net/{currenttime,tlsdial}: add minimum possible time for TLSandrew/current-time

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

4 files changed, 77 insertions, 0 deletions

diff --git a/net/currenttime/currenttime.go b/net/currenttime/currenttime.go
new file mode 100644
index 000000000..7ef2a1c56
--- /dev/null
+++ b/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
+}
diff --git a/net/currenttime/currenttime_test.go b/net/currenttime/currenttime_test.go
new file mode 100644
index 000000000..958a8436e
--- /dev/null
+++ b/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)
+	}
+}
diff --git a/net/currenttime/mintime.txt b/net/currenttime/mintime.txt
new file mode 100644
index 000000000..5da66af78
--- /dev/null
+++ b/net/currenttime/mintime.txt
@@ -0,0 +1 @@
+1741638824797
\ No newline at end of file
diff --git a/net/currenttime/update-current-time.go b/net/currenttime/update-current-time.go
new file mode 100644
index 000000000..717a984ce
--- /dev/null
+++ b/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)
+	}
+}