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.

Fixes golang/go#51994

Change-Id: I83ea80901d4977d8f78523e3d1e16e0a7df5b172
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/395314
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Julie Qiu <julieqiu@google.com>

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