netdev - drivers/net/tun.c: CAP_NET_ADMIN conditional not reached

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Message-ID: <20110728220026.GA25322@pintail.smokva.net>
Date:	Fri, 29 Jul 2011 00:00:27 +0200
From:	Petar Bogdanovic <petar@...kva.net>
To:	netdev@...r.kernel.org
Subject: drivers/net/tun.c: CAP_NET_ADMIN conditional not reached

Hi,

drivers/net/tun.c says:

    1004  if (((tun->owner != -1 && cred->euid != tun->owner) ||
    1005       (tun->group != -1 && !in_egroup_p(tun->group))) &&
    1006      !capable(CAP_NET_ADMIN))
    1007          return -EPERM;

this will skip !capable(CAP_NET_ADMIN) if tun->owner and tun->group are
both -1, which seems to be their default value (see lines 854,855).
ip(8) from iproute2 won't do any TUNSETOWNER/TUNSETGROUP ioctl() if no
uid and/or gid is supplied so when using `ip tuntap' as an unprivileged
user, you'll be unable to create a new tun(4) but won't have any
problems attaching to an existing device created without both user and
group options.

I'm not sure whether this is intentional since it has been around for
years.. although triggering it with tunctl(8) was not possible---tunctl
without `-u' seems to use 0 as the default value for TUNSETOWNER.

		Petar Bogdanovic
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