Re: [PATCH v14 3/4] selinux: teach SELinux about anonymous inodes

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

On Fri, Jan 8, 2021 at 12:33 AM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> From: Daniel Colascione <dancol@google.com>
>
> This change uses the anon_inodes and LSM infrastructure introduced in
> the previous patches to give SELinux the ability to control
> anonymous-inode files that are created using the new
> anon_inode_getfd_secure() function.
>
> A SELinux policy author detects and controls these anonymous inodes by
> adding a name-based type_transition rule that assigns a new security
> type to anonymous-inode files created in some domain. The name used
> for the name-based transition is the name associated with the
> anonymous inode for file listings --- e.g., "[userfaultfd]" or
> "[perf_event]".
>
> Example:
>
> type uffd_t;
> type_transition sysadm_t sysadm_t : anon_inode uffd_t "[userfaultfd]";
> allow sysadm_t uffd_t:anon_inode { create };
>
> (The next patch in this series is necessary for making userfaultfd
> support this new interface.  The example above is just
> for exposition.)
>
> Signed-off-by: Daniel Colascione <dancol@google.com>
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
> ---
>  security/selinux/hooks.c            | 59 +++++++++++++++++++++++++++++
>  security/selinux/include/classmap.h |  2 +
>  2 files changed, 61 insertions(+)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 644b17ec9e63..8b4e155b2930 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2934,6 +2933,63 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
>         return 0;
>  }
>
> +static int selinux_inode_init_security_anon(struct inode *inode,
> +                                           const struct qstr *name,
> +                                           const struct inode *context_inode)
> +{
> +       const struct task_security_struct *tsec = selinux_cred(current_cred());
> +       struct common_audit_data ad;
> +       struct inode_security_struct *isec;
> +       int rc;
> +
> +       if (unlikely(!selinux_initialized(&selinux_state)))
> +               return 0;
> +
> +       isec = selinux_inode(inode);
> +
> +       /*
> +        * We only get here once per ephemeral inode.  The inode has
> +        * been initialized via inode_alloc_security but is otherwise
> +        * untouched.
> +        */
> +       isec->initialized = LABEL_INITIALIZED;
> +       isec->sclass = SECCLASS_ANON_INODE;
> +
> +       if (context_inode) {
> +               struct inode_security_struct *context_isec =
> +                       selinux_inode(context_inode);
> +               if (context_isec->initialized != LABEL_INITIALIZED)
> +                       return -EACCES;
> +               if (context_isec->sclass != SECCLASS_ANON_INODE) {
> +                       pr_err("SELinux:  initializing anonymous inode with non-anonymous inode");
> +                       return -EACCES;
> +               }

This would preclude using this facility for anonymous inodes created
by kvm and other use cases.
Don't do this.

> +
> +               isec->sid = context_isec->sid;
> +       } else {
> +               rc = security_transition_sid(
> +                       &selinux_state, tsec->sid, tsec->sid,
> +                       isec->sclass, name, &isec->sid);
> +               if (rc)
> +                       return rc;
> +       }
> +
> +       /*
> +        * Now that we've initialized security, check whether we're
> +        * allowed to actually create this type of anonymous inode.
> +        */
> +
> +       ad.type = LSM_AUDIT_DATA_INODE;
> +       ad.u.inode = inode;
> +
> +       return avc_has_perm(&selinux_state,
> +                           tsec->sid,
> +                           isec->sid,
> +                           isec->sclass,
> +                           ANON_INODE__CREATE,

FILE__CREATE is perfectly appropriate here, not that it makes any difference.