netdev - Re: [PATCH] selinux: Read sk->sk_family once in selinux_socket

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Message-ID: <CAEjxPJ7+13QMFefy9uKYr449db2pi66CU-0GOFZ+BszpQJcStA@mail.gmail.com>
Date: Tue, 7 Jan 2025 15:16:34 -0500
From: Stephen Smalley <stephen.smalley.work@...il.com>
To: Paul Moore <paul@...l-moore.com>
Cc: Mikhail Ivanov <ivanov.mikhail1@...wei-partners.com>, 
	Mickaël Salaün <mic@...ikod.net>, 
	selinux@...r.kernel.org, omosnace@...hat.com, 
	linux-security-module@...r.kernel.org, netdev@...r.kernel.org, 
	yusongping@...wei.com, artem.kuzin@...wei.com, 
	konstantin.meskhidze@...wei.com
Subject: Re: [PATCH] selinux: Read sk->sk_family once in selinux_socket_bind()

On Fri, Dec 13, 2024 at 3:09 PM Paul Moore <paul@...l-moore.com> wrote:
>
> On Fri, Dec 13, 2024 at 11:40 AM Mikhail Ivanov
> <ivanov.mikhail1@...wei-partners.com> wrote:
> > On 12/13/2024 6:46 PM, Stephen Smalley wrote:
> > > On Fri, Dec 13, 2024 at 5:57 AM Mikhail Ivanov
> > > <ivanov.mikhail1@...wei-partners.com> wrote:
> > >>
> > >> On 12/12/2024 8:50 PM, Mickaël Salaün wrote:
> > >>> This looks good be there are other places using sk->sk_family that
> > >>> should also be fixed.
> > >>
> > >> Thanks for checking this!
> > >>
> > >> For selinux this should be enough, I haven't found any other places
> > >> where sk->sk_family could be read from an IPv6 socket without locking.
> > >>
> > >> I also would like to prepare such fix for other LSMs (apparmor, smack,
> > >> tomoyo) (in separate patches).
> > >
> > > I'm wondering about the implications for SELinux beyond just
> > > sk->sk_family access, e.g. SELinux maps the (family, type, protocol)
> > > triple to a security class at socket creation time via
> > > socket_type_to_security_class() and caches the security class in the
> > > inode_security_struct and sk_security_struct for later use.
> >
> > IPv6 and IPv4 TCP sockets are mapped to the same SECCLASS_TCP_SOCKET
> > security class. AFAICS there is no other places that can be affected by
> > the IPV6_ADDFORM transformation.
>
> Yes, thankfully we don't really encode the IP address family in any of
> the SELinux object classes so that shouldn't be an issue.  I also
> don't think we have to worry about the per-packet labeling protocols
> as it's too late in the communication to change the socket's
> associated packet labeling, it's either working or it isn't; we should
> handle the mapped IPv4 address already.
>
> I am a little concerned about bind being the only place where we have
> to worry about accessing sk_family while the socket isn't locked.  As
> an example, I'm a little concerned about the netfilter code paths; I
> haven't chased them down, but my guess is that the associated
> socket/sock isn't locked in those cases (in the relevant output and
> postroute cases, forward should be a non-issue).
>
> How bad is the performance impact of READ_ONCE()?  In other words, how
> stupid would it be to simply do all of our sock->sk_family lookups
> using READ_ONCE()?

I could be wrong, but I don't think there is any overhead except on Dec Alpha.