net/{currenttime,tlsdial}: add minimum possible time for TLS

This adds a new package, net/currenttime, which is a thin wrapper around
time.Now. If the value returned by time.Now is before a hard-coded value
baked into the binary, that hard-coded value will be returned instead.
In the case where the system has a buggy, malfunctioning, or nonexistent
RTC, this can improve the likelihood that Tailscale will be able to
establish a connection to the control plane (via TLS) and fetch the
server certificate.

As a future TODO: we should cache this value on-disk between process
starts (possibly in the state file?) so that we succeed even if the
Tailscale server certificate has already expired from the perspective of
the minimum time.

Additionally, add a GitHub workflow that bumps the current time to a new
value every 14 days, so that the value stays reasonably up-to-date in
our repository without introducing impurities into the build process.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: If63cf28c4f188993894d3de589fd65ad447def6f

.github/workflows/update-current-time.yml

@@ -0,0 +1,44 @@
+name: update-current-time
+
+on:
+  # allow manual execution
+  workflow_dispatch:
+
+  # run every 14 days
+  schedule:
+    - cron: "0 0 */14 * *"
+
+concurrency:
+  group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  update-flake:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out code
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - name: Get access token
+        uses: tibdex/github-app-token@3beb63f4bd073e61482598c45c71c1019b59b73a # v2.1.0
+        id: generate-token
+        with:
+          app_id: ${{ secrets.LICENSING_APP_ID }}
+          installation_retrieval_mode: "id"
+          installation_retrieval_payload: ${{ secrets.LICENSING_APP_INSTALLATION_ID }}
+          private_key: ${{ secrets.LICENSING_APP_PRIVATE_KEY }}
+
+      - name: Send pull request
+        uses: peter-evans/create-pull-request@dd2324fc52d5d43c699a5636bcf19fceaa70c284 #v7.0.7
+        with:
+          token: ${{ steps.generate-token.outputs.token }}
+          author: Time Updater <noreply+time-updater@tailscale.com>
+          committer: Time Updater <noreply+time-updater@tailscale.com>
+          branch: time-updates
+          commit-message: "net/currentime: update minimum time"
+          title: "net/currentime: update minimum time"
+          body: Triggered by ${{ github.repository }}@${{ github.sha }}
+          signoff: true
+          delete-branch: true
+          reviewers: andrew-d

cmd/derper/depaware.txt

@@ -103,6 +103,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         tailscale.com/kube/kubetypes                                 from tailscale.com/envknob
         tailscale.com/metrics                                        from tailscale.com/cmd/derper+
         tailscale.com/net/bakedroots                                 from tailscale.com/net/tlsdial
+        tailscale.com/net/currenttime                                from tailscale.com/net/tlsdial
         tailscale.com/net/dnscache                                   from tailscale.com/derp/derphttp
         tailscale.com/net/ktimeout                                   from tailscale.com/cmd/derper
         tailscale.com/net/netaddr                                    from tailscale.com/ipn+

cmd/k8s-operator/depaware.txt

@@ -847,6 +847,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         tailscale.com/net/bakedroots                                 from tailscale.com/net/tlsdial+
         tailscale.com/net/captivedetection                           from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/connstats                                  from tailscale.com/net/tstun+
+        tailscale.com/net/currenttime                                from tailscale.com/net/tlsdial
         tailscale.com/net/dns                                        from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns+
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback

cmd/tailscale/depaware.txt

@@ -102,6 +102,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/metrics                                        from tailscale.com/derp+
         tailscale.com/net/bakedroots                                 from tailscale.com/net/tlsdial
         tailscale.com/net/captivedetection                           from tailscale.com/net/netcheck
+        tailscale.com/net/currenttime                                from tailscale.com/net/tlsdial
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback
         tailscale.com/net/dnscache                                   from tailscale.com/control/controlhttp+
         tailscale.com/net/dnsfallback                                from tailscale.com/control/controlhttp+

cmd/tailscaled/depaware.txt

@@ -297,6 +297,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/bakedroots                                 from tailscale.com/net/tlsdial+
         tailscale.com/net/captivedetection                           from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/connstats                                  from tailscale.com/net/tstun+
+        tailscale.com/net/currenttime                                from tailscale.com/net/tlsdial
         tailscale.com/net/dns                                        from tailscale.com/cmd/tailscaled+
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns+
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback

net/currenttime/currenttime.go

@@ -0,0 +1,42 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package currenttime provides a fallback "current time" that can be used as
+// the minimum possible time for things like TLS certificate verification.
+//
+// This ensures that if a Tailscale client's clock is wrong, it can still
+// verify TLS certificates, assuming that the server certificate hasn't already
+// expired from the point of view of the minimum time.
+//
+// In the future, we may want to consider caching the last known current time
+// on-disk to improve the accuracy of this fallback.
+package currenttime
+
+import (
+	_ "embed"
+	"strconv"
+	"time"
+)
+
+//go:embed mintime.txt
+var minTimeUnixMs string
+
+var minCurrentTime time.Time
+
+func init() {
+	ms, err := strconv.ParseInt(minTimeUnixMs, 10, 64)
+	if err != nil {
+		panic(err)
+	}
+	minCurrentTime = time.UnixMilli(int64(ms))
+}
+
+// Now returns the current time as per [time.Now], except that if it is before
+// the baked-in "minimum current time", that value will be returned instead.
+func Now() time.Time {
+	now := time.Now()
+	if now.Before(minCurrentTime) {
+		return minCurrentTime
+	}
+	return now
+}

net/currenttime/currenttime_test.go

@@ -0,0 +1,14 @@
+package currenttime
+
+import (
+	"testing"
+	"time"
+)
+
+func TestMinTime(t *testing.T) {
+	// The baked-in time should always be before the current time.
+	now := time.Now()
+	if !minCurrentTime.Before(now) {
+		t.Fatalf("minCurrentTime is not before the current time: %v >= %v", minCurrentTime, now)
+	}
+}

net/currenttime/mintime.txt

@@ -0,0 +1 @@
+1741638824797

net/currenttime/update-current-time.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ignore
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"time"
+)
+
+func main() {
+	contents := fmt.Sprintf(`%d`, time.Now().UnixMilli())
+	if err := os.WriteFile("mintime.txt", []byte(contents), 0644); err != nil {
+		log.Fatal(err)
+	}
+}

net/tlsdial/tlsdial.go

@@ -29,6 +29,7 @@ import (
 	"tailscale.com/health"
 	"tailscale.com/hostinfo"
 	"tailscale.com/net/bakedroots"
+	"tailscale.com/net/currenttime"
 	"tailscale.com/net/tlsdial/blockblame"
 )
 
@@ -144,6 +145,7 @@ func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
 		opts := x509.VerifyOptions{
 			DNSName:       cs.ServerName,
 			Intermediates: x509.NewCertPool(),
+			CurrentTime:   currenttime.Now(), // helps if the system clock is wrong
 		}
 		for _, cert := range cs.PeerCertificates[1:] {
 			opts.Intermediates.AddCert(cert)