[PATCH v18 05/23] net: Prepare UDS for security module stacking

[Stephen Smalley]

On Wed, Jul 8, 2020 at 8:23 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.
>
> The secid field of the unix_skb_parms structure has been
> replaced with a pointer to an lsmblob structure, and the
> lsmblob is allocated as needed. This is similar to how the
> list of passed files is managed. While an lsmblob structure
> will fit in the available space today, there is no guarantee
> that the addition of other data to the unix_skb_parms or
> support for additional security modules wouldn't exceed what
> is available.
>
> Reviewed-by: Kees Cook <keescook at chromium.org>
> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
> Cc: netdev at vger.kernel.org
> ---

> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 3385a7a0b231..d246aefcf4da 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -138,17 +138,23 @@ static struct hlist_head *unix_sockets_unbound(void *addr)
>  #ifdef CONFIG_SECURITY_NETWORK
>  static void unix_get_secdata(struct scm_cookie *scm, struct sk_buff *skb)
>  {
> -       UNIXCB(skb).secid = scm->secid;
> +       UNIXCB(skb).lsmdata = kmemdup(&scm->lsmblob, sizeof(scm->lsmblob),
> +                                     GFP_KERNEL);
>  }
>
>  static inline void unix_set_secdata(struct scm_cookie *scm, struct sk_buff *skb)
>  {
> -       scm->secid = UNIXCB(skb).secid;
> +       if (likely(UNIXCB(skb).lsmdata))
> +               scm->lsmblob = *(UNIXCB(skb).lsmdata);
> +       else
> +               lsmblob_init(&scm->lsmblob, 0);
>  }
>
>  static inline bool unix_secdata_eq(struct scm_cookie *scm, struct sk_buff *skb)
>  {
> -       return (scm->secid == UNIXCB(skb).secid);
> +       if (likely(UNIXCB(skb).lsmdata))
> +               return lsmblob_equal(&scm->lsmblob, UNIXCB(skb).lsmdata);
> +       return false;
>  }

I don't think that this provides sensible behavior to userspace.  On a
transient memory allocation failure, instead of returning an error to
the sender and letting them handle it, this will just proceed with
sending the message without its associated security information, and
potentially split messages on arbitrary boundaries because it cannot
tell whether the sender had the same security information.  I think
you instead need to change unix_get_secdata() to return an error on
allocation failure and propagate that up to the sender.  Not a fan of
this change in general both due to extra overhead on this code path
and potential for breakage on allocation failures.  I know it was
motivated by paul's observation that we won't be able to fit many more
secids into the cb but not sure we have to go there prematurely,
especially absent its usage by upstream AA (no unix_stream_connect
hook implementation upstream).  Also not sure how the whole bpf local
storage approach to supporting security modules (or at least bpf lsm)
might reduce need for expanding these structures?