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

@@ -607,6 +607,15 @@ func (b *BannerError) Error() string {
 	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 @@ func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, err
 	var perms *Permissions
 
 	authFailures := 0
+	authAttempts := 0
 	noneAuthCount := 0
 	var authErrs []error
 	var calledBannerCallback bool
@@ -645,6 +655,19 @@ userAuthLoop:
 			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 {