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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Kalpesh Solanki wrote:
> 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")

Should this be

Fixes: ff4cc3ac93e1 ("[TUNTAP]: Allow setting the linktype of the tap device from userspace")

> 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;

Thanks, a lot simpler.

But, arg is unsigned long while tun->dev->type is unsigned short.

An arg ARPHRD_IEEE802154 | 0x10000 could pass this check, get and get
assigned while truncated.

Cast to unsigned short in these comparisons?

>  		} else {
>  			ret = call_netdevice_notifiers(NETDEV_PRE_TYPE_CHANGE,
>  						       tun->dev);
> -- 
> 2.53.0
>