[PATCH] selinux: Fix use of KEY_NEED_* instead of KEY__* perms [v2]

[Stephen Smalley]

On Tue, Apr 28, 2020 at 8:54 AM David Howells <dhowells at redhat.com> wrote:
>
> selinux: Fix use of KEY_NEED_* instead of KEY__* perms
>
> selinux_key_permission() is passing the KEY_NEED_* permissions to
> avc_has_perm() instead of the KEY__* values.  It happens to work because
> the values are all coincident.
>
> Fixes: d720024e94de ("[PATCH] selinux: add hooks for key subsystem")
> Reported-by: Paul Moore <paul at paul-moore.com>
> Signed-off-by: David Howells <dhowells at redhat.com>
> ---
>  security/selinux/hooks.c |   23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 0b4e32161b77..4b6624e5dab4 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6539,20 +6539,39 @@ static void selinux_key_free(struct key *k)
>         kfree(ksec);
>  }
>
> +static unsigned int selinux_keyperm_to_av(unsigned int need_perm)
> +{
> +       switch (need_perm) {
> +       case KEY_NEED_VIEW:     return KEY__VIEW;
> +       case KEY_NEED_READ:     return KEY__READ;
> +       case KEY_NEED_WRITE:    return KEY__WRITE;
> +       case KEY_NEED_SEARCH:   return KEY__SEARCH;
> +       case KEY_NEED_LINK:     return KEY__LINK;
> +       case KEY_NEED_SETATTR:  return KEY__SETATTR;
> +       default:
> +               WARN_ON(1);
> +               return 0;
> +       }
> +}
> +
>  static int selinux_key_permission(key_ref_t key_ref,
>                                   const struct cred *cred,
> -                                 unsigned perm)
> +                                 unsigned need_perm)
>  {
>         struct key *key;
>         struct key_security_struct *ksec;
> +       unsigned int perm;
>         u32 sid;
>
>         /* if no specific permissions are requested, we skip the
>            permission check. No serious, additional covert channels
>            appear to be created. */
> -       if (perm == 0)
> +       if (need_perm == 0)
>                 return 0;
>
> +       perm = selinux_keyperm_to_av(need_perm);
> +       if (perm == 0)
> +               return -EPERM;
>         sid = cred_sid(cred);
>
>         key = key_ref_to_ptr(key_ref);

1) Are we guaranteed that the caller only ever passes a single
KEY_NEED_* perm at a time (i.e. hook is never called with a bitmask
of multiple permissions)?  Where is that guarantee enforced?

2) We had talked about adding a BUILD_BUG_ON() or other build-time
guard to ensure that new KEY_NEED_* permissions
are not added without updating SELinux.  We already have similar
constructs for catching new capabilities (#if CAP_LAST_CAP > 63 #error
...), socket address families (#if PF_MAX > 45 #error ...),  RTM_* and
XFRM_MSG* values.