Re: [PATCH v7 3/6] seccomp: add a way to get a listener fd from ptrace

[Christian Brauner]

On Wed, Oct 10, 2018 at 10:45:29AM -0700, Andy Lutomirski wrote:
> On Mon, Oct 8, 2018 at 11:00 AM Tycho Andersen <tycho@xxxxxxxx> wrote:
> >
> > On Mon, Oct 08, 2018 at 05:16:30PM +0200, Christian Brauner wrote:
> > > On Thu, Sep 27, 2018 at 09:11:16AM -0600, Tycho Andersen 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.
> > >
> > > So for the slow of mind aka me:
> > > I'm not sure I completely understand this problem. Can you outline how
> > > sendmsg() and socket() are involved in this?
> > >
> > > I'm also not sure that this holds (but I might misunderstand the
> > > problem) afaict, you could do try to get the fd out via CLONE_FILES and
> > > other means so something like:
> > >
> > > // let's pretend the libc wrapper for clone actually has sane semantics
> > > pid = clone(CLONE_FILES);
> > > if (pid == 0) {
> > >         fd = seccomp(SECCOMP_SET_MODE_FILTER, SECCOMP_FILTER_FLAG_NEW_LISTENER, &prog);
> > >
> > >         // Now this fd will be valid in both parent and child.
> > >         // If you haven't blocked it you can inform the parent what
> > >         // the fd number is via pipe2(). If you have blocked it you can
> > >         // use dup2() and dup to a known fd number.
> > > }
> >
> > But what if your seccomp filter wants to block both pipe2() and
> > dup2()? Whatever syscall you want to use to do this could be blocked
> > by some seccomp policy, which means you might not be able to use this
> > feature in some cases.
> 
> You don't need a syscall at all. You can use shared memory.

Yeah, I pointed that out too in the next mail. :)

> 
> >
> > Perhaps it's unlikely, and we can just go forward knowing this. But it
> > seems like it is worth at least acknowledging that you can wedge
> > yourself into a corner.
> >
> 
> I think that what we *really* want is a way to create a seccomp fitter

I thought about this exact thing when discussing my reservations about
ptrace() but I didn't want to defer this patchset any longer. But I
really like this idea of being able to get an fd *before* the filter is
loaded.

> and activate it later (on execve or via another call to seccomp(),
> perhaps).  And we already sort of have that using ptrace() but a
> better interface would be nice when a real use case gets figured out.