Re: [PATCH 27/59] NET: Store LSM access information in the socket blob for UDS

[Stephen Smalley <stephen.smalley@gmail.com>]

On Tue, Apr 9, 2019 at 5:42 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>
> UNIX domain socket connections don't have sufficient
> space in the socket buffer (skb) secmark for more than
> one Linux security module (LSM) to pass data. Expanding
> the secmark has been ruled out as an option. Store the
> necessary data in the socket security blob pointed to
> by the skb socket.

I don't believe this is correct.  The secid in the unix_skb_parms is
not the same as the secmark in the sk_buff, and I don't know if we are
necessarily prohibited from expanding it.  Also, I don't think you can
just store it in the socket security blob, especially without any form
of locking, as that can be shared across multiple sk_buffs.

>
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> ---
>  include/linux/security.h | 20 +++++++++++++++++++-
>  net/unix/af_unix.c       | 14 ++++++++------
>  security/security.c      | 17 ++++++++++++++++-
>  3 files changed, 43 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index e76d7a9dbe50..c413dcc1905a 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -71,6 +71,7 @@ struct ctl_table;
>  struct audit_krule;
>  struct user_namespace;
>  struct timezone;
> +struct sk_buff;
>
>  enum lsm_event {
>         LSM_POLICY_CHANGE,
> @@ -100,6 +101,22 @@ static inline bool lsm_export_any(struct lsm_export *l)
>                 ((l->flags & LSM_EXPORT_APPARMOR) && l->apparmor));
>  }
>
> +static inline bool lsm_export_equal(struct lsm_export *l, struct lsm_export *m)
> +{
> +       if (l->flags != m->flags || l->flags == LSM_EXPORT_NONE)
> +               return false;
> +       if (l->flags & LSM_EXPORT_SELINUX &&
> +           (l->selinux != m->selinux || l->selinux == 0))
> +               return false;
> +       if (l->flags & LSM_EXPORT_SMACK &&
> +           (l->smack != m->smack || l->smack == 0))
> +               return false;
> +       if (l->flags & LSM_EXPORT_APPARMOR &&
> +           (l->apparmor != m->apparmor || l->apparmor == 0))
> +               return false;
> +       return true;
> +}
> +
>  /**
>   * lsm_export_secid - pull the useful secid out of a lsm_export
>   * @data: the containing data structure
> @@ -143,6 +160,8 @@ static inline void lsm_export_to_all(struct lsm_export *data, u32 secid)
>                       LSM_EXPORT_APPARMOR;
>  }
>
> +extern struct lsm_export *lsm_export_skb(struct sk_buff *skb);
> +
>  /* These functions are in security/commoncap.c */
>  extern int cap_capable(const struct cred *cred, struct user_namespace *ns,
>                        int cap, unsigned int opts);
> @@ -174,7 +193,6 @@ extern int cap_task_setnice(struct task_struct *p, int nice);
>  extern int cap_vm_enough_memory(struct mm_struct *mm, long pages);
>
>  struct msghdr;
> -struct sk_buff;
>  struct sock;
>  struct sockaddr;
>  struct socket;
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 4d4107927ba2..afe9c9f1adeb 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -143,21 +143,23 @@ 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)
>  {
> -       lsm_export_secid(&scm->le, &(UNIXCB(skb).secid));
> +       struct lsm_export *ble = lsm_export_skb(skb);
> +
> +       *ble = scm->le;
>  }
>
>  static inline void unix_set_secdata(struct scm_cookie *scm, struct sk_buff *skb)
>  {
> -       lsm_export_to_all(&scm->le, UNIXCB(skb).secid);
> +       struct lsm_export *ble = lsm_export_skb(skb);
> +
> +       scm->le = *ble;
>  }
>
>  static inline bool unix_secdata_eq(struct scm_cookie *scm, struct sk_buff *skb)
>  {
> -       u32 best_secid;
> -
> -       lsm_export_secid(&scm->le, &best_secid);
> -       return (best_secid == UNIXCB(skb).secid);
> +       return lsm_export_equal(&scm->le, lsm_export_skb(skb));
>  }
> +
>  #else
>  static inline void unix_get_secdata(struct scm_cookie *scm, struct sk_buff *skb)
>  { }
> diff --git a/security/security.c b/security/security.c
> index 69983ad68233..015c38c882ba 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -46,7 +46,22 @@ static struct kmem_cache *lsm_file_cache;
>  static struct kmem_cache *lsm_inode_cache;
>
>  char *lsm_names;
> -static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init;
> +
> +/* Socket blobs include infrastructure managed data */
> +static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init = {
> +       .lbs_sock = sizeof(struct lsm_export),
> +};
> +
> +/**
> + * lsm_export_skb - pointer to the lsm_export associated with the skb
> + * @skb: the socket buffer
> + *
> + * Returns a pointer to the LSM managed data.
> + */
> +struct lsm_export *lsm_export_skb(struct sk_buff *skb)
> +{
> +       return skb->sk->sk_security;
> +}
>
>  /* Boot-time LSM user choice */
>  static __initdata const char *chosen_lsm_order;
> --
> 2.19.1
>