Quoting Eric W. Biederman (ebied...@xmission.com):
>
> Andy Lutomirski <l...@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" <ebied...@xmission.com>
Acked-by: Serge Hallyn <serge.hal...@canonical.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(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index c1bbe86..398a50f 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2781,7 +2781,8 @@ static int mntns_install(struct nsproxy *nsproxy, void
> *ns)
> struct path root;
>
> if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
> - !nsown_capable(CAP_SYS_CHROOT))
> + !nsown_capable(CAP_SYS_CHROOT) ||
> + !nsown_capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> if (fs->users != 1)
> diff --git a/ipc/namespace.c b/ipc/namespace.c
> index cf3386a..7c1fa45 100644
> --- a/ipc/namespace.c
> +++ b/ipc/namespace.c
> @@ -170,7 +170,8 @@ static void ipcns_put(void *ns)
> static int ipcns_install(struct nsproxy *nsproxy, void *new)
> {
> struct ipc_namespace *ns = new;
> - if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
> + if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN) ||
> + !nsown_capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> /* Ditch state from the old ipc namespace */
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index 560da0d..fdbd0cd 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -325,7 +325,8 @@ static int pidns_install(struct nsproxy *nsproxy, void
> *ns)
> struct pid_namespace *active = task_active_pid_ns(current);
> struct pid_namespace *ancestor, *new = ns;
>
> - if (!ns_capable(new->user_ns, CAP_SYS_ADMIN))
> + if (!ns_capable(new->user_ns, CAP_SYS_ADMIN) ||
> + !nsown_capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> /*
> diff --git a/kernel/utsname.c b/kernel/utsname.c
> index f6336d5..08b197e 100644
> --- a/kernel/utsname.c
> +++ b/kernel/utsname.c
> @@ -113,7 +113,8 @@ static int utsns_install(struct nsproxy *nsproxy, void
> *new)
> {
> struct uts_namespace *ns = new;
>
> - if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
> + if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN) ||
> + !nsown_capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> get_uts_ns(ns);
> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> index 2e9a313..8acce01 100644
> --- a/net/core/net_namespace.c
> +++ b/net/core/net_namespace.c
> @@ -649,7 +649,8 @@ static int netns_install(struct nsproxy *nsproxy, void
> *ns)
> {
> struct net *net = ns;
>
> - if (!ns_capable(net->user_ns, CAP_SYS_ADMIN))
> + if (!ns_capable(net->user_ns, CAP_SYS_ADMIN) ||
> + !nsown_capable(CAP_SYS_ADMIN))
> return -EPERM;
>
> put_net(nsproxy->net_ns);
> --
> 1.7.5.4
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