Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

tun: Require CAP_NET_ADMIN to attach to an ownerless device

Twenty years ago I made creating a tuntap device require CAP_NET_ADMIN in commit ca6bb5d7ab22 ("[NET]: Require CAP_NET_ADMIN to create tuntap devices.")

The idea was to allow the permissions on /dev/net/tun to be relaxed to 0666 and thus allow owners of tun devices assigned with TUNSETOWNER to actually open it and get at their devices.

What we missed at the time was that persistent devices with *no* owner were already essentially world-consumable, and anyone who could open the device node could also attach to those.

Even then, using persistent devices with no explicitly set owner — on the (false) presumption that CAP_NET_ADMIN would be required to open them — wasn't an egregious userspace bug. There are cases where it does make sense to separate the creation/configuration of the netdevice, from the process which shovels packets into it.

Since then, TUNSETGROUP was added a year later in commit 8c644623fe7e ("[NET]: Allow group ownership of TUN/TAP devices."). Its commit message noted that if both uid and gid are set, both have to match (in itself a bizarre choice) — but still not that with *neither* set, no permission at all was required.

Then in 2012 commit c8d68e6be1c3 ("tuntap: multiqueue support") allowed new queues to be attached to a device from the moment of its creation, with no way for userspace to close the race between creation and…