ssh/tailssh: always use current time for policy evaluation

Whenever the SSH policy changes we revaluate all open connections to
make sure they still have access. This check was using the wrong
timestamp and would match against expired policies, however this really
isn't a problem today as we don't have policy that would be impacted by
this check. Fixing it for future use.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/tailssh.go

@@ -69,7 +69,7 @@ type server struct {
 }
 
 func (srv *server) now() time.Time {
-	if srv.timeNow != nil {
+	if srv != nil && srv.timeNow != nil {
 		return srv.timeNow()
 	}
 	return time.Now()
@@ -152,10 +152,6 @@ type conn struct {
 
 	insecureSkipTailscaleAuth bool // used by tests.
 
-	// now is the time to consider the present moment for the
-	// purposes of rule evaluation.
-	now time.Time
-
 	connID       string             // ID that's shared with control
 	action0      *tailcfg.SSHAction // first matching action
 	srv          *server
@@ -278,8 +274,9 @@ func (srv *server) newConn() (*conn, error) {
 		return nil, gossh.ErrDenied
 	}
 	srv.mu.Unlock()
-	c := &conn{srv: srv, now: srv.now()}
-	c.connID = fmt.Sprintf("conn-%s-%02x", c.now.UTC().Format("20060102T150405"), randBytes(5))
+	c := &conn{srv: srv}
+	now := srv.now()
+	c.connID = fmt.Sprintf("conn-%s-%02x", now.UTC().Format("20060102T150405"), randBytes(5))
 	c.Server = &ssh.Server{
 		Version:         "Tailscale",
 		Handler:         c.handleSessionPostSSHAuth,
@@ -751,7 +748,7 @@ func (ss *sshSession) vlogf(format string, args ...interface{}) {
 }
 
 func (c *conn) newSSHSession(s ssh.Session) *sshSession {
-	sharedID := fmt.Sprintf("sess-%s-%02x", c.now.UTC().Format("20060102T150405"), randBytes(5))
+	sharedID := fmt.Sprintf("sess-%s-%02x", c.srv.now().UTC().Format("20060102T150405"), randBytes(5))
 	c.logf("starting session: %v", sharedID)
 	return &sshSession{
 		Session:  s,
@@ -1087,7 +1084,7 @@ func (c *conn) ruleExpired(r *tailcfg.SSHRule) bool {
 	if r.RuleExpires == nil {
 		return false
 	}
-	return r.RuleExpires.Before(c.now)
+	return r.RuleExpires.Before(c.srv.now())
 }
 
 func (c *conn) evalSSHPolicy(pol *tailcfg.SSHPolicy, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, ok bool) {

ssh/tailssh/tailssh_test.go

@@ -179,7 +179,6 @@ func TestMatchRule(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			c := &conn{
-				now:  time.Unix(200, 0),
 				info: tt.ci,
 			}
 			got, gotUser, err := c.matchRule(tt.rule, nil)