[CFT][PATCH 5/7] userns: Only allow the creator of the userns unprivileged mappings

[Andy Lutomirski]

On Mon, Dec 8, 2014 at 2:10 PM, Eric W. Biederman <ebiederm at xmission.com> wrote:
>
> If you did not create the user namespace and are allowed
> to write to uid_map or gid_map you should already have the necessary
> privilege in the parent user namespace to establish any mapping
> you want so this will not affect userspace in practice.
>
> Limiting unprivileged uid mapping establishment to the creator of the
> user namespace reduces the set of credentials that must be verified
> can be obtained without privielge, making code verification simpler.
>

s/privielge/privilege/

But I still can't parse that sentence.

The code itself is:

Reviewed-by: Andy Lutomirski <luto at amacapital.net>

> Limiting unprivileged gid mapping establishment (which is temporarily
> absent) to the creator of the user namespace also ensures that the
> combination of uid and gid can already be obtained without privilege.
>
> This is part of the fix for CVE-2014-8989.
>
> Cc: stable at vger.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> ---
>  kernel/user_namespace.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index da1eeb927b21..413f60fd5983 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -812,14 +812,16 @@ static bool new_idmap_permitted(const struct file *file,
>                                 struct user_namespace *ns, int cap_setid,
>                                 struct uid_gid_map *new_map)
>  {
> +       const struct cred *cred = file->f_cred;
>         /* Don't allow mappings that would allow anything that wouldn't
>          * be allowed without the establishment of unprivileged mappings.
>          */
> -       if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1)) {
> +       if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1) &&
> +           uid_eq(ns->owner, cred->euid)) {
>                 u32 id = new_map->extent[0].lower_first;
>                 if (cap_setid == CAP_SETUID) {
>                         kuid_t uid = make_kuid(ns->parent, id);
> -                       if (uid_eq(uid, file->f_cred->euid))
> +                       if (uid_eq(uid, cred->euid))
>                                 return true;
>                 }
>         }
> --
> 1.9.1
>



-- 
Andy Lutomirski
AMA Capital Management, LLC