[CFT][PATCH 6/7] userns: Add a knob to disable setgroups on a per user namespace basis

[Andy Lutomirski]

On Mon, Dec 8, 2014 at 2:11 PM, Eric W. Biederman <ebiederm at xmission.com> wrote:
>
> - Expose the knob to user space through a proc file /proc/<pid>/setgroups
>
>   A value of 0 means the setgroups system call is disabled in the

"deny"

>   current processes user namespace and can not be enabled in the
>   future in this user namespace.
>
>   A value of 1 means the segtoups system call is enabled.
>

"allow"

> - Descedent user namespaces inherit the value of setgroups from

s/Descedent/Descendent/

> --- a/kernel/groups.c
> +++ b/kernel/groups.c
> @@ -222,6 +222,7 @@ bool may_setgroups(void)
>          * the user namespace has been established.
>          */
>         return userns_gid_mappings_established(user_ns) &&
> +               userns_setgroups_allowed(user_ns) &&
>                 ns_capable(user_ns, CAP_SETGID);
>  }

Can you add a comment explaining the ordering?  For example:

We need to check for a gid mapping before checking setgroups_allowed
because an unprivileged user can create a userns with setgroups
allowed, then disallow setgroups and add a mapping.  If we check in
the opposite order, then we have a race: we could see that setgroups
is allowed before the user clears the bit and then see that there is a
gid mapping after the other thread is done.

--Andy


-- 
Andy Lutomirski
AMA Capital Management, LLC