netdev - Possible mistake in commit 3ca459eaba1b ("tun: fix group permission check")

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Message-ID: <CAFqZXNtkCBT4f+PwyVRmQGoT3p1eVa01fCG_aNtpt6dakXncUg@mail.gmail.com>
Date: Mon, 27 Jan 2025 10:10:20 +0100
From: Ondrej Mosnacek <omosnace@...hat.com>
To: Stas Sergeev <stsp2@...dex.ru>, Willem de Bruijn <willemb@...gle.com>, 
	Jason Wang <jasowang@...hat.com>
Cc: Jakub Kicinski <kuba@...nel.org>, network dev <netdev@...r.kernel.org>, 
	Linux Security Module list <linux-security-module@...r.kernel.org>, 
	SElinux list <selinux@...r.kernel.org>
Subject: Possible mistake in commit 3ca459eaba1b ("tun: fix group permission check")

Hello,

It looks like the commit in $SUBJ may have introduced an unintended
change in behavior. According to the commit message, the intent was to
require just one of {user, group} to match instead of both, which
sounds reasonable, but the commit also changes the behavior for when
neither of tun->owner and tun->group is set. Before the commit the
access was always allowed, while after the commit CAP_NET_ADMIN is
required in this case.

I'm asking because the tun_tap subtest of selinux-testuite [1] started
to fail after this commit (it assumed CAP_NET_ADMIN was not needed),
so I'm trying to figure out if we need to change the test or if it
needs to be fixed in the kernel.

Thanks,

[1] https://github.com/SELinuxProject/selinux-testsuite/

-- 
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.