Re: [PATCH v3 1/4] seccomp: add a return code to trap to userspace

[Tycho Andersen]

Hi Jann,

On Sun, Jun 03, 2018 at 08:41:01PM +0200, Jann Horn wrote:
> On Sun, Jun 3, 2018 at 2:29 PM Tycho Andersen <tycho@xxxxxxxx> wrote:
> >
> > This patch introduces a means for syscalls matched in seccomp to notify
> > some other task that a particular filter has been triggered.
> >
> > The motivation for this is primarily for use with containers. For example,
> > if a container does an init_module(), we obviously don't want to load this
> > untrusted code, which may be compiled for the wrong version of the kernel
> > anyway. Instead, we could parse the module image, figure out which module
> > the container is trying to load and load it on the host.
> >
> > As another example, containers cannot mknod(), since this checks
> > capable(CAP_SYS_ADMIN). However, harmless devices like /dev/null or
> > /dev/zero should be ok for containers to mknod, but we'd like to avoid hard
> > coding some whitelist in the kernel. Another example is mount(), which has
> > many security restrictions for good reason, but configuration or runtime
> > knowledge could potentially be used to relax these restrictions.
> >
> > This patch adds functionality that is already possible via at least two
> > other means that I know about, both of which involve ptrace(): first, one
> > could ptrace attach, and then iterate through syscalls via PTRACE_SYSCALL.
> > Unfortunately this is slow, so a faster version would be to install a
> > filter that does SECCOMP_RET_TRACE, which triggers a PTRACE_EVENT_SECCOMP.
> > Since ptrace allows only one tracer, if the container runtime is that
> > tracer, users inside the container (or outside) trying to debug it will not
> > be able to use ptrace, which is annoying. It also means that older
> > distributions based on Upstart cannot boot inside containers using ptrace,
> > since upstart itself uses ptrace to start services.
> >
> > The actual implementation of this is fairly small, although getting the
> > synchronization right was/is slightly complex.
> >
> > Finally, it's worth noting that the classic seccomp TOCTOU of reading
> > memory data from the task still applies here, but can be avoided with
> > careful design of the userspace handler: if the userspace handler reads all
> > of the task memory that is necessary before applying its security policy,
> > the tracee's subsequent memory edits will not be read by the tracer.
> [...]
> > @@ -857,13 +1020,28 @@ static long seccomp_set_mode_filter(unsigned int flags,
> >         if (IS_ERR(prepared))
> >                 return PTR_ERR(prepared);
> >
> > +       if (flags & SECCOMP_FILTER_FLAG_GET_LISTENER) {
> > +               listener = get_unused_fd_flags(O_RDWR);
> 
> I think you want either 0 or O_CLOEXEC here?

Do we? I suppose it makes sense to be able to set CLOEXEC, but I could
imagine a case where a handler wanted to fork+exec to handle
something. I'm happy to make the change, but it's not obvious to me
that it's what we want by default.

> > +out_put_fd:
> > +       if (flags & SECCOMP_FILTER_FLAG_GET_LISTENER) {
> > +               if (ret < 0) {
> > +                       fput(listener_f);
> > +                       put_unused_fd(listener);
> > +               } else {
> > +                       fd_install(listener, listener_f);
> > +                       ret = listener;
> > +               }
> > +       }
> >  out_free:
> >         seccomp_filter_free(prepared);
> >         return ret;
> [...]
> > +static __poll_t seccomp_notify_poll(struct file *file,
> > +                                   struct poll_table_struct *poll_tab)
> > +{
> > +       struct seccomp_filter *filter = file->private_data;
> > +       __poll_t ret = 0;
> > +       struct seccomp_knotif *cur;
> > +
> > +       ret = mutex_lock_interruptible(&filter->notify_lock);
> > +       if (ret < 0)
> > +               return ret;
> > +
> > +       list_for_each_entry(cur, &filter->notifications, list) {
> > +               if (cur->state == SECCOMP_NOTIFY_INIT)
> > +                       ret |= EPOLLIN | EPOLLRDNORM;
> > +               if (cur->state == SECCOMP_NOTIFY_SENT)
> > +                       ret |= EPOLLOUT | EPOLLWRNORM;
> > +       }
> > +
> > +       mutex_unlock(&filter->notify_lock);
> > +
> > +       return ret;
> > +}
> 
> I don't think f_op->poll handlers work like this. AFAIK you're
> supposed to use something like poll_wait() to connect the caller to
> something like a waitqueue head, so that as soon as the file becomes
> ready for reading/writing, any waiting task is notified. See
> eventfd_poll() in fs/eventfd.c for a simple example. AFAICS in the
> current code, seccomp_notify_poll() only works if an event is pending
> at the time seccomp_notify_poll() is called.

Arg. I was trying to avoid adding yet another piece of
synchronization, but perhaps it's not possible. Thanks for pointing
this out.

Tycho