Re: [PATCH v6 3/5] seccomp: add a way to get a listener fd from ptrace

Tycho Andersen
2018-09-06T15:50:08Z

On Thu, Sep 06, 2018 at 05:45:25PM +0200, Jann Horn wrote:
> On Thu, Sep 6, 2018 at 5:29 PM Tycho Andersen <tycho@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.
> [...]
> > +long seccomp_new_listener(struct task_struct *task,
> > + unsigned long filter_off)
> > +{
> > + struct seccomp_filter *filter;
> > + struct file *listener;
> > + int fd;
> > +
> > + if (!capable(CAP_SYS_ADMIN))
> > + return -EACCES;
> > +
> > + 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);
>
> Did you mean to write something like `init_listener(task, filter)` here?

Yes, yes I did. Thanks, Jann.

Tycho