Re: [PATCH 2/4] userns: Require CAP_SYS_ADMIN for most uses of setns.

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Dec 14, 2012 at 2:03 PM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
>
> Andy Lutomirski <luto@amacapital.net> found a nasty little bug in
> the permissions of setns.  With unprivileged user namespaces it
> became possible to create new namespaces without privilege.
>
> However the setns calls were relaxed to only require CAP_SYS_ADMIN in
> the user nameapce of the targed namespace.
>
> Which made the following nasty sequence possible.
>
> pid = clone(CLONE_NEWUSER | CLONE_NEWNS);
> if (pid == 0) { /* child */
>         system("mount --bind /home/me/passwd /etc/passwd");
> }
> else if (pid != 0) { /* parent */
>         char path[PATH_MAX];
>         snprintf(path, sizeof(path), "/proc/%u/ns/mnt");
>         fd = open(path, O_RDONLY);
>         setns(fd, 0);
>         system("su -");
> }
>
> Prevent this possibility by requiring CAP_SYS_ADMIN
> in the current user namespace when joing all but the user namespace.
>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  fs/namespace.c           |    3 ++-
>  ipc/namespace.c          |    3 ++-
>  kernel/pid_namespace.c   |    3 ++-
>  kernel/utsname.c         |    3 ++-
>  net/core/net_namespace.c |    3 ++-
>  5 files changed, 10 insertions(+), 5 deletions(-)

Acked-by: Andy Lutomirski <luto@amacapital.net>