Re: [PATCH v15 05/23] net: Prepare UDS for security module stacking

From: Paul Moore <paul@paul-moore.com>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: casey.schaufler@intel.com, James Morris <jmorris@namei.org>,
	linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
	keescook@chromium.org, john.johansen@canonical.com,
	penguin-kernel@i-love.sakura.ne.jp,
	Stephen Smalley <sds@tycho.nsa.gov>
Subject: Re: [PATCH v15 05/23] net: Prepare UDS for security module stacking
Date: Mon, 9 Mar 2020 21:02:41 -0400	[thread overview]
Message-ID: <CAHC9VhTAk7+Zuh6vcOd=9hb-5aWY6=Rgv2aVqnSCm1q+TCHNGA@mail.gmail.com> (raw)
In-Reply-To: <50b463c1-1ff1-caad-3e4c-6e822e1c4a7a@schaufler-ca.com>

On Mon, Mar 9, 2020 at 8:13 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 3/6/2020 2:14 PM, Paul Moore wrote:
> > On Fri, Feb 14, 2020 at 6:42 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> >> Change the data used in UDS SO_PEERSEC processing from a
> >> secid to a more general struct lsmblob. Update the
> >> security_socket_getpeersec_dgram() interface to use the
> >> lsmblob. There is a small amount of scaffolding code
> >> that will come out when the security_secid_to_secctx()
> >> code is brought in line with the lsmblob.
> >>
> >> Reviewed-by: Kees Cook <keescook@chromium.org>
> >> Reviewed-by: John Johansen <john.johansen@canonical.com>
> >> Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
> >> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> >> cc: netdev@vger.kernel.org
> >> ---
> >>  include/linux/security.h |  7 +++++--
> >>  include/net/af_unix.h    |  2 +-
> >>  include/net/scm.h        |  8 +++++---
> >>  net/ipv4/ip_sockglue.c   |  8 +++++---
> >>  net/unix/af_unix.c       |  6 +++---
> >>  security/security.c      | 18 +++++++++++++++---
> >>  6 files changed, 34 insertions(+), 15 deletions(-)
> > ...
> >
> >> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> >> index 17e10fba2152..59af08ca802f 100644
> >> --- a/include/net/af_unix.h
> >> +++ b/include/net/af_unix.h
> >> @@ -36,7 +36,7 @@ struct unix_skb_parms {
> >>         kgid_t                  gid;
> >>         struct scm_fp_list      *fp;            /* Passed files         */
> >>  #ifdef CONFIG_SECURITY_NETWORK
> >> -       u32                     secid;          /* Security ID          */
> >> +       struct lsmblob          lsmblob;        /* Security LSM data    */
> >>  #endif
> >>         u32                     consumed;
> >>  } __randomize_layout;
> > This might be a problem.  As it currently stands, the sk_buff.cb field
> > is 48 bytes; with CONFIG_SECURITY_NETWORK=n unix_skb_parms is 28 bytes
> > on a 64-bit system.  That leaves 20 bytes (room for 5 LSMs) assuming a
> > tight packing *and* that netdev doesn't swoop in and drop another few
> > fields in unix_skb_parms.
> >
> > This may work now, and you might manage to sneak this by the netdev
> > crowd, but I predict problems in the future.
>
> Do you think that making this a struct lsmblob * instead would make
> the change more likely to be accepted? It would complicate the code
> but remove the issue.

I honestly have no idea anymore when it comes to the netdev crowd.  I
can toss out a few examples, but you've been in this space long enough
to have seen the same things I have wrt to LSMs and the networking
folks.

Regardless of the implementation, I don't think you can embed the
lsmblob struct in the skb.cb; room for five LSMs is likely going to be
a limiting factor.  Once you settle on that, no matter what you do for
a reference, pointer/index/etc., the problems are all roughly the
same.  The trick is to find out what netdev will begrudgingly accept,
and for that I'm afraid you'll need to ask them directly.

-- 
paul moore
www.paul-moore.com