Re: [PATCH 2/2] Notify container-init parent a 'reboot' occured

Serge Hallyn
2011-08-11T21:10:37Z

Quoting Daniel Lezcano (daniel.lezcano@free.fr):
> When the reboot syscall is called and the pid namespace where the calling
> process belongs to is not from the init pidns, we send a SIGCHLD with CLD_REBOOTED
> to the parent of this pid namespace.
>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>

...

> +void do_notify_parent_cldreboot(struct task_struct *tsk, int why, char *buffer)
> +{
> + struct siginfo info = { };
> + struct task_struct *parent;
> + struct sighand_struct *sighand;
> + unsigned long flags;
> +
> + if (tsk->ptrace)
> + parent = tsk->parent;
> + else {
> + tsk = tsk->group_leader;
> + parent = tsk->real_parent;
> + }
> +
> + info.si_signo = SIGCHLD;
> + info.si_errno = 0;
> + info.si_status = why;
> +
> + rcu_read_lock();
> + info.si_pid = task_pid_nr_ns(tsk, parent->nsproxy->pid_ns);
> + info.si_uid = __task_cred(tsk)->uid;

This eventually should become:

info.si_uid = user_ns_map_uid(task_cred_xxx(t, user_ns),
current_cred(), current_uid());

I've got a first-stab patch at converting the rest of
kernel/signal.c in http://kernel.ubuntu.com/git?p=serge/userns-2.6.git

> + rcu_read_unlock();
> +
> + info.si_utime = cputime_to_clock_t(tsk->utime);
> + info.si_stime = cputime_to_clock_t(tsk->stime);
> +
> + info.si_code = CLD_REBOOTED;
> +
> + sighand = parent->sighand;
> + spin_lock_irqsave(&sighand->siglock, flags);
> + if (sighand->action[SIGCHLD-1].sa.sa_handler != SIG_IGN &&
> + sighand->action[SIGCHLD-1].sa.sa_flags & SA_CLDREBOOT)
> + __group_send_sig_info(SIGCHLD, &info, parent);
> + /*
> + * Even if SIGCHLD is not generated, we must wake up wait4 calls.
> + */
> + __wake_up_parent(tsk, parent);
> + spin_unlock_irqrestore(&sighand->siglock, flags);
> +}

...

> @@ -426,10 +434,18 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
> {
> char buffer[256];
> int ret = 0;
> + struct pid_namespace *pid_ns = current->nsproxy->pid_ns;
> +
> + /* We only trust the superuser with rebooting the system. */
> + if (!capable(CAP_SYS_BOOT)) {

Doesn't this mean that an unprivileged task in a container can shut
down the container?

The pidns->user_ns patch I sent earlier today gives you what you need
so that you can add

if (!ns_capable(current_pid_ns()->user_ns, CAP_SYS_BOOT)
return -EPERM;

right here to prevent that.

> + /* If we are not in the initial pid namespace, we send a signal
> + * to the parent of this init pid namespace, notifying a shutdown
> + * occured */
> + if (pid_ns != &init_pid_ns)
> + pid_namespace_reboot(pid_ns, cmd, buffer);
>
> - /* We only trust the superuser with rebooting the system. */
> - if (!capable(CAP_SYS_BOOT))
> return -EPERM;
> + }
>
> /* For safety, we require "magic" arguments. */
> if (magic1 != LINUX_REBOOT_MAGIC1 ||
> --
> 1.7.4.1
>
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