa"])
+	clientConf := &ClientConfig{
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		User:            someUsername,
+	}
+
+	go func() {
+		_, chans, reqs, err := NewServerConn(c1, serverConf)
+		if err != nil {
+			t.Errorf("server handshake: %v", err)
+			userCh <- "error"
+			return
+		}
+		go DiscardRequests(reqs)
+		for ch := range chans {
+			ch.Reject(Prohibited, "")
+		}
+	}()
+
+	conn, _, _, err := NewClientConn(c2, "", clientConf)
+	if err != nil {
+		t.Fatalf("client handshake: %v", err)
+		return
+	}
+	conn.Close()
+
+	got := <-userCh
+	if got != someUsername {
+		t.Errorf("username = %q; want %q", got, someUsername)
+	}
+}