net: tap_open(): set sk_uid from current_fsuid()

Commit 66b2c338 initializes the "sk_uid" field in the protocol socket
(struct sock) from the "/dev/tapX" device node's owner UID. Per original
commit 86741ec2 ("net: core: Add a UID field to struct sock.",
2016-11-04), that's wrong: the idea is to cache the UID of the userspace
process that creates the socket. Commit 86741ec2 mentions socket() and
accept(); with "tap", the action that creates the socket is
open("/dev/tapX").

Therefore the device node's owner UID is irrelevant. In most cases,
"/dev/tapX" will be owned by root, so in practice, commit 66b2c338 has
no observable effect:

- before, "sk_uid" would be zero, due to undefined behavior
  (CVE-2023-1076),

- after, "sk_uid" would be zero, due to "/dev/tapX" being owned by root.

What matters is the (fs)UID of the process performing the open(), so cache
that in "sk_uid".

Cc: Eric Dumazet <edumazet@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Pietro Borrello <borrello@diag.uniroma1.it>
Cc: netdev@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: 66b2c338 ("tap: tap_open(): correctly initialize socket uid")
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2173435


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>