[PATCH v4 3/4] seccomp: add a way to get a listener fd from ptrace

[Jann Horn]

On Fri, Jun 22, 2018 at 12:04 AM Tycho Andersen <tycho at tycho.ws> wrote:
> As an alternative to SECCOMP_FILTER_FLAG_GET_LISTENER, perhaps a ptrace()
> version which can acquire filters is useful. There are at least two reasons
> this is preferable, even though it uses ptrace:
>
> 1. You can control tasks that aren't cooperating with you
> 2. You can control tasks whose filters block sendmsg() and socket(); if the
>    task installs a filter which blocks these calls, there's no way with
>    SECCOMP_FILTER_FLAG_GET_LISTENER to get the fd out to the privileged task.
[...]
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index bbc24938c51d..b68a5d4a15cd 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1743,6 +1743,34 @@ static struct file *init_listener(struct task_struct *task,
>
>         return ret;
>  }
> +
> +long seccomp_new_listener(struct task_struct *task,
> +                         unsigned long filter_off)
> +{
> +       struct seccomp_filter *filter;
> +       struct file *listener;
> +       int fd;
> +
> +       filter = get_nth_filter(task, filter_off);
> +       if (IS_ERR(filter))
> +               return PTR_ERR(filter);
> +
> +       fd = get_unused_fd_flags(0);
> +       if (fd < 0) {
> +               __put_seccomp_filter(filter);
> +               return fd;
> +       }
> +
> +       listener = init_listener(task, task->seccomp.filter);
> +       __put_seccomp_filter(filter);
> +       if (IS_ERR(listener)) {
> +               put_unused_fd(fd);
> +               return PTR_ERR(listener);
> +       }
> +
> +       fd_install(fd, listener);
> +       return fd;
> +}

I think there's a security problem here. Imagine the following scenario:

1. task A (uid==0) sets up a seccomp filter that uses SECCOMP_RET_USER_NOTIF
2. task A forks off a child B
3. task B uses setuid(1) to drop its privileges
4. task B becomes dumpable again, either via prctl(PR_SET_DUMPABLE, 1)
or via execve()
5. task C (the attacker, uid==1) attaches to task B via ptrace
6. task C uses PTRACE_SECCOMP_NEW_LISTENER on task B
7. because the seccomp filter is shared by task A and task B, task C
is now able to influence syscall results for syscalls performed by
task A

Unless I'm missing something, you might have to add some extra
security check here: Either a check to ensure that no other task is
using the same seccomp filter, or (as a last resort) a check for
capable(CAP_SYS_ADMIN).