tun_device: enforce ioctl restrictions

Require all SELinux domains which have permission to perform ioctls on
/dev/tun explicitly specify what ioctls they perform. Only allow the
safe defaults FIOCLEX and FIONCLEX, which are alternate, uncommon ways
to set and unset the O_CLOEXEC flag.

Remove app's ability to issue *any* ioctls on /dev/tun, period. Add
neverallow assertions (compile time assertion + CTS test) to prevent
regressions.

Limit system_server's ability to perform ioctls on /dev/tun to FIOCLEX,
FIONCLEX, TUNGETIFF, and TUNSETIFF. Testing and source code examination
shows that only TUNGETIFF and TUNSETIFF are used by system_server.

The goal of this change is to put SELinux ioctl controls in place for
/dev/tun, so we don't have to maintain the custom kernel patch at
https://android.googlesource.com/kernel/common/+/11cee2be0c2062ba88f04eb51196506f870a3b5d%5E%21

Delete the neverallow assertion in isolated_app.te. This is already
covered by the assertion present in app_neverallows.te.

Test: cts-tradefed run cts -m CtsHostsideNetworkTests -t com.android.cts.net.HostsideVpnTests
Test: cts-tradefed run cts -m CtsHostsideNetworkTests
Test: cts-tradefed run cts -m CtsNetTestCases
Bug: 111560739
Bug: 111560570
Change-Id: Ibe1c3a9e880db0bee438535554abdbc6d84eec45

private/app_neverallows.te

@@ -112,8 +112,35 @@ neverallow { all_untrusted_apps -mediaprovider } {
 # No untrusted component should be touching /dev/fuse
 neverallow all_untrusted_apps fuse_device:chr_file *;
 
-# Do not allow untrusted apps to directly open tun_device
-neverallow all_untrusted_apps tun_device:chr_file open;
+# Do not allow untrusted apps to directly open or
+# issue ioctls to the tun_device
+neverallow all_untrusted_apps tun_device:chr_file { open ioctl };
+# Additionally, assert that the following ioctls are never reachable.
+# This should already be blocked by the neverallow rule above, but this
+# is added for robustness, and to prove equivalence to the kernel patch at
+# https://android.googlesource.com/kernel/common/+/11cee2be0c2062ba88f04eb51196506f870a3b5d%5E%21
+neverallowxperm all_untrusted_apps tun_device:chr_file ioctl {
+  SIOCGIFHWADDR
+  SIOCSIFHWADDR
+  TUNATTACHFILTER
+  TUNDETACHFILTER
+  TUNGETFEATURES
+  TUNGETFILTER
+  TUNGETSNDBUF
+  TUNGETVNETHDRSZ
+  TUNSETDEBUG
+  TUNSETGROUP
+  TUNSETIFF
+  TUNSETLINK
+  TUNSETNOCSUM
+  TUNSETOFFLOAD
+  TUNSETOWNER
+  TUNSETPERSIST
+  TUNSETQUEUE
+  TUNSETSNDBUF
+  TUNSETTXFILTER
+  TUNSETVNETHDRSZ
+};
 
 # Only allow appending to /data/anr/traces.txt (b/27853304, b/18340553)
 neverallow all_untrusted_apps anr_data_file:file ~{ open append };

private/isolated_app.te

@@ -57,9 +57,6 @@ unix_socket_connect(isolated_app, traced_producer, traced)
 ##### Neverallow
 #####
 
-# Do not allow isolated_app to directly open tun_device
-neverallow isolated_app tun_device:chr_file open;
-
 # Isolated apps should not directly open app data files themselves.
 neverallow isolated_app { app_data_file privapp_data_file }:file open;
 

private/system_server.te

@@ -339,6 +339,7 @@ allow system_server audio_device:chr_file rw_file_perms;
 
 # tun device used for 3rd party vpn apps
 allow system_server tun_device:chr_file rw_file_perms;
+allowxperm system_server tun_device:chr_file ioctl { TUNGETIFF TUNSETIFF };
 
 # Manage system data files.
 allow system_server system_data_file:dir create_dir_perms;

public/app.te

@@ -334,7 +334,7 @@ allow appdomain runas_exec:file getattr;
 
 # Apps receive an open tun fd from the framework for
 # device traffic. Do not allow untrusted app to directly open tun_device
-allow { appdomain -isolated_app -ephemeral_app } tun_device:chr_file { read write getattr ioctl append };
+allow { appdomain -isolated_app -ephemeral_app } tun_device:chr_file { read write getattr append };
 
 # Connect to adbd and use a socket transferred from it.
 # This is used for e.g. adb backup/restore.

public/domain.te

@@ -300,6 +300,10 @@ allowxperm domain devpts:chr_file ioctl unpriv_tty_ioctls;
 # named pipes, and named sockets). We start off with a safe set.
 allowxperm domain { file_type fs_type domain dev_type }:{ dir notdevfile_class_set blk_file } ioctl { FIOCLEX FIONCLEX };
 
+# If a domain has ioctl access to tun_device, it must clearly enumerate the
+# ioctls used. Safe defaults are listed below.
+allowxperm domain tun_device:chr_file ioctl { FIOCLEX FIONCLEX };
+
 # Allow a process to make a determination whether a file descriptor
 # for a plain file or pipe (fifo_file) is a tty. Note that granting
 # this whitelist to domain does not grant the ioctl permission to