tailcfg, ssh/tailssh: make SSHUser value '=' map ssh-user to same local-user

Updates #3802

Change-Id: Icde60d4150ca15c25d615a4effb3d3c236f020a8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh.go

@@ -624,10 +624,14 @@ func matchRule(r *tailcfg.SSHRule, ci *sshConnInfo) (a *tailcfg.SSHAction, local
 }
 
 func mapLocalUser(ruleSSHUsers map[string]string, reqSSHUser string) (localUser string) {
-	if v, ok := ruleSSHUsers[reqSSHUser]; ok {
-		return v
+	v, ok := ruleSSHUsers[reqSSHUser]
+	if !ok {
+		v = ruleSSHUsers["*"]
+	}
+	if v == "=" {
+		return reqSSHUser
 	}
-	return ruleSSHUsers["*"]
+	return v
 }
 
 func matchesPrincipal(ps []*tailcfg.SSHPrincipal, ci *sshConnInfo) bool {

ssh/tailssh/tailssh_test.go

@@ -153,6 +153,18 @@ func TestMatchRule(t *testing.T) {
 			ci:       &sshConnInfo{uprof: &tailcfg.UserProfile{LoginName: "foo@bar.com"}},
 			wantUser: "ubuntu",
 		},
+		{
+			name: "ssh-user-equal",
+			rule: &tailcfg.SSHRule{
+				Action:     someAction,
+				Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+				SSHUsers: map[string]string{
+					"*": "=",
+				},
+			},
+			ci:       &sshConnInfo{sshUser: "alice"},
+			wantUser: "alice",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

tailcfg/tailcfg.go

@@ -1573,6 +1573,8 @@ type SSHRule struct {
 	// actual user that's logged in.
 	// If the map value is the empty string (for either the
 	// requested SSH user or "*"), the rule doesn't match.
+	// If the map value is "=", it means the ssh-user should map
+	// directly to the local-user.
 	// It may be nil if the Action is reject.
 	SSHUsers map[string]string `json:"sshUsers"`