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

[Paul Moore]

On Fri, Feb 14, 2020 at 6:42 PM Casey Schaufler <casey at 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 at chromium.org>
> Reviewed-by: John Johansen <john.johansen at canonical.com>
> Acked-by: Stephen Smalley <sds at tycho.nsa.gov>
> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
> cc: netdev at 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.

-- 
paul moore
www.paul-moore.com