[CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished

[Andy Lutomirski]

On Mon, Dec 8, 2014 at 2:07 PM, Eric W. Biederman <ebiederm at xmission.com> wrote:
>
> setgroups is unique in not needing a valid mapping before it can be called,
> in the case of setgroups(0, NULL) which drops all supplemental groups.
>
> The design of the user namespace assumes that CAP_SETGID can not actually
> be used until a gid mapping is established.  Therefore add a helper function
> to see if the user namespace gid mapping has been established and call
> that function in the setgroups permission check.
>
> This is part of the fix for CVE-2014-8989, being able to drop groups
> without privilege using user namespaces.
>
> Cc: stable at vger.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> ---
>  include/linux/user_namespace.h | 9 +++++++++
>  kernel/groups.c                | 7 ++++++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index e95372654f09..41cc26e5a350 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -37,6 +37,15 @@ struct user_namespace {
>
>  extern struct user_namespace init_user_ns;
>
> +static inline bool userns_gid_mappings_established(const struct user_namespace *ns)
> +{
> +       bool established;
> +       smp_mb__before_atomic();
> +       established = ACCESS_ONCE(ns->gid_map.nr_extents) != 0;
> +       smp_mb__after_atomic();
> +       return established;
> +}

I don't think this works on all platforms.  ACCESS_ONCE is not atomic
in the smp_mb__before_atomic sense.

> +
>  #ifdef CONFIG_USER_NS
>
>  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> diff --git a/kernel/groups.c b/kernel/groups.c
> index 02d8a251c476..e0335e44f76a 100644
> --- a/kernel/groups.c
> +++ b/kernel/groups.c
> @@ -6,6 +6,7 @@
>  #include <linux/slab.h>
>  #include <linux/security.h>
>  #include <linux/syscalls.h>
> +#include <linux/user_namespace.h>
>  #include <asm/uaccess.h>
>
>  /* init to 2 - one for init_task, one to ensure it is never freed */
> @@ -217,7 +218,11 @@ bool may_setgroups(void)
>  {
>         struct user_namespace *user_ns = current_user_ns();
>
> -       return ns_capable(user_ns, CAP_SETGID);
> +       /* It is not safe to use setgroups until a gid mapping in
> +        * the user namespace has been established.
> +        */
> +       return userns_gid_mappings_established(user_ns) &&
> +               ns_capable(user_ns, CAP_SETGID);
>  }
>
>  /*
> --
> 1.9.1
>

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC