[PATCH net v3] tun: disallow setting IEEE 802.15.4 and 6LoWPAN link types via TUNSETLINK

[Kalpesh Solanki <skalpesh942@gmail.com>]

A userspace application can set the link type of a TUN/TAP interface
to ARPHRD_IEEE802154, ARPHRD_IEEE802154_MONITOR, or ARPHRD_6LOWPAN
via ioctl(fd, TUNSETLINK, arg) because tun_get_addr_len() explicitly
recognized these types.

However, TUN devices cannot function as genuine IEEE 802.15.4 or 6LoWPAN
interfaces:

1. They lack wpan_phy, channels, PAN IDs, and MLME operations, and tun
   never initializes dev->ieee802154_ptr or dev->ml_priv. When an
   unprivileged user creates a TUN interface in a user+net namespace,
   changes its link type to ARPHRD_IEEE802154, and issues a netlink dump
   request (such as IEEE802154_LLSEC_LIST_KEY),
   ieee802154_llsec_dump_table() unconditionally dereferences
   dev->ieee802154_ptr / ieee802154_mlme_ops(dev), triggering a supervisor
   null-pointer dereference / general protection fault. Similar
   dereferences occur across net/ieee802154/nl-phy.c (ieee802154_del_iface)
   and net/ieee802154/socket.c.

2. Similarly, the 6LoWPAN subsystem assumes via lowpan_dev(dev) that
   netdev_priv(dev) is struct lowpan_dev. On a TUN device, netdev_priv(dev)
   is struct tun_struct. On netdev events (such as NETDEV_DOWN in
   net/6lowpan/core.c:lowpan_device_event), it clears bits in
   lowpan_dev(dev)->ctx.table flags, which corrupts private memory inside
   tun_struct.

Rather than scattering checks across subsystems, fix the issue at the
source: disallow setting ARPHRD_IEEE802154, ARPHRD_IEEE802154_MONITOR, and
ARPHRD_6LOWPAN via TUNSETLINK by returning -EINVAL.

Fixes: cca8ea3b05c9 ("net: tun: set tun->dev->addr_len during TUNSETLINK processing")
Reported-by: syzbot+9ea34da5ca6100593c90@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9ea34da5ca6100593c90
Signed-off-by: Kalpesh Solanki <skalpesh942@gmail.com>
---
v2 -> v3:
- Include ARPHRD_6LOWPAN alongside ARPHRD_IEEE802154 as agreed with Willem
  de Bruijn, preventing Sashiko AI flags and addressing the underlying
  lowpan_dev(dev) memory corruption.
- Minimize code churn as advised by Willem de Bruijn: drop unrelated
  USHRT_MAX check and preserve tun_get_addr_len(), headers, and existing
  block indentation.
- Fresh standalone thread as advised by Jakub Kicinski.
- Link to v1 and v2 discussion:
  https://lore.kernel.org/all/20261101092219.1278751-1-skalpesh942@gmail.com/

v1 -> v2:
- Instead of adding NULL checks in net/ieee802154/nl-mac.c, disallow setting
  ARPHRD_IEEE802154 and ARPHRD_IEEE802154_MONITOR link types via TUNSETLINK in
  drivers/net/tun.c, as discussed with Andrew Lunn and identified in Sashiko
  bot review. Virtual IEEE 802.15.4 interfaces are properly provided by
  mac802154_hwsim and fakelb; TUN devices cannot function as genuine 802.15.4
  radios and lack private 802.15.4 contexts.
- Add an explicit bounds check (arg > USHRT_MAX) to prevent integer truncation
  bypasses when casting 64-bit ioctl arg to 16-bit dev->type.
- Remove ARPHRD_IEEE802154 and ARPHRD_IEEE802154_MONITOR from tun_get_addr_len()
  and drop unused <linux/ieee802154.h> header.

 drivers/net/tun.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index dcec7311e60..05eb28578b8 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -3362,6 +3362,10 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
 			netif_info(tun, drv, tun->dev,
 				   "Linktype set failed because interface is up\n");
 			ret = -EBUSY;
+		} else if (arg == ARPHRD_IEEE802154 ||
+			   arg == ARPHRD_IEEE802154_MONITOR ||
+			   arg == ARPHRD_6LOWPAN) {
+			ret = -EINVAL;
 		} else {
 			ret = call_netdevice_notifiers(NETDEV_PRE_TYPE_CHANGE,
 						       tun->dev);
-- 
2.53.0