Re-add tun/tap to default device rules

Since v1.2.0 was released, a number of users complained that the removal
of tun/tap device access from the default device ruleset is causing a
regression in their workloads.

Additionally, it seems that some upper-level orchestration tools
(Docker Swarm, Kubernetes) makes it either impossible or cumbersome
to supply additional device rules.

While it's probably not quite right to have /dev/net/tun in a default
device list, it was there from the very beginning, and users rely on it.
Let's keep it there for the sake of backward compatibility.

This reverts commit 2ce40b6ad7.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/devices/devicefilter_test.go

@@ -120,14 +120,21 @@ block-8:
         51: MovImm32 dst: r0 imm: 1
         52: Exit
 block-9:
-// /dev/pts (c, 136, wildcard, rwm, true)
+// tuntap (c, 10, 200, rwm, true)
         53: JNEImm dst: r2 off: -1 imm: 2 <block-10>
-        54: JNEImm dst: r4 off: -1 imm: 136 <block-10>
-        55: MovImm32 dst: r0 imm: 1
-        56: Exit
+        54: JNEImm dst: r4 off: -1 imm: 10 <block-10>
+        55: JNEImm dst: r5 off: -1 imm: 200 <block-10>
+        56: MovImm32 dst: r0 imm: 1
+        57: Exit
 block-10:
-        57: MovImm32 dst: r0 imm: 0
-        58: Exit
+// /dev/pts (c, 136, wildcard, rwm, true)
+	58: JNEImm dst: r2 off: -1 imm: 2 <block-11>
+        59: JNEImm dst: r4 off: -1 imm: 136 <block-11>
+        60: MovImm32 dst: r0 imm: 1
+        61: Exit
+block-11:
+        62: MovImm32 dst: r0 imm: 0
+        63: Exit
 `
 	var devices []*devices.Rule
 	for _, device := range specconv.AllowedDevices {

libcontainer/specconv/spec_linux.go

@@ -315,6 +315,23 @@ var AllowedDevices = []*devices.Device{
 			Allow:       true,
 		},
 	},
+	// The following entry for /dev/net/tun device was there from the
+	// very early days of Docker, but got removed in runc 1.2.0-rc1,
+	// causing a number of regressions for users (see
+	// https://github.com/opencontainers/runc/pull/3468).
+	//
+	// Some upper-level orcherstration tools makes it either impossible
+	// or cumbersome to supply additional device rules, so we have to
+	// keep this for the sake of backward compatibility.
+	{
+		Rule: devices.Rule{
+			Type:        devices.CharDevice,
+			Major:       10,
+			Minor:       200,
+			Permissions: "rwm",
+			Allow:       true,
+		},
+	},
 }
 
 type CreateOpts struct {