ssh: cap total userauth attempts per server connection

serverAuthenticate only bounded real failures via MaxAuthTries.
PartialSuccessError responses and the publickey isQuery short-circuit
both kept the loop running without incrementing authFailures, so a
client could keep the server processing SSH_MSG_USERAUTH_REQUEST
messages indefinitely on a single connection.

Add an unconditional cap, maxAuthServerAttempts = 128, on the total
number of userauth requests handled per connection. The counter is
incremented at the top of the loop before method dispatch, so every
method and every isQuery / partial-success path counts. When the cap
is exceeded the server sends SSH_MSG_DISCONNECT with reason 2 ("too
many authentication attempts"), mirroring the MaxAuthTries handling.
The bound is well below OpenSSH's hard cap of 1024 but above any
realistic multi-step auth flow.

Change-Id: I56779fc55cd00ddfd32ec938f8de3a49be0145dc
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781903
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

ssh/server.go

diff --git a/ssh/server.go b/ssh/server.go
index 0192a67..9292f0b 100644
--- a/ssh/server.go
+++ b/ssh/server.go
@@ -607,6 +607,15 @@
 	return b.Err.Error()
 }
 
+// maxAuthServerAttempts caps the total number of SSH_MSG_USERAUTH_REQUEST
+// messages the server will process on a single connection, regardless of
+// outcome (failure, partial success, public key query, or none). It is a
+// backstop against clients that drive the authentication loop indefinitely
+// without ever incurring a real failure — for example by repeatedly
+// triggering PartialSuccessError or by spamming public key offer queries —
+// neither of which increment the MaxAuthTries failure counter.
+const maxAuthServerAttempts = 128
+
 func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
 	if config.PreAuthConnCallback != nil {
 		config.PreAuthConnCallback(s)
@@ -617,6 +626,7 @@
 	var perms *Permissions
 
 	authFailures := 0
+	authAttempts := 0
 	noneAuthCount := 0
 	var authErrs []error
 	var calledBannerCallback bool
@@ -645,6 +655,19 @@
 			return nil, &ServerAuthError{Errors: authErrs}
 		}
 
+		if authAttempts >= maxAuthServerAttempts {
+			discMsg := &disconnectMsg{
+				Reason:  2,
+				Message: "too many authentication attempts",
+			}
+			if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
+				return nil, err
+			}
+			authErrs = append(authErrs, discMsg)
+			return nil, &ServerAuthError{Errors: authErrs}
+		}
+		authAttempts++
+
 		var userAuthReq userAuthRequestMsg
 		if packet, err := s.transport.readPacket(); err != nil {
 			if err == io.EOF {

ssh/server_test.go

diff --git a/ssh/server_test.go b/ssh/server_test.go
index 502a25b..01e262b 100644
--- a/ssh/server_test.go
+++ b/ssh/server_test.go
@@ -166,6 +166,52 @@
 	}
 }
 
+func TestMaxAuthServerAttempts(t *testing.T) {
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	invocations := 0
+	clientConf := &ClientConfig{
+		User:            "user",
+		HostKeyCallback: InsecureIgnoreHostKey(),
+		AuthCallback: func(ctx *ClientAuthContext) (AuthMethod, error) {
+			invocations++
+			return PublicKeys(testSigners["rsa"]), nil
+		},
+	}
+
+	go NewServerConn(c1, alwaysPartialPubKeyServer())
+
+	_, _, _, err = NewClientConn(c2, "", clientConf)
+	if err == nil {
+		t.Fatal("expected the server to disconnect after exceeding the attempts cap")
+	}
+	// The error must be the server's disconnect, not the client-side
+	// "too many authentication attempts (N), aborting" bound. The
+	// disconnect format is set by disconnectMsg.Error().
+	if !strings.Contains(err.Error(), "ssh: disconnect") ||
+		!strings.Contains(err.Error(), "too many authentication attempts") {
+		t.Fatalf("expected server disconnect, got: %v", err)
+	}
+	// Server-side request sequence per AuthCallback iteration:
+	//   1 "none"       (initial, before any AuthCallback)
+	//   2 pubkey query (AuthCallback #k, server returns OK)
+	//   3 pubkey signed (AuthCallback #k, server returns partial)
+	// So after k complete callbacks the server has processed 1+2k requests.
+	// The (k+1)-th callback's query is processed as request number 2(k+1);
+	// the subsequent signed request is rejected by the cap check
+	// (authAttempts >= maxAuthServerAttempts), so 2(k+1) = maxAuthServerAttempts
+	// and invocations = maxAuthServerAttempts / 2.
+	expected := maxAuthServerAttempts / 2
+	if invocations != expected {
+		t.Errorf("AuthCallback invoked %d times; want %d", invocations, expected)
+	}
+}
+
 func TestMaxAuthTriesFirstNoneAuthErrorIgnored(t *testing.T) {
 	username := "testuser"
 	serverConfig := &ServerConfig{