cmd/tailscaled, version/distro: default to userspace-networking on Crostini

cros-garcon NULL-derefs on cold-boot netlink enumeration when
tailscale0 is present, preventing the Crostini container and
ChromeOS Terminal from starting cleanly. This is an upstream
ChromiumOS bug in cros-garcon; tailscaled can work around it
by defaulting to userspace-networking mode on Crostini.

Tailscale SSH continues to work via tailscaled's netstack.
Users can override with --tun=tailscale0 on ChromeOS builds
where cros-garcon is fixed.

Crostini is detected via /opt/google/cros-containers/bin/garcon,
which is present in every Crostini penguin container.

ssh/tailssh extends the existing Debian default-PATH case to
cover Crostini, since Crostini is Debian-based and benefits
from the same SSH PATH defaults.

RELNOTE: Crostini now defaults to userspace-networking.

Fixes #19488
Updates #12090

Signed-off-by: ferrumclaudepilgrim <ferrumclaudepilgrim@users.noreply.github.com>

cmd/tailscaled/tailscaled.go

@@ -87,6 +87,16 @@ func defaultTunName() string {
 			// See https://github.com/tailscale/tailscale-synology/issues/35
 			return "tailscale0,userspace-networking"
 		}
+		if buildfeatures.HasNetstack && distro.Get() == distro.Crostini {
+			// cros-garcon NULL-derefs on cold-boot netlink interface
+			// enumeration when tailscale0 is present, preventing the
+			// Crostini container and ChromeOS Terminal from starting
+			// cleanly. Default to userspace-networking until the
+			// upstream ChromiumOS bug is fixed.
+			// See https://github.com/tailscale/tailscale/issues/12090
+			// See https://issuetracker.google.com/issues/517069318
+			return "userspace-networking"
+		}
 	}
 	return "tailscale0"
 }

ssh/tailssh/user.go

@@ -93,7 +93,7 @@ func defaultPathForUser(u *user.User) string {
 	}
 	isRoot := u.Uid == "0"
 	switch distro.Get() {
-	case distro.Debian:
+	case distro.Debian, distro.Crostini:
 		hi := hostinfo.New()
 		if hi.Distro == "ubuntu" {
 			// distro.Get's Debian includes Ubuntu. But see if it's actually Ubuntu.

version/distro/distro.go

@@ -19,6 +19,7 @@ type Distro string
 
 const (
 	Debian    = Distro("debian")
+	Crostini  = Distro("crostini") // ChromeOS Crostini Linux container; Debian-based
 	Arch      = Distro("arch")
 	Synology  = Distro("synology")
 	OpenWrt   = Distro("openwrt")
@@ -84,6 +85,11 @@ func linuxDistro() Distro {
 		// Currently supported product families:
 		// - UDM (UniFi Dream Machine, UDM-Pro)
 		return UBNT
+	case have("/opt/google/cros-containers/bin/garcon"):
+		// ChromeOS Crostini ships /opt/google/cros-containers/bin/garcon
+		// in every penguin container. MUST be checked before Debian since
+		// Crostini is Debian-based.
+		return Crostini
 	case have("/etc/debian_version"):
 		return Debian
 	case have("/etc/arch-release"):